Have you ever noticed the “A” setting on the ISO dial on your Canon DSLR? “A” stands for auto ISO. Some people love it, some people hate it. Most people don’t fully understand it (and maybe that is why they hate it in the first place!). In this blog post I will try to explain where auto ISO makes perfect sense (and where it does not).

What people tend to think of auto-ISO

Photographers who were active in the analog film times and have no technical background, often think of auto-ISO as something that simply shouldn’t be allowed. They all but call it blasphemy to have this feature on a camera. For all they know, the ISO value is the sensitivity of your film roll, something you carefully choose before a shoot and live with during that shoot. But the fact of the matter is, that in digital cameras the ISO value is nothing more than a signal amplifier inside your camera, just like your stereo if you will. And with that… The camera can so easily change this while shooting. The people at Canon figured… So why not have the ISO setting determined by the light metered from the camera? And so auto-ISO was born.

Auto ISO is actually an extremely useful feature, and it makes perfect sense from a technical perspective. I’m sorry for those who say letting the camera change ISO as you shoot is unnatural and should be removed from the camera’s list of features… I think it is the one single thing that will get you the technical optimum for your gear while not going “manual everything” with all of its limitations in agility.

How auto ISO works and some quirks

We’ll get to how auto-ISO works in a minute. First I need to address this “weird behavior” of the “400 ISO thingy”: People often use auto-ISO in conjunction with Av or Tv, without really knowing what they are actually doing from a technical standpoint. Sure they set this mode with some target in their minds, but at the same time they give the body some “freedom” you should not give the camera most of the time. As a result, a lot of Canon bodies may “just decide” to click to 400 ISO for no apparent reason. But is there really no reason why it does this?

Let’s take a look at Av and Tv modes:

  • In Av mode, you SET the aperture and the camera meters light and calculates the correct shutter speed for you;
  • In Tv mode, you SET the shutter speed and the camera meters light and calculates the correct aperture.

Makes perfect sense so far right? We fix one variable, and the camera uses light metering to set the second one for us.

But when we enable auto-ISO in either of these modes, all of a sudden we now have TWO variables the camera can set. So what should the camera do? Let’s take Av as an example: You SET the aperture, and the camera meters light. But now it could adjust the ISO and/or the shutter speed. So which one to choose?

In the consumer/prosumer bodies there is some compromise / default used that should work “in most circumstances”. That is where the 400 ISO comes in. In more expensive bodies you get more features to deal with this. Not sure how far that goes… Never owned a 1D body, but just to throw in a nasty one: what happens to a 1D when you set bracketing in M mode with auto-ISO enabled…???

The 400 ISO was probably chosen because it is the most sensitive setting in which most/any DSLR body generates very acceptable images still. I like to think of it that Canon kindly reminds us that we really should make up our minds, and chooses the “safest ISO” they could think of.

How to properly use auto ISO with full control

So how to go about this “400 ISO feature”? The answer is simple: Make sure the camera only has ONE variable to vary… Also known as “M” or Manual mode. In manual mode, you fix both aperture and shutter speed, and the camera then varies the ISO until the image is properly exposed.

Canon almost got this right; because if I look at it from a pure technical perspective, I should be able to preset under – or overexposure as well while in “M” with auto-ISO. That is something I really really miss; why can you preset for -1eV under exposure in Av and Tv mode, but not in M mode with auto-ISO enabled? This clearly shows one of two things: 1) That people at Canon do think out of the box, but at the same time are somewhat limited in their views, or 2) In the amount of work they are willing to put into modifying their software: before auto-ISO came about, M naturally did not have the ability to under/overexpose – it still hasn’t today. Hmmmm…. 😉

I myself use auto-ISO a lot, but always in manual mode. And here is why I use auto-ISO: As soon as you push the lens limits in any way it starts to make perfect sense. Imagine you have your gear, and for the shot you want to take there isn’t really enough light (I bet you never came upon that one right 😉 ). In this case you KNOW what shutter speed you’d need at a minimum (either because of the focal length and the fact you have or do not have IS on the lens), and/or because of the movement of the subject you want to capture. You also KNOW what aperture you’ll need. Either opened fully (as to catch most light), or stopped down to some value to increase lens quality or increase your Depth of Field (DOF).

