Can anyone explain ISO, f stop, shutter speed relationship

[rhoraq]

I'm new to this & confused on the relationship btwn ISO, f stop, & shutter speed. The ISO confuses me the most. I currently own a Sony A350. And when is it a good idea to raise the ISO? I was always afraid to mess with it b/c i thought it would bring a lot of noise into the pic and make it grainy. I'm confused now. I do mainly portrait photography. Thanks for any help.

[OsmosisStudios]

Shutter speed is the duration that the sensor is exposed to light. It's measured in seconds or fractions of seconds. Ex.: 1/60 is 1/60th of a second.

Aperture is the opening of a lens and acts like your pupil in your eye. The larger the opening, the more light comes in. The smaller the opening, the less light comes in. Aperture is expressed as a fraction as well, but it may not always seem logical. You'll learn the stops as they come.

ISO is the sensitivity to light. On film it meant larger "sections" to the film for the light to hit. On digital it means the "boosting" of the signal from the sensor to the processor.

Shutter speed controls movement. If you have a slower shutter speed, objects that move will blur. A shorter shutter speed will freeze objects.
Aperture controls the Depth of Field, or the amount of the image in front/behind your focus point. If you focus on a fencepost that is 10 feet away, it controls how much of the fence is in focus in front and behind that post.
ISO is the third factor. It doesnt affect anything really directly, but higher ISO settings do bring in more "Noise" or grain.

I tend to leave my ISO alone as much as possible and play with shutter speed and aperture (the first more than the second). I only raise the ISO when my combination of shutter speed and aperture isnt giving me the right exposure.

[zona5101]

someone was kind enough to post this link in another thread. Plenty to read and you can absorb it at your own pace...

Ultimate Exposure Computer

[dcclark]

Osmosis and zona gave good explanations. Here's just another way I like to think about shutter speed, aperture, and ISO.

Each of these (shutter speed, aperture, ISO) affects how much light makes it in to your final image. The balance among the 3 is something that you need to choose, based on the particular photo you're trying to take. Each of your three variables has a "trade-off" which you must decide on for yourself. Here's a cheat sheet:

Shutter speed: longer speed = more light, shorter speed = less light. Trade-off: longer speed also means the possibility of motion blur (which may or may not be desirable), while shorter speed may freeze action.

Aperture: more open aperture = more light, more closed aperture = less light. Trade-off: open apertures have a shallow depth of field, which means only a very small area can be in focus. This is desirable sometimes (often for portraits or macros), but undesirable others (such as many landscapes).

ISO: higher ISO = more sensitivity to light (you can think of this as "more light"), lower ISO = less sensitivity to light. Trade-off: higher ISO leads to more noise in the image, which can result in lost detail and distracting blobs.

Hope this helps!

[inkista]

Two more points to add.

1) the sensor in your dSLR is much larger than the one in your P&S camera, so your pixels are bigger, too. This has an effect on high iso noise. You probably won't be bothered by the noise from iso until you go up to iso 800 or higher, unless you're really sensitive to it.

2) Each of those three settings has a different tradeoff, but they're interconnected because they all contribute light to the overall exposure. And in terms of exposure, you can trade the settings off in equivalent units of light called an EV (exposure value), or "stop".

A stop is a doubling/halving of the light.

So, if you halve your iso (-1 EV) and double the shutter speed (+1 EV), you'd get the same exposure. If you double your iso (+1EV) and halve the shutter speed (-1EV), same thing.

The math is a little harder with aperture, because the aperture is given as an f-number, not as the actual area of the opening (which is what's directly proportional to the amount of light being let in). So, what's actually doubling is the square of the f-number. Basically, what you want to do is memorize the full stop f-number scale:

1.0 | 1.4 | 2 | 2.8 | 4 | 5.6 | 8 | 11 | 16 | 22

Going down/up one number on that scale is the aperture equivalent of halving/doubling the iso or shutter speed. So you can trade stops among all three settings, depending on which tradeoffs/effects are most important to you.

The analogy some people make is to think of a proper exposure as a certain sized bucket, and the light as water. You want to fill the bucket to get a good exposure. Your shutter speed is how long you turn the faucet on. The aperture is the width of the pipe. And the iso is the water pressure.

[JFSanders]

Originally Posted by zona5101

someone was kind enough to post this link in another thread. Plenty to read and you can absorb it at your own pace...

Ultimate Exposure Computer

Great explanation and very useful too! Thanks for paying it forward.

[vickistark]

however I think I need to read the STOP portion a few more times for that to sink in. From what I read I am assuming that if I need a large dof for say a landscape shot and I don't want to loose the light I would choose a small aperture which is a large f stop and then go down in shutter speed to level out the exposure. Is that correct in theory?

[autofocus]

Originally Posted by vickistark

however I think I need to read the STOP portion a few more times for that to sink in. From what I read I am assuming that if I need a large dof for say a landscape shot and I don't want to loose the light I would choose a small aperture which is a large f stop and then go down in shutter speed to level out the exposure. Is that correct in theory?

your assumption is correct...the smaller the f/stop (larger number) the longer the shutter will have to stay open to expose the image correctly. Here's what I think might help you..in most circumstatnces, ISO to me is always the last part of the equation to consider. I will always try to keep that set at it's lowest setting the camera supports. However, there will be times when lighting conditions are such that (because of the equipment you are using as in too slow a lens) it will be near impossible to get the shot, and the only way to get it will be to raise the ISO up, then you do so regardless of noise.

[vickistark]

cool, I keep reading and picking up tidbits here and there and I think I am starting to understand the exposure triangle and the trade offs you have to take into consideration a lot better now. With that said I have a better understanding of the manual manipulation of my camera and I think I am ready to start playing with settings and see what I come up with. Thanks loads for all the info and I look forward to getting a little feedback in the near future!

[Elmo]

try this.

You need a bucket of water.

ISO is like the bucket size. how much water does it take to get a bucket ful.

Then to get the bucket full of water you use a water hose. How long you need the water to run (Shutter speed) is a function of how big the water hose is (f-Stop).

Keep in mind is water pressure is low (less brightness) you can get a bigger hose or wait longer to get your bucket full.