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Reversing the Inverse Square Law

A Guest Post by Dustin Diaz from Flash Bullet Photography

dps-intro.jpg

Admittedly, one of the most boring subjects in photography is the inverse square law. But before you begin scratching your head and yelling out “the inverse-what?” Just hold on a sec.

First off, one does not need to memorize the laws of light to go and start taking pictures, or even to become a professional. But anyone with an SLR that truly wants to master all the variables in an exposure, you should at least know about it, and have a good sense of how it works. And of course the main purpose to know about the inverse square law (if you haven’t figured it out already) is when making the dive into flash photography.

Therefore, if you’re one of those who claim “I don’t shoot with flash, I only use available light,” then have fun on your little pedestal making up excuses on why you pretend not to be interested in flash photography. Sure your brand new Canon 5Dmkii or Nikon D3 with a 50mm f/1.4 lens is going to destroy the darkness with stunning images at ISO 3200, but we are talking about professional studio portraiture / magazine quality photography that utilizes flash to create amazingly sharp, colorful, and beautifully lit photos. Not to mention, your flashes are “always available” — use them to your advantage.

Alright, now that we have that out of the way, let’s look at some basics that we might already know.

Shutter Speed, Aperture, and ISO sensitivity

Shutter Speed

By now everyone knows that the longer the shutter speed, the more light you let in. Inversely, the faster the shutter, you let in less light. And of course, stops of light (in terms of shutter speed) work in factors of two. That means if you double a shutter speed of 1/100, it becomes 1/50. That is ‘one stop of light’ brighter. Inversely, if you cut it in half from 1/100 → 1/200, you have made your exposure ‘one stop darker’.

Great, let’s move on.

Aperture

dps-1.jpg

dps-1-histogram.png

SB-900 @ 1/2 power – 3ft away


We should all have our f stops memorized. If not, it’s a simple scale where we can double our numbers, starting at f/1, and each time we double, we will increase by two stops of light. Therefore:

1 → 2 → 4 → 8 → 16 → 32

To calculate our increments in-between, we simply multiply each stop by our magic number 1.4 (which is, coincidentally, the rounded number of the square root of 2 (which is ~ 1.414)). This makes our first stop easy. 1 x 1.4 = 1.4! We can now fill in our gaps accordingly!

1.0 → 1.4 → 2.0 → 2.8 → 4 → 5.6 → 8 → 11 → 16 → 22 → 32

We now have an aperture scale displaying full one stop increments. Just for kicks, let’s have a brief look at what a classic third aperture scale looks like (since many of you will be working with these numbers on your SLR):

… → 1.4 → 1.6 → 1.8 → 2.0 → 2.2 → 2.5 → 2.8 → …

Ok, I get the aperture numbers, remind me of the exposure relationship?

Let’s say we had a correct exposure of 1/125 at f/8. Your model sitting their patiently is waiting for their beautiful portrait to be taken. You (the photographer) have made a design decision to go for a more shallow depth of field. So you drop your aperture to f/4. That’s two stops of light brighter. f/8 → f/5.6 → f/4. So to compensate, you speed up your shutter by two stops. 1/125 → 1/250 → 1/500. Easy peezy, makes sense, been there done that.

ISO Speed

dps-2.jpg

dps-2-histogram.png

SB-900 @ 1/32 power – 3ft away


We all know that our lowest ISO produces our cleanest files. Our light stops will look fairly similar and our scale usually looks something like this:

100 → 200 → 400 → 800 → 1600

Some of the latest cameras like the Nikon D700/D3, or the Canon 5Dmkii go up to crazy ISO’s like 25,600, which is four stops brighter than ISO 1600!

Jumping quickly back to our example, if we were originally at ISO 400 and needed to drop two stops of light, we could have simply went from ISO 400 → 200 → 100

Ok, now what about this flash thing?

Flashes have stops of light too! These increments are measured in half increments, just like shutter speed. This is what we call the “flash power,” or rather, how much light it spits out. Our scale looks like this:

1/1 → 1/2 → 1/4 → 1/8 → 1/16

From left to right, we say “full power,” “half power,” “quarter power,” etc. Some flashes can let out light as dim as 1/128 like the SB-800/900 or the Canon 580EXii.

One of the main things you should consider when buying a flash, is to look at how powerful they are. Meaning, “how bright will this be at full power compared to this other flash at full power?” We can tell how powerful they are by looking at the flash Guide Number. This number is extremely important to know when buying a flash. Almost nearly as important as knowing how many millimeters your lens is. For example, you wouldn’t go out and buy a 200mm lens without knowing it’s, well, 200mm! Right?

What is a Guide Number?

In simple mathematical terms, it is aperture multiplied by distance, in which your flash can properly expose a subject at a given distance (aperture and distance). The standard Guide Numbers you should be looking for are measured at ISO 100 (film speed), at the 35mm head position, at full 1/1 power.

Let’s take a basic Nikon flash, for example the SB-600 has a G.N. of 98. To make our math easier, let’s just round it off to 100. What this means is the following. Take a look at our ascii diagram. We have a flash (F), a subject (S). They are twenty feet apart in distance.

/
(F)——————————20′——————————(S)
\

To properly expose our subject, we would need an aperture of f/5.

f/5 × 20′ = 100 G.N.