Now that you have determined both your shutter speed and aperture, the “M” mode makes most sense. But now you have a new issue: What ISO to choose? Right now, auto-ISO should make more than perfect sense. The camera will figure this out for you. The technology is there, so why not use it. The result is images with absolute control for aperture and shutter speed, and the best-you-can-get ISO, meaning minimal noise.

Some examples where auto-ISO makes sense

Some examples where auto-ISO makes perfect sense. I’ll start off with the obvious ones, then I’ll move up to the less obvious ones which will show just how handy auto ISO can be.

Example 1: Shooting humming birds
I want to capture a hummingbird in flight with its wings nearly frozen. I know from previous shoots that I need to shoot with a shutter speed of 1/1600th. So I set my camera to Tv, 1/1600th, and estimate the ISO at 800. I come home with the perfect shot, taken at 1/1600th, ISO 800 and f/9.

But is this shot perfect? Let’s see how we would do on manual with auto-ISO: I know I need 1/1600th, and there really isn’t that much light. So I set the camera to “M”, 1/1600th and f/7.1 because I know my lens delivers tack sharp images from that aperture on. I also enable auto-ISO, and I return with the same, no an even more perfect shot taken at 1/1600th, f/7.1, ISO 400.

See where this is going? The second attempt delivered a shot not at 800 ISO, but at 400 ISO which rendered a much better image with far less noise (this is an actual example of me shooting with a 7D and a 100-400L).

Example 2: Wildlife capture in South Africa
I am shooting wildlife in Africa with my 100-400L and the amount of light is less than optimal. Again, this is where auto-ISO comes in: At 400mm I need to shoot at at least 1/800th. I choose f/7.1 as a minimum aperture because the 100-400L is only REALLY sharp from that aperture on. So I set my camera to manual using these settings. But now I have no ability to adjust the exposure.

Auto-ISO will fix this for me: It will raise the ISO, but no further than needed, and it will do so dynamically. So more light means less noise, less light means more noise, but still it is the technical optimum of what my gear can handle.

I hear some people saying: Just use a faster lens. A 400mm f/2.8 would have allowed you to decrease the ISO, and a better body to match it will decrease the noise further down at that ISO. Even though that is very true, there still is the technical optimum to achieve: a faster lens “would allow me to lower the ISO” – so why not have the camera lower the ISO for you automatically??

Another way of getting to this “optimum technical setting”

Another option that would solve this puzzle (but in a slightly different way) would be if I could tell the camera to for example “never go under 1/800th” and use Av, or “never go under f/7.1” and use Tv. I used to think THAT would be the killer feature for me to have that would allow me to have an auto-metered perfect shot. But as it turns out… Auto ISO is actually a smarter one.

Because even if the camera could “clip” the shutter speed and aperture I’d still have to fix my ISO value to “something that makes sense”, which means whatever I choose there, I will probably either get more noise than required (when I set my ISO too high), or an aperture/shutter speed that I did not really want to get (when I set my ISO too low meaning under exposure).


In my opinion, auto-ISO is a great feature when used in “M” mode. And as soon as you know the limitations of your gear. It is SO important that you know this. Shutter speeds are ruled by lens focal length and the optional IS feature, and/or by movement of the subject. Aperture is ruled by lens quality (“from aperture x the lens is tack sharp”), or a required DOF (also see my DOF calculator)

With auto ISO you can then auto meter and get away with the lowest ISO possible. This “lowest ISO” has the same advantage for any camera (or camera brand for that matter): Higher ISO means more noise. So the lower ISO you can get away with, you should. Auto-ISO makes exactly this possible, the easy way.