ISO: Why You Care

In the last lesson, you learned that ISO was sort of like turning up the volume on your stereo, in that it magnified the light as it came into your camera. That sounds like a pretty good way to get a brighter picture in low light, right? It is, but unfortunately there’s a cost associated with it.

Here are two utterly unremarkable photos that I took of the corner of my office building:

The first one was taken at ISO 100, the lowest my Canon Digital Rebel XT will go, and the second one was taken at ISO 1600, which is coincidentally the highest ISO that camera can do. You probably don’t see much difference in them at this size, but I promise you that they’re very different images.

Let’s go back to music for a bit. If you have a very quiet recording on your stereo and you turn it up, the sound gets louder. Something else happens, too– any background noises in the recording get magnified. If you’re listening to a tape (remember those?) the hiss of the tape gets louder. If there’s a small bit of noise in the background, it gets louder.

Just like sound recordings, our digital images exhibit something called noise, and that noise makes the images look grittier and grainier at high ISOs. For the following images, I’ve magnified everything to 2x the original size, and cropped down to a small section of the image for illustration purposes. First, let’s look at the upper left corner of the sky at ISO 100, 400, and 1600:

As you can see, the sky in the ISO 100 image is very smooth. At ISO 400 you can start to see it getting gritty, and at ISO 1600 the image looks pretty bad. That’s noise.

Here are a few more examples, at ISO 100 and ISO 1600:

In all three cases, the ISO 100 image looks pretty smooth, while the ISO 1600 image is very grainy.

It’s important to know that different cameras can have very different performance characteristics at higher ISOs. Inexpensive point & shoot cameras tend toward really horrible noise as the ISO increases, though I’m sure there are exceptions. Consumer-grade Canon digital SLRs are known for performing reasonably well at high ISOs, and their higher-end cameras often do exceptionally well. I have gotten some excellent low-noise shots out of my 5D at ISO 1600. Of course, I didn’t use that camera to take these test images, since the difference in quality would have been much more difficult to show.

If you’d like to see the full resolution versions of the originals:

ISO 100
ISO 400
ISO 1600

That’s great, but how should use use the ISO setting on your camera? My strategy is very simple. For typical shooting I always leave my camera set on ISO 100 until I find myself unable to get the aperture and shutter speed I need to get the shot. If there’s not enough light to shoot at ISO 100, I turn the ISO up until I get to an acceptable shutter speed and aperture. If I know I’m going to be shooting in a relatively dark environment like a nightclub, I just set the ISO to the highest setting I can and cope with whatever noise I get, since I already know that lower ISOs probably won’t be useful.

It can be useful to consider what you want to do with the images you shoot. If you’re just going to use them on the web, then the extra noise at higher ISOs won’t be much of a consideration, since the noise won’t be all that visible. Likewise, if you’re printing small images, noise won’t affect the output too much. However, if you’re planning to print enlargements, or you crop the image down significantly before printing it or displaying it on the web, noise may be a factor.

I hope that ISO has been demystified for you. Has it?

A quick update from mid-2010: This article was written a couple of years ago.  Since then, dSLR technology has improved considerably.  Many newer dSLRs can shoot very good images at ISO 800   or even 1600, and some produce usable shots at 3200 or even higher.  All of the concepts remain the same, but the top end has moved a bit.  I expect that will keep happening in the future.

Next lesson:  How much more light?  How much less?