Now if we moved our subject 20′ further…

/
(F)—————————————————————40′—————————————————————(S)
\

To compensate or this distance, we would need to let in more light! This means we’ll have to open up our aperture to f/2.5

f/2.5 × 40′ = 100 G.N.

If we kept our aperture at f/5, we would underexpose our subject. Likewise, if we opened up to much, for example, to f/1.4, we would overexpose our subject. Got it?

Ok, I’m ready for that inverse square thingy

Alright, if you insist: The inverse square law states that “the intensity of light radiating from a point source is inversely proportional to the square of the distance from the source.

Therefore an object twice as far away, receives only 1/4 the amount of light. Or if it’s twice as close, then it’s 4x as bright. Each of these is a difference of two stops.

This is confusing, I know. But luckily, and coincidentally, we already know a little about this already with our aperture scale. Just know that light has “depth” in the same way that our focal plane does. By now you know that the closer you get to an object, the shallower your depth of field looks, of which we can conclude that all objects behind your subject “quickly fall out of focus.” This same exact rule applies to light (Thank heavens!). Let’s look at a simple example, and we’ll use the numbers on our aperture scale to make it easy.

In this illustration, we added a background (B):

/
(F)——————————4′——————————(S)——————————4′——————————(B)
\

Let’s assume these settings:

ISO 100
f/4
flash is at 1/4 power

It is safe to assume that our background is two stops underexposed. (4′ → 5.6′ → 8′)

Now what if we moved our subject two feet closer toward the flash?

/
(F)————2′————(S)————————————————6′—————————————————(B)
\

Since we just moved our subject “twice as close” we made it two stops brighter! Therefore to make up for this overexposure, we need to do one of two things:

A) Dial down the flash two stops
1/4 → 1/8 → 1/16
or
B) Close down the aperture
f/4 → f/5.6 → f/8

Each will keep your subject properly exposed, but it’s an artistic decision for you to make if you want to keep your shallow depth of field (option A), or remove more ambient light and get a slightly sharper image (option B). Also pay close attention that due to the inverse square law, our background is now “four stops” underexposed!

8′ → 5.6′ → 4′ → 2.8′ → 2′

Cool! What flash do I buy? The one with the higher Guide Number, right?

Well, kind of, yes. There are some cases where newer, more expensive flashes will have lower Guide Numbers than say for instance, the 30 year old Vivitar 285HV for $89 (via B&H) that has a G.N. of 120 whereas we’ll see Canon selling their flagship 580EXii for over $400 which has the same G.N. Or even the new Nikon SB-900 has a G.N. of 111. What’s up with that? Well, clearly if you buy a new Nikon or Canon speedlight for your system, it will talk to your camera with all the latest and greatest i/e-TTL technology, the stops of light will range from full 1/1 power to 1/128, whereas the Vivitar only has stops of 1/1 → 1/2 → 1/4 → 1/16 (yes, it skips 1/8). The SB-900 will have a 17-200mm range of light to cover larger or tighter spaces. It has gel detection, three different patterns to throw at your subjects, it even has super quick recycling time.

But hey, don’t knock on the Vivitar; off camera, that’s a helluva bargain for that much power! Hook it up to your camera with a PC chord, or a pair of Pocket Wizards, and you’re golden.

With all that’s been said, go out, get a flash (if you don’t already have one), throw it on manual, and experiment by taking a ton of pictures.

This article has been republished for use at Digital Photography School.

About the Author: Dustin Diaz is an Engineer at Twitter Inc. and a professional photographer with his wife Erin Caton. Together they run Flash Bullet Photography in San Francisco.

Read more from our Tips & Tutorials category.

Guest Contributor This post was written by a guest contributor to DPS. Please see their details in the post above.

Become a Contributor: Check out Write for DPS page for details about how YOU can share your photography tips with the DPS community.

  • Zero-Equals-Infinity

    Thanks again Dustin; and your photos speak volumes.

    felipe, I am not qualified to answer your question in the general sense. There are several complicating factors which make it difficult to answer. One thing I can say is that using a current manufacturer’s flash with a current manufacturer’s camera will be safe. Third party flashes, such as Metz and Sigma will provide information about compatibility with their flash units, (otherwise nobody would buy them, not knowing if they would work.) It is an entirely different scenario if you are looking to hook up a 30 year old flash you picked up at a garage sale. You often can, but you will be paying for that bargain flash by having to spend time researching, (and the glory of the web is that somebody, somewhere, will all but certainly have done the leg work, if you can find his information with a well targeted search.)

    I own a Metz 54 MZ4-i, which I bought it a couple of years ago because it had a great feature set, and it provides the ability to change a module to make it compatible with other cameras. I did this when I bought a Canon G10, (which is a great camera), and before I bought my Nikon D3x. The joy is that I can use the full feature set of the flash on both cameras, and I all I had to do was buy a Nikon module (about $90.00).

    Today, I would likely buy the SB-900 flash units from Nikon due to the versatility, ease and power of Nikon’s Creative Lighting System (CLS). That system makes it possible to have studio quality lighting on location, in a small, low-weight package. The main drawback is that these small strobes do not have the output and cycling speed of studio light, but they are reasonably priced and very portable.

  • http://www.flickr.com/photos/trazomfreak Amanda K. Yepiz

    Will there be a test on this next week?

