First, I apologize for the horrible joke. Perhaps someone slipped me decaf instead this morning. The thing is, I have to talk about math, and I don’t math. It makes my head hurt. This is simple math, and once you understand it, you don’t have to think about it every time you take a picture.
We can get that out of the way first, so here goes:
When you think about your shutter speed, you need to the whole numbers you see are actually fractions represented by whole numbers (some DSLRs do display the numbers as fractions, so if yours does – mine doesn’t – disregard this). So, if you turn your DSLR to “S” (Nikon) or “TV” (Canon) for shutter/time value priority shooting (where you set the shutter speed and your camera chooses an appropriate aperture to partner with your settings), you’ll see “250” or “1000” or some other whole number. However, you need to know what those numbers mean – they mean 1/250 of a second, or 1/1000 of a second.
So if you want a fast shutter speed, choose the biggest whole number, which is the smallest fraction (4000, or 1/4000 of second, for example). If you want a slow shutter speed, go for the smallest number.
The “smallest number” part gets a little trickier, because when you get to a one second shutter speed, you’ll go from 1/2 to 1 to 2, which means you’ll move (or step) from 2 to 1 to 2… but the first “2” is 1/2 a second, while the second “2” is two seconds.
How can you tell the difference? I mean, you’re already thinking about white balance and ISO and composition and now I’m telling you you need to not only think about fractions that look like whole numbers, you need to know the difference between which whole number is which, and you’re thinking that automatic – or hell, even the program setting – is looking pretty damn good.
Take a deep breath. It’s going to be OK, I promise. Because once you get out of the fractions, your camera should – should – put a “seconds” mark next to the real whole number. So, the 1/2 second exposure looks like this:
…but the two second exposure should look like this:
…as will every REAL whole number in your set of shutter speeds.
So that’s how shutter speeds look. But how do they work?
Ah, see, that’s the easy part. The shutter speed is the amount of time the curtains (or doors) over your lens stay open. Remember, your camera has two ways of allowing light to hit your image sensor: The size of the lens opening (called aperture) and the amount of time that opening allows in light (called shutter speed).
The faster your shutter speed (1/4000 of a second, or 4000), the less light gets to your image sensor; the slower your shutter speed (20 seconds, or 20″), the more light gets to your image sensor.
Of course, longer shutter speeds allow the image sensor to record more motion, and since we’re discussing a still camera and not a video camera, that’s not necessarily great. Faster shutter speeds capture instances of motion longer ones cannot.
The best way to illustrate the difference, of course, is with photos:
When dealing with wildlife, pets, kids – anything that’s prone to movement, really – faster shutter speeds capture crisp images. Slower shutter speeds allow for more light, but also more blur. It can be a nice effect with water, but not so much when your kid has four hands in a photo.
Slower shutter speeds often require a tripod and a remote shutter release or the two-second time delay on your camera (so you don’t shake the camera when you press the shutter release), while faster shutter speeds need lots more light to get a good exposure. Remember, when using shutter priority, your camera will set what it thinks is the appropriate aperture setting, which can affect your depth of field. So pay attention, and upload your photos to your computer and look at them up-close – and examine what settings you used by viewing them in iPhoto and clicking the information icon (if you have a OS X) or by right-clicking and selecting “properties” (if you have a Windows-based machine). This will help you understand your results an how to improve them.