Photography 101.7 - ISO

Photography 101.7 – ISO

Photo 101.7 ISO

Photo: Rainer Ebert - CC license

The following post is from Australian photographer Neil Creek who just launched a free background image site featuring his photography, and is developing his blog as a resource for the passionate photographer.

Welcome to the seventh lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.

This week’s lesson is ISO.

Here’s What We’ve Covered Previously in this Series:

Lesson 1: Light and the Pinhole Camera
Lesson 2: Lenses and Focus
Lesson 3: Lenses, Light and Magnification
Lesson 4: Exposure and Stops
Lesson 5: Aperture
Lesson 6: Shutter

In previous lessons we have talked about the basic theory of how a camera works, including some basic optics, and introduced the idea of exposure and how we control it with the exposure triangle. In this lesson we will be drawing upon what we have learned to understand the third point on the exposure triangle – ISO – and how it works to create your photo.

ISO is probably the most mysterious and complicated aspects of modern photography. ISO simply stands for International Organisation for Standards, and refers to – in simplest terms – the sensitivity of the camera’s sensor. Confusingly, there are several different standards, some which measure different things, and only a few completely agree with each other. To understand how to use ISO in your photography, you don’t need to know anything about that. You should understand how the camera’s sensor chip works, however.

The Sensor Chip

Fig 1.7.0 A typical digital camera sensor and mount.

Fig 1.7.0 A typical digital camera sensor and mount.

The sensor inside your digital camera actually works on the same principle as a solar cell. When light hits the chip, a tiny electric current is generated: the brighter the light, the stronger the current. Instead of there being one giant solar panel generating lots of electricity, there are millions of extremely tiny solar cells collecting very small and precisely measured amounts of light. These photosites will eventually create the pixels in the image you capture.

It should be noted that a photosite is not the same as a pixel. Several photosites will add their captured light, which is filtered and processed and eventually combined to make a pixel. That’s a complex topic for discussion in another lesson.

All of this carefully measured electrical current, which reflects light intensity, is measured and stored by the camera’s circuitry. This data is called the signal. The signal, however, must compete with the noise inherent in all electrical equipment.

Signal vs Noise

Fig 1.7.1 Noise is at a constant background level to the signal.

Fig 1.7.1 Noise is at a contstant background level to the signal.

An unavoidable fact of electronics is noise. While the sensor is measuring the tiny electronic currents generated by the light, there is also a tiny electric current that comes from other places. This unwanted current is called noise, and it mostly comes from the ambient heat of the sensor. The difference between the value of the signal and the value of the noise is called the Signal to Noise Ratio. When the ratio becomes smaller, the noise is more apparent, and the signal may get lost in it.

There are two ways that the signal to noise ratio can become low: by a drop in the signal, or an increase in the noise. In modern cameras, the noise is mostly a constant value, so we only need to worry when the signal drops – that is, when we’re photographing a dark subject. The graph to the right may help to visualise the situation.

The ISO’s Effect on your Photos

Sometimes we aren’t fortunate enough to be shooting with enough light to be able to ignore the noise. When we need to keep a fast shutter speed, or there’s not enough light even with the aperture wide open, we can increase the ISO setting on the camera. When we do so, we are increasing the signal gain. Essentially this is like turning up the volume. All of the values of the measured current (whether from light or noise) are increased. Each doubling of the ISO value, is a doubling of the gain: a doubling of the measured current in the chip.

Fig 1.7.2 - As the ISO setting increases so does the noise, until it overwhelms the signal.

Fig 1.7.2 - As the ISO setting increases so does the noise, until it overwhelms the signal.

Doubling the light in your photo is a pretty easy way to make an otherwise under exposed photo bright enough, but it comes at a cost: you lower the signal to noise ratio, and the noise becomes more apparent. Imagine you’re in a candle-lit room, and to take your photo, you have to set your camera to ISO1600 to get a correct exposure. You have now increased the noise value – which at 100ISO would be invisible – sixteen times, resulting in a noisy, grainy mess.