  • Felipe

    The problem with buying a camera manufacturer’s flash for me is the cost. The nikon SB-900 you mentioned (US$469 on Amazon.com) is worth US$1229.35 here in Brazil.
    Consequently, I am forced to look into other brands, especially Chinese ones. The problem with that is some of them don’t have any documented specs, neither on the box they come in, nor on the internet.
    If there’s no way to find out the trigger voltage by measuring, then it looks like I have a good reason to just shoot available light. :(

  • Dave Graham

    Dustin,

    Super presentation re: flash exposure!

    One correction in your basic presentation on apertures would help avoid confusing the novice. One of my students challenged my presentation on apertures/stops, in all good faith, by quoting your words:

    “We should all have our f stops memorized. If not, it’s a simple scale where we can double our numbers, starting at f/1, and each time we double, we will increase by two stops of light. Therefore:

    1 ? 2 ? 4 ? 8 ? 16 ? 32″

    The novice often confuses the “aperture number” (1, 2, 4, 8, 16, 32 etc.) with the reciprocal nomenclature (f/1, f/2, f/4, f/8, f/16. f/32 etc…) which is the accurate reflection of the relationship between aperture diameter and lens focal length.

    So, in fact “…we can double our numbers, starting at f/1., and each time we double, we will…” DECREASE (not increase) “…by two stops of light”.

    Best regards and congrats on an otherwise superb article which sharpened my perception of several aspects of flash photography.

    Dave Graham

  • Zero-Equals-Infinity

    Forgive my naivete felipe, but is it illegal or difficult to buy one over the net from the U.S. and get it shipped in, (saving ~750.00 USD?) Unless government is in bed with the importer, or intent on applying taxes which amount to tariffs, (with resulting problems to trade agreements), it would seem that the price difference should be self-correcting via web-sales. Otherwise, the difference would make a grey or black market all but inevitable. I have to assume that the government is taking a role in this. You may be forced to look at the less legitimate means to get your gear, though I expect no reply to this message if it is true for self-evident reasons.

    Good luck, and sorry to read about your challenges.

  • Felipe

    Amazon.com for some reason won’t ship the 580EXII to Brazil (in my post I mentioned the SB-900 just because you mentioned it previously). Other sites I tried won’t ship it here either. :/

    Hypothetically, if they DID ship it here, this is how I have to calculate the actual amount I’d pay:
    $445 – the flash itself
    + $60 – shipping&handling, just guessing here.
    + insurance – no idea how much, it’s usually included automatically in S&H.
    * 1.7 – The government charges 70% import tax
    = (445+60)*1.7 = $858 USD
    If I were to buy it in my hometown it’d be a crazy amount (around what I mentioned in the previous post), and if I were to ship it in from São Paulo it’d be around $910.

    While canon/nikon flashes are far better than generic flashes, $40 on dealextreme sounds tempting.

  • Nino

    Hi Dustin.

    Amazing article thank you very much. How do your example work with an ISO of 200. My Nikon D300 only has ISO 200 as the lowest number.

  • TomL

    Dustin:

    thank you for an extremely informative piece. It takes many of us to a new level of knowledge – thanks.
    Great comments by all added to the scope of the article.
    These are super masterclasses.

  • Sandeepan Kundu

    Really interesting. More helpful for beginners like me.

  • Robert

    Interesting article- My ideas added-
    For any fixed light source (flash on full power, tungsten bulb, bed lamp, studio lights, etc) the aperture scale when expressed as DISTANCE (you can use feet or metres) gives you an easyguide to positioning lights.
    Examples-.
    One light positioned at 2.8 metres then moved out to 4 metres REDUCES its illumination by ONE stop.
    A light at 5.6 feet moved to 4 feet will INCREASE its illumination on the subject by ONE stop.
    A light at 8 feet moved to 16 feet will REDUCE its illumination by TWO stops.
    TWO equal output lights- place one at 4 metres and the other at 5.6 metres will be 1 stop less.
    -
    The scale to remember is the most common 1 -1.4 -2 -2.8 -4 5.6 -8 -11 -16 -22 -32 -45 etc
    This simple use of the inverse square law is so easy to remember and most useful when using two equal and fixed light sources in a studio.

  • http://djib.fr/ djib

    Awesome article! Thanks for taking the time to write it.

    You should add something about exposure (or write another article ;) ). Zero-Equals-Infinity’s comment was a great help for me.

  • Payal

    Hi,

    Thanks for the article. I am one of those people who are terrified of flash photography and stick to available light as much as possible. Hopefully this will open me up to experimenting with flash.

    Just one pointer though, I would think instead of using 2 as a calculative reference it would help if you used any other number (say 3).. since 2*2=4 same as 2^2 :)

    Great Article! Thanks

    Cheers!
    Payal

  • rex

    Dustin , yr article was VERY helpful, I also found the repltes , Very helpful, folks I thank you ALL. Rex

  • damian young

    you know, i was supprised to see the ISL on here. even X-ray techs utilize it! except with with kilovoltage and miliamperage. mAs

  • http://www.jamesbrandon.cc/blog James Brandon

    Superb article Dustin. Love diving into the technical side of things and this article was a great resource for that. Keep up the good work. Their will always be people who point out errors instead of paying attention to there own! I just ignore them!

