ISO Explained


Figure 1

Stripped to its basics, photography’s all about light: how much, how long, and at what intensity. The aperture, which we covered earlier in the week, controls “how much.” The shutter, which we’ll be taking up next Tuesday, controls “how long.” That leaves the intensity of, or more accurately,
your camera’s sensitivity to, light, which is controlled by its ISO settings.

Let’s get the jargon out of the way first, shall we? ISO simply stands for International Organization for Standardization, a body that sets standards for film speed and sensor sensitivity. ISO actually carries over from the film days (and the numbers correspond to ASA numbers, from the system that ISO replaced). Time was, if you were shooting outdoors, you’d buy ISO 100 or 200, use 400 for indoors, and use 800 or 1600 for low light or situations when you wanted to shoot at a much higher shutter speed in broad daylight.

Just the same as low-ISO film had a very fine grain, the “grain” in digital (more often referred to as “noise”) is much less noticeable at a low ISO than it is when you “push” the ISO, using the upper reaches of your camera’s ISO capability. The reason for this is that you’re increasing the sensor’s sensitivity to input in general; not only does light get picked up, but the sensor is also “reading” stray electrical impulses and incorporating those into your image.

Of course, as with any other feature or setting on your camera, a little experimentation is in order. You always want to know what a setting will do not only for, but also to, your photos. Higher ISO settings, as previously mentioned, introduce varying levels of noise, and loss of detail, into your photos. Whether you’re using specialized software, or just your camera’s native noise reduction, you can keep a lid on the noise to some degree, but you can’t recover lost detail. It’s up to you, therefore, to see how your camera behaves at about 1,000 and above, and to decide whether these are tradeoffs you’re willing to make.

Figure 3

Let me give you an example. Noise is generally going to show in darker areas of the photo, though if the ISO’s set high enough, it’ll be noticeable throughout the photo. Figure 1 was shot in Aperture priority at ISO 400, since I wanted an f/16 aperture to make sure the church’s steeple would be as sharp as the rose window and other parts closer to me. The shot’s reasonably clear of noise. As you’ll see if you peek at the EXIF data, the camera chose a shutter speed of 13 seconds. I happened to have the camera on a tripod at the time, so for me, that was an acceptable shutter speed. But suppose you don’t own a tripod, or don’t have it with you? A higher shutter speed is going to be necessary to reduce motion blur, but if I try a higher shutter speed at f/16, my photo’s going to be terribly underexposed. The compromise lies in our ISO settings. There’s a dizzying array of ISO numbers on your camera, with some cameras starting as low as 50, and going as high as 25,600. Here’s all you need to remember: with each doubling of your ISO number, you’re also doubling your sensor’s sensitivity to light. So going from ISO 800 to ISO 1600, for instance, is one full stop of light; in other words, I’ve got twice the light sensitivity I had before.

Figure 4

So. I’m not willing to compromise on aperture because I don’t want to sacrifice my depth of field. I probably can’t squeeze more than a third of a stop out of my shutter speed without the photo being underexposed. The solution, therefore, is to boost the ISO. We’ll try going up two stops, to ISO 1600 (figure 2). Notice that the aperture stays the same, but the shutter speed has been “sped up” to 8/10ths of a second. If I want to go faster still (maybe it’s cold out and my hands are shaky; it’s not like I have to worry about the church scuttling off), I can push to ISO 3200 (Figure 3, ½ second). Or maybe my subject isn’t quite as well-lit as this church, and I have no tripod (I know I already said I had one… play along for a minute). I can push my ISO all the way up to 25,600, and, hey presto, my subject is there in the frame, plain as day (Figure 4)… but I’ve sacrificed image quality for not much of a gain shutter speed.

Speaking of degradation, let’s take a closer look at the noise, shall we? Figure 5 shows a 100% crop (300×300 pixels) from a section of the shot in figure 4. Not a great look, is it?

Figure 5

Noise isn’t your friend. I’m also going to end on the assumption that if you’re making a photo of something, you want the end result to be reasonably close to what you’re seeing in front of you. Besides the noise and lost detail that higher ISO introduces, it also affects the look of the light in the scene, making the night look brighter, overall, than it actually is. Your best bet, therefore, is a support (a tripod, or, barring that, a monopod) with a nice, long shutter speed and the lowest-possible ISO. Being the realist that I am, I know you won’t always have a tripod. The next best thing is to push the ISO only as far as it needs to go, and shoot with as slow a shutter speed as your handholding technique will allow.

A note on sensor size and ISO: because a larger sensor can accommodate larger pixels, it’s possible to get higher, cleaner ISO performance out of a full-frame sensor than out of an APS-C sensor, while both of those will outperform a compact, and all three will beat the pants off your iPhone. It’s not uncommon for current cameras, like Nikon’s D7000 and Canon’s 60D, to be capable of up to a staggering 25,600 ISO, although the level of noise present in the photo (and the amount of detail that’s lost) makes it impractical. Realistically, newer cameras can be expected to give acceptable results up to about ISO 3200, with the caveat that your mileage may vary.