Technique / Discuss

chadmiller    says:

If your eyesight is not perfect, you may sometimes find yourself squinting at difficult-to-read things. The lens part of your eye isn't good enough to focus on some things, so long ago you accidentally discovered that closing your eyelids until they're almost shut make things clearer. How?

Throw out everything you know about modern cameras with their focusing voodoo, and let's consider the simplest and earliest camera form -- pinhole cameras. Pinhole cameras are just boxes with film on the inside of one side and a tiny hole in the opposite side. You point the box at something, uncover the hole, wait a while, and eventually cover back up the hole and take out the exposed film. In the photo you just took, everything was sharp, as long as the box and the subject were (relatively) still, no matter how near or far away.

As photography became more widespread and attention spans became shorter, it became obvious that the "wait a while" step was the inconvenient part of taking a photo. Folks tried tricks like exploding some flash-powder so that more light came through the tiny hole, which worked passably well, but nearby things tended to catch aflame.

The next step for getting the same amount of light on the film in a short period of time was to make the "pin-hole" large. This also worked, but it had a terrible side effect: Blur!

Think of yourself as a single, miniscule spot on the film, looking out through the "pin-hole" to the outside world. When the hole is small, you see only a tiny speck of Universe "out there," so you have no choice about what color/shade to become -- the tiny speck of light you see is what you'll represent on film, and it doesn't even matter how far away that yellow thing is, as it's all you can see anyway. But, if the hole is wide, then you can look through it to see a big ol' area out there. Now, you can see a bit of a yellow flower, some green leaves, and brown bark of a tree, and some white of a cloud far behind it. What color are you going to turn? You must average all that you can see together, yielding an ugly greenish umber color. Congratulations -- you're a blurry spot.

What was a photographer to do?! A few opticians came to the rescue and demonstrated how a few glass elements can be used in front a large hole to move some things into focus. Not all things, note, as glass elements can only properly redirect and "focus" light from some things that are a particular, specific distance away. To choose how far away this "focus plane" is, one could move the glass elements to and fro.

Now, thanks to those clever opticians, the photographer could use a really big hole (which we'll call an "aperture" from now on) to get a lot of light, and still have something in focus. But, if he didn't really need so much light and if he was feeling nostalgic for the days when his pin-hole camera had everything in focus, he could close down the hole a bit -- the smaller he makes the aperture, the more stuff was in focus in his photo.

When you squint your eyes to see better, your eyelids are just like the aperture of your camera lens. When "squinted", the normal focus-plane becomes very thick. Depending on the aperture size, the focus plane can be very thin or infinitely wide. In photography, we call the focus plane's depth "the depth of field".

On point-and-shoot cameras, the aperture is permenently "squinted" shut pretty tight, and almost everything is in focus. On more complex cameras, one can open up the aperture to get more light, but the side-effect is that the depth of feild becomes very short. (Note: When the depth is short, getting the focus right is very important!)



On most cameras that are not point-and-shoot cameras, there's a setting for "f-stop," which is a ratio of the lens' length to the size of the aperture. The larger the number of the f-stop, the smaller the hole. (If you're math-minded, think of the number as the denominator of a fraction, under "1".) The ratio system is an effort to make all f-stops of a particular value let through the same amount of light, regardless of the lens' other characteristics.

The aperture acts as yet another focuser, so to speak, outside of the normal glass focus-elements of your lens. In the sundry kinds of photos you take, the depth of the field in focus can make artistic statements.

On landscape shots, the "depth of field" is usually very wide, so a small aperture is advisable, requiring a faster film or a slower shutter speed to compensate for the lower light level.



But, in general, when taking a portrait photo, one wants to push the background and extreme foreground out of focus so that the subject of your photo (and more specifically, usually the subject's eyes) is the only thing that catches the viewer's attention.

[tags: composition, exposure]
Posted at 8:04PM, 13 November 2004 PST ( permalink )