There’s an under-utilized button on the front of your SLR, that, until now, you’ve only used to blind your friends. If you’ve got your pop-up strobe out, there’s a button on the front of your camera, next to the lens, reachable with your right hand. It makes your strobe go all crazy, and no, that’s not just to prank your friends, and make them blind.
It’s the depth of field preview button, and it helps you get the most out of your DSLR.
Why would anyone design a complicated moving mirror and shutter system like the SLR? Because it allows for a statistically insignificant shutter lag, and at the same time, allows the photographer to see through the lens(Hey! Wait! That’s what TTL means!). Rangefinders are fantastic because their mirrorless design allows for a more compact camera, but this comes at a cost. You lose the ability to actually see through the lens. This can cause some problems with the image being from a slightly different angle than the viewfinder, but it also has the larger drawback of not being able to see the depth of field before you shoot.
Depth of field, or DoF, is, to put it simply, the not-blurry part of your shot. It’s a flat plane that is perpendicular to the image sensor(or film!) unless you have a special lens or camera that lets you do fun things.
Two things control DoF: Distance and aperture. The closer to your camera you focus, the thinner that slice of focused area will be, and the farther away you focus, then the deeper it will be. This is why you can focus on two different mountains at once, but that picture you took had Mom looking all sharp and crisp, but Uncle Mike was all blurry.
The other controlling factor is your aperture, the hole in your lens. The aperture is expressed as a fraction, so as the f-number gets bigger, the actual hole gets smaller. Hence, a really large aperture would be 1, and a small aperture would be 64. Yes, it seems backwards.
One way to solidify this in your mind is to use the DoF preview button – but not as intended. With your pop-up strobe safely tucked away, and your aperture opened up as wide as it will go(set to the lowest f-number), say f/1.8, or f/2.8, or f/3.5, look into the barrel of the lens and press the DoF preview button. You won’t see much happen. Now, set the aperture as small as it will go, f/16, f/22, or whatever, and press the DoF button again. This time you should see the blades of the aperture shoot in, and leave only a tiny hole in the middle of the lens for light to get through.
Now, a wide-open lens lets in a lot of light, but the DoF will be a very thin slice. Sometimes you’ll get an eye, but the nose will be blurry. When you stop down to f/16, you will get much less light, but you will have a large slice of your photo in focus. Photography is always about compromising between aperture, shutter speed, and ISO to get the shot you’re looking for.
Maybe you’ve got a great feel for your camera, and you don’t need to check exactly what’s in focus, or maybe the edges of your DoF aren’t that important to you, but let’s get back to that photo of Mom and Uncle Mike. Now, before you take the photo, you can push the DoF preview button, and you can see exactly where your shot gets blurry. You can adjust your focus to get them both nice and sharp, or, if that’s impossible, ask them to be nice and stand on the same plane. Maybe you won’t always use the DoF preview button, but it doesn’t hurt to know why it’s there.
Also, if you didn’t know about the blind-your-friends trick… you didn’t learn it from me.