I thought Auto ISO was going to be a great feature…until I started using it!

I borrowed a Canon 5D Mark II from Canon Professional Services a few times and wanted to try out the auto ISO feature, which covers 100-3200 ISO. I mean, how many times have I suddenly gone from a low-light situation to a bright-light situation while photographing (e.g., a bridal couple coming out from inside a dark church to the bright sunshine outdoors) and forgot or didn’t have a chance to change the ISO from 1600 or 3200 to 200 or 100?

I normally shoot in either aperture priority or manual exposure mode. It turns out that the 5D Mark II defaults to 400 ISO in manual mode (i.e., it doesn’t really do anything in manual mode). In aperture priority, it will look for a shutter speed + ISO combination that gives the “proper” exposure given the aperture you’ve selected. This would be OK if the camera didn’t “think” that a shutter speed as low as 1/20 or 1/15 sec was acceptable in low light, or that a high ISO of 800 or 1600 in conjunction with an unnecessarily high shutter speed of 1/4000 or 1/8000 sec was just peachy when you’ve moved outdoors into plenty of daylight.

The problem is that you can’t set a minimum or maximum shutter speed on the 5D Mark II, so you can’t prevent it from deciding to use unacceptably slow shutter speeds or unnecessarily high ISOs.

With the Canon 1D Mark IV, not only *can* you set minimum and maximum shutter speeds, but auto ISO still works in manual exposure mode! When I discovered this (auto ISO in manual mode), I thought I would never have a reason to use aperture priority with the 1D. Heck, you can decide your ideal aperture and shutter speed, and then let the camera choose the lowest ISO that still yields a proper exposure….like having simultaneous aperture and shutter speed priority with auto exposure!

It turns out there’s one little “fly in the ointment” with auto ISO in manual mode on the 1D: you can’t use the “*” button to fix exposure and recompose the shot like you can in aperture or shutter priority. I would think this is something Canon could change with a simple firmware update (Canon, are you listening?).

There’s another potential problem with using auto ISO in manual mode on the 1D that I’ll mention a little later in this post; but the purpose of this post is to discuss making auto ISO useful, regardless of which camera you own. So, back to the question…

How do you make auto ISO actually useful?

It depends on your camera. Here’s a short decision tree I’ve devised:

A) If you have a camera that allows the setting of minimum and maximum shutter speeds and you plan to shoot in something other than shutter priority mode, set these min and max shutter speeds accordingly to keep the camera from selecting too low shutter speeds or too high ISOs when set for auto ISO. I find that the 1D actually tends to seek a low ISO if possible, so I set the min possible shutter speed to something like 1/60 sec and don’t worry about the max shutter speed. A little testing with your own camera should allow you to figure out the best settings for it.

B) If you can’t set minimum or maximum shutter speeds, you may be better off using shutter priority (versus aperture priority, program, full auto, or what have you). If you’re controlling the shutter speed, you can make sure it’s fast enough to keep your dark indoor images from being blurred due to camera shake or movement of your subject. Also, you can make sure it’s not so fast that it forces the camera to choose higher than necessary ISOs when there’s plenty of light (e.g., outdoors on a bright day). You have to be somewhat careful to change it from slower speeds to higher speeds when you step outdoors into bright conditions from a dark indoor situation. If your lens is able to constrict to small apertures–like f/16, f/22 or even smaller–you should be fine if you forget initially. If you don’t want the aperture to be so small (e.g., because you don’t want the depth of field to be large), make sure to increase the shutter speed in the new brighter scene as soon as you can.

The main reason to use auto ISO is to have your camera automatically use the lowest ISO possible given the prevailing light level so that image noise is kept to a minimum. Each camera comes with its own algorithm that decides what combination of aperture size, shutter speed, and ISO to use for any particular shot. If you shoot in full auto mode, the camera chooses some combination of all three parameters to get a good exposure. If you shoot aperture priority with auto ISO, you’ve taken away aperture size as a parameter and the camera will vary only shutter speed and ISO to get the correct exposure. If you shoot shutter priority with auto ISO, you’ve taken away shutter speed as a parameter and the camera will vary only aperture size and ISO to get the correct exposure.

What’s at issue here is what the camera decides to do with the 2-3 parameters it’s varying. In automatic mode, your camera is varying all 3 parameters and you’ve got little control over its decisions. Being able to specify a minimum and/or maximum shutter speed can give you *some* control, but not many cameras include this ability.

If you fix the aperture size (via aperture priority) at, say, f/4, the camera could decide to drop the ISO down to 100 and the shutter speed to 1/15 sec to get a proper exposure. It’s difficult to get a non-blurry image at 1/15 sec, especially if the subject is a person or animal which seldom stay perfectly still.

If you fix the shutter speed (via shutter priority) at, say, 1/125 sec, the camera could decide to set the aperture to f/8 and raise the ISO to 3200, even though the lens is capable of an aperture of f/2.8 or larger; this larger aperture could bring the ISO down to 400 or lower, which most photographers I know would prefer.

Unfortunately, most cameras don’t let you tweak their internal algorithms so that it makes its decisions directly in line with your preferences.

The reason I got excited about the 1D Mark IV’s ability to shoot auto ISO in manual mode is that you can fix *both* the aperture size and the shutter speed, and only allow the ISO to vary to obtain the correct exposure. I found two problems with it however. First, there’s the problem I mentioned where you can’t press the “*” button, which fixes the exposure, and then recompose the shot (something that Canon may be able to change in the firmware). The other potential problem is if it’s very bright outside, the camera may not be able to find a ISO low enough to get a proper exposure with your selected aperture + shutter settings, leading to a potentially badly over-exposed image.

Ultimately, the best way to make auto ISO work for you is test it with your own camera in different modes with different settings and see which combination gets closest to the way you want your camera to operate. You may find a combination that gets agreeably close to your preferences. Or you may–as many have–simply give up on auto ISO altogether!

If auto ISO is important to you, it would be best to test various cameras first–perhaps using some of the tips I’ve mentioned in this post–before you actually make your camera purchase.

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