Shooting at high ISO settings is one of the most challenging technical issues in photography. Noise can look ugly and obscure detail in your photos, but sometimes the light is so poor that you have to accept the noise or get no photo at all. The newest breed of cameras are able to get incredibly high signal to noise ratios, and let photographers get clearer images than ever before in very dark conditions. Compact cameras, with their tiny sensors are always going to perform relatively poorly at high ISO.

Much can also be done about noise in your photos in the processing stage, but that’s a topic for a later lesson. As always, one should struggle to do the best one can in the camera, before resorting to post-processing to fix problems.

This is All Too Confusing

I warned you! ISO and noise are difficult concepts, but the good news is that there’s a simple take-away lesson from all this:

Noise is ugly. Avoid noise by shooting at low ISO settings. Only increase your ISO if there is no other way to get enough light for a good exposure.

The good news is that most DSLRs are very good at handling noise at low ISO settings (100-400) so you don’t need to worry about them too much. When you start to get into the medium (800-1600) to high (1600+) ISO settings, does noise begin to become obnoxious. If the alternative is missing a great shot though, don’t be afraid to crank up the ISO.

The forward march of technology is very exciting, as new technologies and techniques ever improve the sensitivity of camera sensors. Even though ISO is a bear the photographer must wrestle, it’s getting friendlier and cuddlier every year.


A series of images taken with the Canon 5D MkII, changing the ISO and other setting to keep a constant exposure. Noise increases dramatically at higher ISO settings.

Fig 1.7.3 A series of images taken with the Canon 5D MkII, changing the ISO and other setting to keep a constant exposure. Noise increases dramatically at higher ISO settings.

High ISO was required to balance low ambient light with flash, to fill in shadows. © Neil Creek

High ISO was required to balance low ambient light with flash, to fill in shadows. © Neil Creek

In order to get a bright photo of the stars without trailing, high ISO was necessary. © Neil Creek

In order to get a bright photo of the stars without trailing, high ISO was necessary.
© Neil Creek

Without a tripod, this shot in very low light was only possible with high ISO. © Neil Creek

Without a tripod, this shot in very low light was only possible with high ISO.
© Neil Creek


Homework for this lesson is fairly simple. ISO is simply a matter of “turning up the brightness” on your photos, so it’s not really complicated in practice. What you should do, however, is experiment with your camera on various ISO settings and get a feel for how images look. If you know that ISO 1600 looks terrible on your camera, then you’ll be more likely to try to find other ways to get more light on the subject than just be lazy and increase the ISO. On the other hand, you’ll also know when it’s worth pushing it all the way just to catch the photo that can’t be missed.


Read more from our Tips & Tutorials category

Neil Creek is a professional photographer from Melbourne, Australia. He has been shooting with a DSLR since 2004, and blogging about his experiences since 2006. Neil has authored five ebooks and a video training course, all designed to help others improve their photography. View Neil's folio at his home page. Learn about his publications here.

Some Older Comments

  • Lumix Cameras March 12, 2011 12:30 pm

    Good examples using the different ISO settings. I was surprised to see there was no noise on the image at 800 ISO because many other cameras show noise at that ISO setting. However I think modern digital cameras have a higher tolerance to noise these days.

  • Frankie January 11, 2011 11:40 pm

    Thanks, very helpful. Looking forward to experimenting with this.

  • malayvahan July 27, 2009 09:35 pm

    i wish i could attend the classes

  • Char May 7, 2009 10:07 pm

    Thank you for the valuable lessons. Looking forward to the next one.

  • Sarah April 20, 2009 12:46 pm

    I've just started playing around a lot with ISO lately. So many people seem to want to leave it down around 100, when I generally keep it at 200-400. I figured out why however at my son's last tball practice - my children are just too fast. I need fast shutter speeds to capture them and unless it's super sunny out, that means I need the 400 to catch them in action and have a lot fewer blurry shots. After photographing them so much I realize why I'm always so tired at the end of the day - they never hold still! *L*

  • Sybren April 17, 2009 06:19 pm

    What I miss in this bog post is what I miss in most explanations of ISO: the effect on the colours. Higher ISO doesn't just give you more noise, it also makes the colours less saturated, more bland. This may be an important factor for you to choose a lower ISO setting.