  • http://dpshots.com DP Shots

    That’s something unique and very useful. Appreciate the effort.

  • http://www.flickr.com/photos/macuache Macuache

    Excellent explanation, thank you so much!

  • Michael Mark

    PLEASE! The phrase “twice as close” describes an impossibility. “Twice” means “two times”. You can be two times the distance, two times as fast (4mph is two times as fast as 2mph). You can be one half the distance, or one half as far, or one half the speed, or one half as tall. You CAN be twice the distance or twice as tall. You CAN’T be twice as short, though, just as you can’t be twice as close.

    Basic.

  • http://dustindiaz.com Dustin Diaz

    Michael,
    Congrats on trolling the internet a year after an article was posted. Good on ya. You deserve a pat on the back

  • http://Flickr.com/scullionsnaps Kieran

    I am really bad at math and this artical (although makes sense) has went straight over my head.
    Well I ever understand the correct procedure in making good pictures?
    Or will I forever be : shooting, reviewing, deleting ?

Some older comments

  • Kieran

    October 27, 2011 08:24 pm

    I am really bad at math and this artical (although makes sense) has went straight over my head.
    Well I ever understand the correct procedure in making good pictures?
    Or will I forever be : shooting, reviewing, deleting ?

  • Dustin Diaz

    October 27, 2011 03:13 pm

    Michael,
    Congrats on trolling the internet a year after an article was posted. Good on ya. You deserve a pat on the back

  • Michael Mark

    October 27, 2011 01:00 pm

    PLEASE! The phrase "twice as close" describes an impossibility. "Twice" means "two times". You can be two times the distance, two times as fast (4mph is two times as fast as 2mph). You can be one half the distance, or one half as far, or one half the speed, or one half as tall. You CAN be twice the distance or twice as tall. You CAN'T be twice as short, though, just as you can't be twice as close.

    Basic.

  • Macuache

    January 5, 2011 02:41 am

    Excellent explanation, thank you so much!

  • DP Shots

    October 20, 2010 05:30 pm

    That's something unique and very useful. Appreciate the effort.

  • James Brandon

    October 16, 2010 01:35 am

    Superb article Dustin. Love diving into the technical side of things and this article was a great resource for that. Keep up the good work. Their will always be people who point out errors instead of paying attention to there own! I just ignore them!

  • damian young

    October 7, 2010 12:22 pm

    you know, i was supprised to see the ISL on here. even X-ray techs utilize it! except with with kilovoltage and miliamperage. mAs

  • rex

    October 3, 2010 03:48 pm

    Dustin , yr article was VERY helpful, I also found the repltes , Very helpful, folks I thank you ALL. Rex

  • Payal

    September 30, 2010 01:27 pm

    Hi,

    Thanks for the article. I am one of those people who are terrified of flash photography and stick to available light as much as possible. Hopefully this will open me up to experimenting with flash.

    Just one pointer though, I would think instead of using 2 as a calculative reference it would help if you used any other number (say 3).. since 2*2=4 same as 2^2 :)

    Great Article! Thanks

    Cheers!
    Payal

  • djib

    September 28, 2010 03:25 am

    Awesome article! Thanks for taking the time to write it.

    You should add something about exposure (or write another article ;) ). Zero-Equals-Infinity's comment was a great help for me.

  • Robert

    September 27, 2010 03:13 pm

    Interesting article- My ideas added-
    For any fixed light source (flash on full power, tungsten bulb, bed lamp, studio lights, etc) the aperture scale when expressed as DISTANCE (you can use feet or metres) gives you an easyguide to positioning lights.
    Examples-.
    One light positioned at 2.8 metres then moved out to 4 metres REDUCES its illumination by ONE stop.
    A light at 5.6 feet moved to 4 feet will INCREASE its illumination on the subject by ONE stop.
    A light at 8 feet moved to 16 feet will REDUCE its illumination by TWO stops.
    TWO equal output lights- place one at 4 metres and the other at 5.6 metres will be 1 stop less.
    -
    The scale to remember is the most common 1 -1.4 -2 -2.8 -4 5.6 -8 -11 -16 -22 -32 -45 etc
    This simple use of the inverse square law is so easy to remember and most useful when using two equal and fixed light sources in a studio.

  • Sandeepan Kundu

    September 27, 2010 02:59 pm

    Really interesting. More helpful for beginners like me.

  • TomL

    September 26, 2010 11:20 pm

    Dustin:

    thank you for an extremely informative piece. It takes many of us to a new level of knowledge - thanks.
    Great comments by all added to the scope of the article.
    These are super masterclasses.

  • Nino

    September 26, 2010 04:21 pm

    Hi Dustin.

    Amazing article thank you very much. How do your example work with an ISO of 200. My Nikon D300 only has ISO 200 as the lowest number.

  • Felipe

    September 26, 2010 01:57 pm

    Amazon.com for some reason won't ship the 580EXII to Brazil (in my post I mentioned the SB-900 just because you mentioned it previously). Other sites I tried won't ship it here either. :/

    Hypothetically, if they DID ship it here, this is how I have to calculate the actual amount I'd pay:
    $445 - the flash itself
    + $60 - shipping&handling, just guessing here.
    + insurance - no idea how much, it's usually included automatically in S&H.
    * 1.7 - The government charges 70% import tax
    = (445+60)*1.7 = $858 USD
    If I were to buy it in my hometown it'd be a crazy amount (around what I mentioned in the previous post), and if I were to ship it in from São Paulo it'd be around $910.

