Auto ISO

ISO is an extremely valuable tool in digital photography.  It's great to be able to adjust the sensitivity of your sensor as the lighting conditions change without having to change an entire roll of film.  But as convenient as it is (especially compared to film,) it would be great if it could be something we never had to think about. That's where the auto ISO setting comes in.

This subject is a little complicated.  To fully appreciate what auto ISO does, you have to understand the advantages and disadvantages of changing ISO as well as the affect shutter speed has on your images.  If this is beyond you right now, keep reading and just use the settings I recommend.  Even if you don't understand it all yet, it will help you get better pictures consistently.  For background, read What is ISO and ISO and Digital Noise as well as How to Choose a Shutter Speed Part One and Two.

Recent Nikon cameras (I don't know for sure about Canon) have an "auto" setting for ISO that allows you to set a range for your ISO and then never think about it again.  That's too strong.  With auto ISO you hardly ever have to think about your ISO setting.  When turned on, auto ISO works by starting at whatever ISO you have set and gradually bumping it up if there isn't enough light in the scene you are photographing.  Additionally, you can set a maximum so that your camera won't take your ISO higher than you are comfortable.

Here is how to set it on a Nikon D40:

1. Push the "MENU" button on the back, navigate to the "Custom Settings Menu" (the pencil), then select "10 ISO auto."  Select "On" and push "OK."
2. Then select "10 ISO auto" again.  This time select "Max. sensitivity" and choose the highest ISO that doesn't produce enough noise to bother you.  (Read ISO and Digital Noise if you need help deciding.  I would choose 800 on the D40.)
3. Then select "10 ISO auto" again.  This time select "Min. shutter speed" and choose the minimum shutter speed that would produce sharp images with your lens.  (Read How to Choose a Shutter Speed Part One and Two if you need help deciding.  I would choose 1/30s or 1/60s depending on the lens.  Choose 1/60s if you have a VR telephoto lens and don't want to ever change your ISO again.)
4. Push the "MENU" button on the back, navigate to the "Shooting Menu" (the camera), then select "ISO sensitivity."  Choose the lowest number there is (200 on the D40, 100 on some other cameras.)  Ignore the "Auto" in this menu - it's not the same thing.

Here's what will happen: your camera will shoot at it's "base ISO" (which you set in step 4) most of the time.  When it's too dark and your shutter speed starts to slow down, it won't go slower than whatever you set in step 3.  Instead, your camera will increase your ISO, but it won't go past the maximum you set in step 2.  This is great because it allows the camera to make the decision you would make without you having to do anything.  You have enough freedom to limit the camera's choices, but you also get to take advantage of your camera's intelligence.

If you can't see "10 ISO auto," read Expanding the Menu.
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How to Choose an ISO

Read background on ISO here and here.

As I've said before, your camera will preform best at its lowest ISO.  Therefore, it is best to set your camera to the lowest ISO setting you can under the conditions you are photographing.  However, there are times when it is valuable, or even essential, to increase your ISO in order to capture a descent image.  Consider this situation . . .

I took this picture at ISO 200 with a shutter speed of 1/2.5 and aperture of f/5.

If you look closely you can see that the picture is blurry because the shutter speed is too slow (see How to choose a Shutter Speed for more on this.)

If I kept all the other settings the same and increased my shutter speed, it would produce a darker image, which is not what I wanted.  Another option (in theory) would be to increase the shutter speed (letting less light in) and make the aperture wider (letting more light in) to produce the same exposure with a faster shutter speed.  Unfortunately, in this situation, the aperture was already as wide as it could go.  Another option is to increase the ISO, making the sensor more sensitive meaning that less light is required to produce an equally bright image.  Doing this would allow for a faster shutter speed (to eliminate the blur) without making the picture darker.

So I took this picture at ISO 400, shutter speed 1/4 and aperture f/5.

Still Blurry.  Still need a faster shutter speed.  Now ISO 800, shutter speed 1/10 and aperture f/5.

Better, but still a little blurry.  Finally, ISO 1600, shutter speed 1/20 and aperture f/5.

This one is less blurry, but now digital noise is starting to degrade the quality of the image.
So what's the lesson here?  There aren't any settings that would have been quite perfect in this situation.  It's just too dark to get a great image - the real answer would be to use a flash or a different lens.  But hopefully you can see when and why you might increase your ISO to get proper exposure.  You can see in these examples both the benefit of being able to increase your ISO and the cost of doing so.

ISO and Digital Noise

If you read this post about ISO, you should have a basic understanding of what ISO can do for you when you are photographing.  And while a basic understanding is good, it is still basic.  When it comes to using the ISO setting on your camera, it is important to understand not only the advantages of being able to control ISO, but also the disadvantages of doing so.

As I said in my original ISO post, increasing your ISO allows you to take pictures in darker settings without having to adjust your shutter speed or aperture.  The problem with increasing your ISO is that your camera will produce more "digital noise."  Your camera will do its best work at its minimum ISO and as you increase the ISO, you will increase the digital noise.  What does this mean?  Take a look at the pictures below.

I took a picture of this scene at every ISO setting on the Nikon D40 and then zoomed in so you can see how the ISO affects digital noise.

Try to ignore the blurriness (especially at ISO 400) which I will explain tomorrow.  Notice the increasing discolored grainy dots as the ISO goes up - that's digital noise and it can ruin a picture.  This is why you need to be careful as you increase your ISO in darker situations.  Note that ISO 3200 is called "HI" on the D40, probably to deter you from using it.
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The Exposure Trinity - The 3 Components of Exposure

This post has been moved here.

What is ISO?

The ISO setting on your camera controls the sensitivity of your digital sensor. Increasing the ISO (changing it to a higher number) makes your sensor more sensitive to light and lowering the ISO makes the sensor less sensitive. This has an important effect on exposure because changing your ISO changes the amount of light your sensor needs to produce a correctly exposed image.

In the real world, here's what this means: when you set your ISO higher you can take pictures in darker settings (all else being equal.) However, increasing your ISO also has a negative effect on the quality and clarity of your images. (Later we will discuss this relationship between ISO and digital noise.)