  • tzjiang April 17, 2009 02:45 am

    Nice Article. I agree with Tigraine. Neil, can you post the EXIF data(at least the ISO) for those 2 pictures. Only the stars has EXIF.


  • Macro Photography April 17, 2009 01:26 am

    These are great lessons. I learned some great and valuable information.

  • Tom April 16, 2009 11:32 pm

    Fletch, thanks for the feedback.
    You are obviously right. Exposure compensation will necessarily change aperture and shutter speed. Thanks for clarifying that one for me.
    And yes, we certainly need some way of determining the sensitivity of the sensor, whatever we may want to label the parameter. Myself, I did not think of the possibility of having an analogue adjustment of sensitivity, but I do look forward to it!
    Again, thank you for the clarification :-)

  • Jeffrey Kontur April 16, 2009 11:16 pm

    @Tom- Exposure Compensation is a slightly different animal than ISO. ISO is part of the exposure triangle (the other two parts being aperture and shutter speed.) From these, your camera determines "proper" exposure. Exposure compensation simply tells your camera to deliberately deviate from "proper" exposure and by how much. Essentially, you are telling the camera to deliberately over or under expose an image from what it thinks is the correct exposure. If you changed the ISO, you would change your camera's calculation of what aperture and shutter speed to use so that it could still arrive at a "proper" exposure but it would not know that you wish for it to purposely over or under expose from this baseline. After all, the camera is just a machine.

  • Fletch April 16, 2009 10:59 pm

    Tom - Exposure compensation is dfiferent from ISO. Exposure compensation will change shutter speed and aperture as well as ISO giving you an exposure that is different to the camera's metered exposure.

    ISO is still relevant in digital as you need some way of decribing the sensitivity of the sensor to light. It dies not need to be described in the same units as film and in 1 stop increments but since this is already familiar to photogrphers it makes sense to stick to it. However the is no reason that the sensor gain has to be constrained to 1 stop (or half stop/third stop) increments so mabe we will see a camera with continuously variable ISO like the volume dial on a sterio.

  • Tom April 16, 2009 06:06 pm

    First of all: thanks for a good article!
    DPS is always well worth a read, I would guess for photographers of all levels.
    I have one question/thought - from a technical perspective, isn't ISO an outdated concept on digital cameras?
    It made sense with film, as different types of film have different sensitivities. But for digital, it's basically just about adding amplification to the signal from the sensor.
    And also, will not the exposure compensation we may use on our DSLR's act in much the same way?
    To my limited understanding, this also ought to raise the noise level as this is also an amplification of the signal from the sensor?
    Therefore, do we actually need the separation between ISO and exposure compensation? Nowadays isn't ISO just a way of keeping film users in an "familiar landscape", without no real meaning?
    Again, it's just a thought, and I might've gotten a few things wrong, but I'd appreciate some input on this.

  • johnny April 16, 2009 05:07 pm

    Good article. I like technical explanations of cameras we use. It's good to understand your tool.

  • Steve Berardi April 16, 2009 12:52 pm

    just one tiny thing to add - avoid using any ISO that ends in "50" .. on most cameras these ISOs are just digital interpolations, and not really a "real" ISO value. using one of these is like using the digital zoom versus optical zoom on a P&S camera..

    this isn't true for all DSLRs--some of them DO have REAL ISO50.. so just google your camera to find out :)

  • Camera Fanatic April 16, 2009 03:39 am

    First of all, great blog.

    If you need a solid, reliable, and stylish point-and-shoot ultracompact digital camera that produces high-quality images, then the new Canon PowerShot SD1100IS may be the best choice. It’s on sale right now at Amazon, so don’t miss out; you can find a link here:

    [edited: sorry, please don't use affiliate links disguised with tinyurls here... thanks]

    I am an advanced amateur photographer and own 2 Canon digital cameras (G2 and 20D). Both have served me well over the years but recently I have found myself needing a decent ultracompact camera that I can easily carry with me at all times for unexpected photo-ops.

    Other current Canon models that I also researched before my purchase of the "bohemian brown" SD1100IS included the SD950IS and the SD1000.