    While canon/nikon flashes are far better than generic flashes, $40 on dealextreme sounds tempting.

  • Zero-Equals-Infinity

    September 26, 2010 02:57 am

    Forgive my naivete felipe, but is it illegal or difficult to buy one over the net from the U.S. and get it shipped in, (saving ~750.00 USD?) Unless government is in bed with the importer, or intent on applying taxes which amount to tariffs, (with resulting problems to trade agreements), it would seem that the price difference should be self-correcting via web-sales. Otherwise, the difference would make a grey or black market all but inevitable. I have to assume that the government is taking a role in this. You may be forced to look at the less legitimate means to get your gear, though I expect no reply to this message if it is true for self-evident reasons.

    Good luck, and sorry to read about your challenges.

  • Dave Graham

    September 26, 2010 02:19 am

    Dustin,

    Super presentation re: flash exposure!

    One correction in your basic presentation on apertures would help avoid confusing the novice. One of my students challenged my presentation on apertures/stops, in all good faith, by quoting your words:

    "We should all have our f stops memorized. If not, it’s a simple scale where we can double our numbers, starting at f/1, and each time we double, we will increase by two stops of light. Therefore:

    1 ? 2 ? 4 ? 8 ? 16 ? 32"

    The novice often confuses the "aperture number" (1, 2, 4, 8, 16, 32 etc.) with the reciprocal nomenclature (f/1, f/2, f/4, f/8, f/16. f/32 etc...) which is the accurate reflection of the relationship between aperture diameter and lens focal length.

    So, in fact "...we can double our numbers, starting at f/1., and each time we double, we will..." DECREASE (not increase) "...by two stops of light".

    Best regards and congrats on an otherwise superb article which sharpened my perception of several aspects of flash photography.

    Dave Graham

  • Felipe

    September 25, 2010 11:31 pm

    The problem with buying a camera manufacturer's flash for me is the cost. The nikon SB-900 you mentioned (US$469 on Amazon.com) is worth US$1229.35 here in Brazil.
    Consequently, I am forced to look into other brands, especially Chinese ones. The problem with that is some of them don't have any documented specs, neither on the box they come in, nor on the internet.
    If there's no way to find out the trigger voltage by measuring, then it looks like I have a good reason to just shoot available light. :(

  • Amanda K. Yepiz

    September 25, 2010 05:22 pm

    Will there be a test on this next week?

  • Zero-Equals-Infinity

    September 25, 2010 01:59 pm

    Thanks again Dustin; and your photos speak volumes.

    felipe, I am not qualified to answer your question in the general sense. There are several complicating factors which make it difficult to answer. One thing I can say is that using a current manufacturer's flash with a current manufacturer's camera will be safe. Third party flashes, such as Metz and Sigma will provide information about compatibility with their flash units, (otherwise nobody would buy them, not knowing if they would work.) It is an entirely different scenario if you are looking to hook up a 30 year old flash you picked up at a garage sale. You often can, but you will be paying for that bargain flash by having to spend time researching, (and the glory of the web is that somebody, somewhere, will all but certainly have done the leg work, if you can find his information with a well targeted search.)

    I own a Metz 54 MZ4-i, which I bought it a couple of years ago because it had a great feature set, and it provides the ability to change a module to make it compatible with other cameras. I did this when I bought a Canon G10, (which is a great camera), and before I bought my Nikon D3x. The joy is that I can use the full feature set of the flash on both cameras, and I all I had to do was buy a Nikon module (about $90.00).

    Today, I would likely buy the SB-900 flash units from Nikon due to the versatility, ease and power of Nikon's Creative Lighting System (CLS). That system makes it possible to have studio quality lighting on location, in a small, low-weight package. The main drawback is that these small strobes do not have the output and cycling speed of studio light, but they are reasonably priced and very portable.

  • Kamal Kamil

    September 25, 2010 08:52 am

    What 4' stand for? Is it 4 meter or 4mm on your lens?

  • Kamal Kamil

    September 25, 2010 08:50 am

    Thanks for the informative article and also the comment rom all of you. All the information is very useful with me, since I'm a beginner in flash photography. Thank you very much

  • Tammy Snyder

    September 25, 2010 08:33 am

    That's the best explanation yet!
    Super(b)!

  • David Hardwick Photography

    September 25, 2010 05:08 am

    This article is a great teaching aid. Thanks for your hard work.

  • Felipe

    September 25, 2010 01:18 am

    Awesome article, it cleared some things up in my head. :)
    Regarding that last section, one thing I felt was missing is a little warning on the matter of hotshoe trigger voltages. I know the article is just about light specifically, but when answering the question "what flash should I buy?" voltage is an important concern if you're going to use a cable or put the flash directly on the camera for some reason.
    Anybody know if you can just put an analog voltmeter on the flash's contacts to find out its peak voltage?

  • Dustin Diaz

    September 25, 2010 01:17 am

    Zero-Equals-Infinity,
    You're right! Your additions were spot on.

    jlmiller,
    Best of luck :)

    Shelley J,
    Much appreciation. That says a lot that it was helpful. I banged my head on this subject for several weeks and writing it down became helpful even for myself.

