My blogging friend Steph emailed me today to ask, “How do I take a picture of my kids when they’re moving and not have it be blurry? Example: say they are jumping and I want to catch them in the air.  When I do it, they are blurry.” This entry is for her and those like her who need a primer in exposure time.

Short answer, and often the long answer: shutter speed. For the beginner, shutter speed is exactly what you would imagine, the amount of time the image sensor is exposed to light. When I am teaching, I try to give out a few guidelines when people ask me the same kinds of questions as Steph did. I explain that 1/1000th of a second is a very short amount of time, and 30 seconds, in photography anyway, is a very long amount of time.

I thought of this just last weekend when my wife Abby and I were shooting pictures together at a nearby spa that has been a client of ours for several years. We were in one of their elegant rooms, which is very quiet, and Abby started shooting with her D70S. Immediately I could tell that her shutter speeds were too long for her to hand hold her camera without a tripod. I could tell because I could hear two clicks. If you can discern more than a single click, the shutter speed is longer than about a 1/30th of a second, and that’s, well, a long time in photography. Over the years, I’ve gotten to where I can usually tell within a stop or two what your shutter speed is, if it’s longer than a 1/30th of a second.

So what does that mean in the real world? Well, Steph‘s kids move, in photographic terms, very fast, as do most kids. The benchmark for “fast” shutter speeds, ones that pretty much assure you of freezing the motion of human beings, is 1/1000th of a second. For you math nurds, that’s one millisecond, and it’s a pretty short period of time. Since it is such a short amount of time, an exposure like that simply doesn’t let all that may photons travel through and strike the sensor. Thus, some situations don’t allow us to use super-short exposures.

Thus begins the eternal compromise in photography. What can we do to “freeze” the action? On a sunny day, we can set the shutter speed to 1/1000th of a second, set our ISO to 200 or so, and realistically get an aperture value around f/4 or f/5.6. But on a gloomier day, that combination of exposure settings will render an image that’s too dark. What do we do? Increase the ISO, open the lens aperture, lower the shutter speed, or some combination of all three. As things get darker, like when the sun goes down or we go inside, eventually we are going to run out of options; the lens is all the way open, the ISO is all the way up, and the shutter speed? Well, we can keep increasing the shutter speed indefinitely, can’t we? But of course, we are unable to freeze the action of our subjects.

One alternative for Steph might be to use flash. The duration of modern electronic flash hovers around a 1/1500th of a second, which is a very short amount of time. There are two disadvantages to flash. One is that your average flash takes four or five seconds to “recycle” and be ready to flash again. The other is the dreadfully unflattering look of direct flash. Thus enters bounce flash, which I discussed some here.

Point-and-shoot cameras have another factor that tends to complicate this formula, and that is shutter lag, the time that passes after you push the shutter release but before the camera takes the picture. Digital SLRs don’t have much shutter lag, but small cameras can lag up to as much as a second before taking the picture. Using them in the case of shooting action requires some mental calculation and anticipation.

I know this seems like a lot to someone like Steph who really just wants pictures of her kids having fun, but once you play around with shutter speeds enough, it will “click,” and suddenly a whole new world of imaging becomes possible.