    Here is my take on the SD1100IS:

    - 8MP CCD sensor with DigicIII processor (excellent resolution images with good dynamic range)
    - Solid construction (most of body made of anodized aluminum)
    - Feels sturdy and well-balanced in the hands
    - Easy to use (logical user-interface) with minimal need to consult owner's manual for basic operation
    - Multiple shooting modes to fit variety of situations (action/sports mode is a glaring omission but read section below to see possibly why)
    - Advanced metering system with accurately exposed pics in even "tricky" situations (great balance of highlights and shadows)
    - Tack-sharp images (much more so with sufficient lighting and use of built-in flash)
    - Macro mode can result in stunning close-ups with outstanding level of detail
    - Optical IS feature helpful when shooting in either low-light conditions with flash off or at telephoto lengths
    - Fast start-up with acceptable shutter-lag (when not using flash)
    - Bright 2.5" LCD monitor (100% coverage, 230k pixels) made of polycrystalline silicon; fairly scratch-resistant (can't vouch if this applies to keys and coins)
    - Optical viewfinder (though only a tiny peephole, it is essential when LCD glare and washout become an issue shooting in bright sunlight or when LCD cannot be used as battery power is nearly depleted)
    - Camera made in Japan (at least those from the 1st shipment; this easily may be subject to change)

    - Lack of manual control over aperture, shutter speed, and focusing (for the obssessive control-freaks)
    - Noise is noticeable beginning at ISO 400 (ISO 800 still useable but probably for only 4x6 images; ISO 1600 mostly unuseable)
    - Fastest shutter speed is 1/1500 sec (not fast enough to stop action for some sporting activities)
    - Auto-focus speed inadequate to follow fast-moving subjects
    - Shutter-lag accentuated with flash on (precious Canon moments lost while waiting for flash to recharge)
    - Cannot adjust focus or optical zoom while shooting in movie mode (focus is fixed for distance selected at first frame, and digital zoom is permitted instead, resulting in significant image quality deterioration)
    - Battery/memory card cover and hinge made of plastic (no safety latch that needs to be de-activated first before sliding cover out, in order to prevent accidental opening)
    - Minor vignetting and chromatic aberration (albeit, difficult not to expect from compact p&s)
    - Pincushion and barrel distortion at the extremes of the focal lengths
    - No RAW shooting mode

    Battery power in camera mode with LCD monitor on is mostly as advertised, allowing for approximately 240 images. If your budget permits, I recommend investing in a few spare batteries as backups and replacing the supplied 32MB memory card with a pair of 4GB SDHC memory cards--vital purchases if you plan to use the movie mode frequently.

    Overall Impression:
    Even with some serious limitations inherent to virtually all digital cameras in this class, I am recommending the Canon PowerShot SD1100IS. It does what it's supposed to do. This camera allows one to take beautiful photographs in an ultracompact, reliable, and elegant device that is both easy and fun to use.

  • Tigraine April 16, 2009 01:32 am

    Thanks for the good article. Especially the graphs made it easier to understand.

    Just one note on the linked Flickr images:
    Maybe you'd like to keep the metadata about the shot in tact so people could see what camera/lens/setting was used in the shot (only the nightsky shot has metadata to it).

    greetings, Tigraine

  • dcclark April 16, 2009 01:28 am

    Well done. I'm always amused at the fact that noise/grain on film and noise/grain in digital sensors managed to end up being so similar. It's one of those (more-or-less) coincidences that is very useful in understanding photography as a whole, film or digital.

    I do have one bit of parting advice: as annoying as grainy, noisy high ISO shots are... if you see a shot and it's about to get away from you, shoot with whatever you've got!. It's better to have the photo than to miss it. Plus, sometimes grain can be kind of nice in a photo...

  • Jack Fussell April 16, 2009 12:41 am

    Another great installment in this series. Since understanding the exposure triangle photography makes much more sense. I think of it like the triangle that you use when playing pool (to set up the balls). If you move one corner of the triangle on the pool table the other two have to adjust their position as well.

    With exposure it's the same thing. If I adjust any of the three settings, ISO, aperture size or shutter speed I have to adjust the others to compensate. Good stuff Neil!

  • irieness April 16, 2009 12:35 am I understand how noise is generated. Very well written explanation. Thanks for the info!