    Ivor,
    I would suggest having a look at the plethora of strobist photos with informational setups taken during my 365 of 2009. http://www.flickr.com/photos/polvero/sets/72157615873440953/

  • Ivor

    September 24, 2010 07:05 pm

    Dustin!

    Truly excellent and informative - real clarity of writing and thought.

    Could I suggest that your next article is some examples anyone could shoot to illustrate each of your points?

  • Shelley J

    September 24, 2010 02:52 pm

    Hi Dustin as some one said its an amazing article, very well written and very helpful for slow learners like me. I too learn more on the job I have already read it twice and will a few more times to really get the picture. I must say tho' my interest in flash photography has suddenly surfaced after having died out since I left studio work. Thanx

  • jlmiller

    September 24, 2010 12:51 pm

    Thanks for the refresher course, perfect timing also because I'm shooting a cover using a technique of making the background 2-3 stop darker and out of focus while making the subject .5-1 stop brighter than norm. This is being shot around 4pm with sun at 45 degrees just before sunset.

  • Zero-Equals-Infinity

    September 24, 2010 11:48 am

    One thing that was not mentioned has to do with shutter speed. The usual relationship between shutter speed and aperture does not apply with flash photography, (but keep in mind how much available lighting you have as well.) For a night shot or any shot in which available light is well below the illumination to be provided by the flash, shutter speed will have little or no affect on exposure, though you do have to be aware of something called the shutter synchronization speed. Just follow the rule of never using a shutter speed which is faster than the synchronization speed. For most cameras this will be 1/200 or 1/250 of a second, and you should check your camera's manual.

    Flash duration is usually extremely short. An excellent discussion of this for the Canon 580EX is provided at http://www.photosbykev.com/wordpress/2008/07/12/canon-580ex-flash-duration/

    Since flash duration is usually well above the synchronization speed, the entire output of even full output from the flash will have reached the sensor while the shutter is open. This means that with respect to light from the flash, there will be no change of exposure from changing the shutter speed from say 1/250 to 1/32 of a second. You would think that this change would result in 3 stops more light, and it does not, at least with respect to the light from the flash. However, the contribution from available light will be 3 stops more, since the available light is constant. Assuming our night shot, 3 more stops of available light might bring a little detail out of deep shadows, but not much.

    An area where the contribution of available light is important is when you are using the flash to fill in a bit of the shadow details, or to light up a person who is backlit by the sun. (Ever see those photos where a person's face is severely underexposed because the sun is behind them. That is where you want to use your flash to increase the exposure of the face.) The upper-end Nikon, Canon and Metz flashes will help you out a lot in combination with a camera which uses TTL exposure, though you can always take a test shot at 1/4 power to see how the subject looks and adjust the output up or down manually if you want to avoid the technical considerations. If the face is blown out, dial it down a stop or two, and if the shadows are too pronounced power up the flash a stop or two, and/or open the aperture, or get closer.

    Fortunately, none of this is rocket science. Where things get tricky with lighting is where you start dealing with glare, and reflective versus non-reflective subjects. Metal and glass can be particularly vexing, so just be thankful that in the majority of outdoor settings with people, animals and flora, this is not an issue.

  • Speedy

    September 24, 2010 10:43 am

    Hi Dustin, thank you for your follow-up explanation. If the G.N. 100 is not applicable to that example then I get it - I just assumed it was the example extended. Upon re-read I also get that the calculation of my question-post was fundamentally flawed (I confused stops and factors), and contains typos (sloppy!). Anyway, thanks again for the article and the explanation. BTW, nice photos!

  • Emma

    September 24, 2010 10:15 am

    Thanks Dustin. They way you spelled out the meanings of the numbers has really helped and I now can't wait to get my camera out and actually put the theory to practice!

  • kirpi

    September 24, 2010 06:43 am

    Thanks for posting. Flash is a tricky tool, and off camera flash is twice as tricky and twice as useful, as well :-)

  • WOG

    September 24, 2010 04:26 am

    Excellent article very helpful & informative now I realize that I really didn't properly understand the thousands of Tutorials and articles on the subject.

  • Tony Woods

    September 24, 2010 04:01 am

    Hi Dustin

    That's about the best and easiest explanation I've ever read. Many thanks for posting it for everyone. Love you website too!!

  • Deck

    September 24, 2010 03:21 am

    Very technical, but something I will be coming back to for reference, this is good stuff for me, great work Sir :)

  • Phil

    September 23, 2010 11:04 pm

    It's an excellent article, and in fact, I plan to go take some photos this weekend, and I will probably print your article out to take with me. I have become comfortable with aperture-priority, and it's time to progress.

    As to the lower-case p, this Web site did that, as it just did on a reply I posted to another article. I typed the P upper-case. I am typing it upper-case on this reply, too. Look at all the other lower-case letters on respondents' names. Hmmm ...

  • Dustin Diaz

    September 23, 2010 08:54 am

    Hi phil,
    Do you always spell your name starting with a lowercase letter? Wonderful. Now go take some pictures.

  • Phil

    September 23, 2010 05:45 am

    Great article. Grammatical comment:

    "Your model sitting their patiently is waiting for their beautiful portrait to be taken."

