Why I use AUTO ISO
Ok, so on a brightly lit day with the sun high in the clear, blue sky, there's little reason to go beyond ISO 100 or 200 when outside. Anything more is just asking for noise and a lower dynamic range. But in anything other than ideal situations, letting the camera pick an ISO setting for me significantly raises the number of keeper shots from any particular shoot, especially when the capture is absolutely critical. Noise can be reduced quite a bit using today's software, so while an image captured at ISO 1600 or 3200 may require a bit more work to make acceptable, having the image itself is better than not having the image at all.
That's really the only real reason I use AUTO ISO: to get the image. I can massage it all I want later in Lightroom or Photoshop or any number of post-processing tools, but if I don't get a good image in the first place, then no amount of processing will create one from thin air.
Getting Rid of the Blur
The biggest reason to use AUTO ISO is help reduce motion blur from those pesky, shaky hands of ours. I'm not exactly shake-free, by any means, and while AUTO ISO isn't a panacea, it will do its very best to make sure the shutter speed never falls below the inverse of my focal length when I'm shooting in Aperture Priority mode (Av on your camera's dial). Which means that I have a pretty good chance at getting a blur-free shot. (Of course, this doesn't account for moving subjects.) When shooting in Shutter Priority (Tv) mode, the camera won't let the shutter speed fall below the selected shutter speed, so if I want to capture every image at 1/1000s, the camera will adjust its ISO accordingly to allow the shot at the desired shutter speed. This is particularly useful when working with sporting events where you want the action to be frozen and sharp.
When Not to Use AUTO ISO
I've mentioned both Av and Tv modes above, but what about Manual (M)? Here's when you might get into unexpected exposures if you use AUTO ISO while also using Manual mode. Some cameras will simply set your ISO at a fixed value (like 400), and others will adjust the ISO for you (essentially reacting like Shutter Priority mode, except you also select your Aperture). Neither option is all that great -- both are a bit unexpected, and unless you catch it quickly, chances are good that you won't end up with what you intended to capture. For those cameras that adjust the ISO, it also means you'll end up with radically different exposures for the same shutter and aperture settings, something else you may not desire. So, just something to remember: when shooting manual, make sure you manually select your ISO as well. (Of course, this rule can be broken if you really did want to shoot with AUTO ISO on Manual mode. I'll leave that to you.)
The other time I use manually selected ISOs is when I'm looking at a decent period of time where the light will remain the same, and where the scene is unlikely to change dramatically. At this point I'll drop to the desired ISO (like ISO 100 outside) -- this way I can ensure that the camera doesn't go off and decide to pick ISO 1600 for a shot or two. Usually the camera is pretty good at knowing when to use a low or high ISO, but just like any other type of auto-exposure, the camera can guess wrong.
Don't Forget Metering Modes
Your camera is figuring out the AUTO ISO setting based upon the same metering it is using to determine Aperture and/or Shutter Speed, so if the camera meter decides on a funky exposure, the AUTO ISO will be similarly funky. That's probably pretty obvious, but is something to remember.
AUTO ISO Range
Just an FYI: some cameras will let you select an AUTO ISO range. The Olympus PEN E-P1, for example, lets you select a minimum and maximum ISO of anything within the camera's minimum and maximum ISO settings. This means that one time I may only wish to go to ISO 1600, but another day I may want to allow the camera to go all the way to ISO 6400. Some cameras won't make this a range, but instead use expansion modes you have to enable. And some will simply use the entire range available regardless of what you want.
Hey! The Camera Picked an ISO I Can't Pick!
So you get home and review your images and you start seeing images with ISO 64 or ISO 250 or ISO 5000, but when you look at your camera, you find that you can't pick those ISO values yourself. That usually means you're not using the "pro" version of the camera where you can select those settings. Thankfully most camera manufacturers let their non-"pro" cameras pick the ISOs in-between even if you can't do it yourself. Consider it one of many incentives to upgrade! ;-)
- AUTO ISO isn't bad -- but understand what's going on underneath.
- When in Av mode, the camera will select the minimum ISO that allows a shutter speed faster than the inverse of your focal length.
- Of course, if things are too dark, the shutter speed may still be too slow to capture a blur-free image. Time to pull out your flash!
- When in Tv mode, the camera will select an ISO and aperture that will capture a properly exposed image at your selected shutter speed.
- Of course, if things are too dark or bright, and the camera can't adjust both ISO and aperture to avoid the problem, you'll end up with an under or over-exposed image.
- When in Manual, don't use AUTO ISO.
- Unless, you really, really, really want to, and know what to expect.
- See if you can select the ISO range so you can either limit the maximum ISO or expand it, depending on the conditions you'll be shooting in.
- Don't be surprised to see ISO numbers you can't physically select on the camera body.
- When things get too light or dark, the camera will still have problems. AUTO ISO can't fix everything.
So, with that, I'm going to zip up my bag of secrets and put it away for now. I'll bring it out sometime in the future an share another "horrible" secret, but until then, keep writing with light!