    1. First "their" should be "there."

    2. A model is a he or she, not a they. Therefore the possessive is his or her, preferably his in standard English usage. (We do not have a gender-neutral possessive pronoun for people; his comes closest.)

    Thank you.

  • Wendy

    September 22, 2010 07:41 pm

    thanks Dustin, I will have to try this out with gear in hand. I have recently been experimenting with OCF and have not been happy with the results. This is very detailed technical information, and for me, quite complicated, but I will have another go.

  • Arun

    September 22, 2010 04:08 am

    Dustin, I've been a big fan of your strobist shots! That's how I started getting interested in Flash Photography. I guess this post is the first of its kind.

    I was lucky to get a mentor who was a very knowledgeable photographer, especially when it came to strobes and flashes. Some of what you just put in numbers, I saw it with my eyes. Nothing better to see it, seeing is believing!

    To put things into perspective,
    Everyone of us need to find the guide numbers of our flash units we work with. Make those charts of 'Distance of Subject * Right Aperture'. How does one get the "Right Aperture"? It's by using a Light meter if you really want to learn it the right way. Takes a bit of your time and patience, but pays off in the end for good!

    In the event that you don't have a light meter, use other ways to approximate a perfect exposure. Approximating could be done by taking 3 test shots, and finding which was closest to perfect exposure. The Zone system of lighting also really helps understand right exposure, hence helping you find the 'right exposure' easier.

    Once you have this chart, you're good. Given any situation, you can handle the lighting more effectively than before.

    Note: Mind you, the Guide Numbers change with angles of the Flash. So, if apart from being directly lit, you're trying to use bounce or bounce+direct, you've got to recalculate!!!

    P.S. Speedy, 100GN was an arbitrary assumption. And the way Dustin calculated 32 is how it works. In case this helps, when I did do something like what I just wrote here, I got about 120 GN for my Canon 580EXII under my conditions.

  • Dustin Diaz

    September 21, 2010 07:41 pm

    Wow, What incredibly positive comments everyone!

    Hi Speedy,
    Take the G.N 100 out of the equation. It was meant to be used as an example in the previous paragraph and explains why you've been tripped up a bit.

    The idea in that example is to make the general assumption "lets say you had the correct exposure at ƒ/4, ISO 100, at 1/4 power - 4 feet away" for whatever reasons. As in, perhaps, maybe you're shooting through a softbox or bouncing off an umbrella (which tends to eat up stops of light). The flash and or modifiers are not taken into account.

    Nevertheless, you could reverse Engineer a GN based on those numbers. If my calculations are correct, you'd be using a flash with a GN of 32. And here's why.

    1/4 power is 2 stops less than 1/1 power. So you'd have to recalculate your aperture from ƒ/4 to ƒ/8. Thus your GN is ƒ/8 x 4 feet = 32.... which really, is a pretty weak flash.

    Hope this helps.

  • rabanne

    September 21, 2010 07:11 pm

    Only two words:

    Thank you.

  • rabanne

    September 21, 2010 07:08 pm

    Only two words:

    Thank you.

  • Speedy

    September 21, 2010 05:40 pm

    Thanks for the article. That was really very informative. However, please explain one of your examples a bit further. The example where the subject is 4' from the camera, and the background is another 4' further, makes no sense to me. In your examples you are using a flash with guide-number (G.N.) 100. Suppose the flash is at full power. With the subject 4' away from the camera we'd want an aperture of f/25, because (in your notation) f/25 x 4' = 100 G.N.. However, in your example you are using f/4. That means you are (25/4)^2 = 39.0625 stops over-exposed. Therefore I figure you'd want your flash power set to 1/40 (well, 1/30.0625), not 1/4, to get the right exposure, because at a constant distance from a light source the illuminance is linearly proportional to the power of the light source. Please tell me where my thinking is going wrong.

  • Speedy

    September 21, 2010 05:37 pm

    Thanks for the article. That was really very informative. However, please explain one of your examples a bit - the example where the subject is 4' from the camera, and the background is another 4' further, makes no sense to me. In your examples you are using a flash with guide-number (G.N.) 100. Suppose the flash is at full power. With the subject 4' away from the camera we'd want an aperture of f/25, because (in your notation) f/25 x 4' = 100 G.N.. However, in your example you are using f/4. That means you are (25/4)^2 = 39.0625 stops over-exposed. Therefore I figure you'd want your flash power set to 1/40 (well, 1/30.0625), not 1/4, to get the right exposure, because at a constant distance from a light source the illuminance is linearly proportional to the power of the light source. Please tell me where my thinking is going wrong.

  • Speedy

    September 21, 2010 05:35 pm

    Thanks for the article. That was really very informative. However, please explain one of your examples a bit - the example where the subject is 4' from the camera, and the background is another 4' further, makes no sense to me. In your examples you are using a flash with guide-number (G.N.) 100. Suppose the flash is at full power. With the subject 4' away from the camera we'd want an aperture of f/25, because (in your notation) f/25 x 4' = 100 G.N.. However, in your example you are using f/4. That means you are (25/4)^2 = 39.0625 stops over-exposed. Therefore I figure you'd want your flash power set to 1/40 (well, 1/30.0625), not 1/4, to get the right exposure, because at a constant distance from a light source the illuminance is linearly proportional to the power of the light source. Please tell me where my thinking is going wrong.

  • hfng

    September 21, 2010 04:33 pm

    Your article is amazing. Your photos are amazing. You're just amazing!

  • Tony

    September 21, 2010 02:52 pm

    I apologise if I sound dumb but does it also mean that if it's 3 times further away therefore it's 3 stops darker?

  • Liju Augustine

    September 21, 2010 01:50 pm

    Great article. Now I have no excuse for blaming the highlights on my SB900.

    Thanks
    Liju Augustine
    http://lfotos.wordpress.com

  • Yvette

    September 21, 2010 12:43 pm

    Definitely the information that I have been looking for. Flash photography has been eluding me, but now I have a technical foundation to build upon instead of the wishy washy explanations I have been getting. Thanks. And, phew, for having my Electrical Engineering background as well!

  • Tim

    September 21, 2010 10:23 am

    This is the best explanation I've read on this subject. Thank you!

  • mark chan

    September 21, 2010 06:52 am

    wheeww.. that was a great article! i learned a lot. I am a radiographer likely a photographer. got a lot of background regarding the inverse square law. thanksss for this article.

  • Marti

    September 21, 2010 06:26 am

    THANK YOU!
    Am trying to understand this so I can take the CPP exam. Math has always been a hangup for me and you made this make sense!!!

  • Nikki

    September 21, 2010 06:18 am

    Bravo! So, so helpful and useful. I'm printing this article off, folding it up and sticking into my camera bag. Good job and thanks!

  • Greg

    September 21, 2010 06:11 am

    Great Article. Question though: when you say: "...it’s a simple scale where we can double our numbers, starting at f/1, and each time we double, we will increase by two stops of light..." don't you mean decrease? Or does the nomenclature refer to the fact that the light required to get a correct exposure at f/2 (as opposed to f/1) must be twice as strong (than for f/1)?

  • miguev

    September 21, 2010 05:47 am

    Great article, I knew about this stuff from the old times when I had to compensate light loss when using loooong extension tubes for macro, but hadn't thought of the effects on the background on a strobist set up. It hadn't occurred to me that the same flash could light both the subject and the background... gotta try that out :)

  • Dustin Diaz

    September 21, 2010 04:30 am

    Hey guys,
    Thanks for the feedback so far :) It is indeed a difficult subject and I admit I had my own stumbling blocks learning this when I first dove into flash photography.

    Wayne,
    Excellent! Glad this makes sense to a fellow Eng.

    cheers folks.

  • Dustin Diaz

    September 21, 2010 04:28 am

    Hey guys,
    Thanks for the feedback so far :) It is indeed a difficult subject and I admit I had my own stumbling blocks learning this when I first dove into flash photography.

    Wayne,
    Excellent! Glad this makes sense to a fellow Eng.

    cheers folks.

  • Marty Hirst

    September 21, 2010 04:09 am

    Thanks Dustin, that was a superb and concise explanation of what is for many a very complicated concept. All my ducks are now in a row.

  • John Harris

    September 21, 2010 03:14 am

    Great explanation, its been so long that I actually thought about it, I needed a refresher.
    Thanks
    John

  • Scott

    September 21, 2010 03:11 am

    Great article. Torqued my head a bit and will take a few re-reads with gear in hand to get it, but this is the kind of information I'm looking for.

  • Wayne

    September 21, 2010 03:04 am

    Thankfully, I'm an Electrical Engineer and have has a ton of physics, so this all make perfect sense. Dustin has does a great job explaining this both mathematically and visually. The section on over/under exposure of the background was VERY helpful. Thank you very much for publishing this!

  • Parvez

    September 21, 2010 02:30 am

    Very nice explaination! clean clear in the heads now :)
    Thank you Dustin!

  • Killian

    September 21, 2010 02:18 am

    This article was definitely among the more "technologically advanced" ones posted here, but very informative. I'll need to re-read a few more times (I'm a very visual learner, so getting a concept strictly through reading is hard for me), but I've got a basic hold of what you're saying.

    Thank you for a simple explanation of a very complex subject!

    Kudos!

  • Shaun Fisher

    September 21, 2010 02:14 am

    Thanks for posting! This is awesome :) Its something I know I know but could never explain it!

  • Martin C

    September 21, 2010 02:12 am

    A good posting and quite well explained Ive just started in the OCF aspect so am always ready to listen. It don't have to be expensive to get into and the new range of flashes from Yongnuo such as the YN468 That gives you TTL as well for under £100. also you can get their poverty wizards RF 602 for the fraction of the price as Pocket wizards and so far touchwood just as reliable.
    The most important thing is to experiment and have fun and you can get great results and open up a new avenue for your photography

  • fortunato_uno

    September 21, 2010 01:39 am

    Man, my heads still spinnig! Great article. It's something I've been checking into for some time. I know that the flash is an esential tool in photography, and have been working hard to learn to use the pop-up as an entry to flash photography. This clearifies a lot of what I've had to figure out.
    I hope that you'll follow this with information on the whole second curtain vs first curtian, and maybe the flash exsposure compensation settings (which I've found very usefull with a pop-up).

  • Erica Hanks

    September 21, 2010 01:19 am

    Perfect timing! My SB-600 will be arriving tomorrow! Thanks for the great info!

  • Shaun Fisher

    September 21, 2010 01:06 am

    Thanks for this. Its something that I know I know but could never have written it out like this :)

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