Monday, March 24, 2008

Understanding Exposure As A Water Faucet

One of the best explanations of exposure I heard (or perhaps read) was to think of it like letting water through a faucet into the sink. If light is represented by water coming out of a faucet, then a properly exposed image would have a definitive (for the purposes of this write-up anyway) amount of water that is needed. An over-exposed image would have too much water, an under-exposed image would have too little.

Now to get that needed amount of water, there are two factors in effect in your water faucet. The first is the length of time you let the water run for (shutter speed), the second is how much water you let through, ie: the size of the pipe (aperture).

So it doesn't matter how you manipulate these two factors, as long as the correct amount of water ends up in the sink when you're done. So a properly exposed image can be made by opening up the faucet really wide and letting a quick burst of water through, or opening up the faucet just a tiny bit, and letting it run a long time. Either way you get the same amount of water in the sink, and your image is properly exposed.

The beauty is, for the most part, your camera will calculate the amount of water itself and you can use either the "Tv" or "Av" (sometimes called just "A" and "T") to control the shutter speed (time the water runs) or aperture (amount of water let through) respectively.

Now there are a few obvious things I'm completely ignoring here. The first is ISO, or the sensitivity of your film. For the purposes of this beginner info though, we're going to assume that the camera is dealing with the ISO, and that while you can set this, this is something that the camera factors in when you ask it to calculate what the proper exposure is. I'm also assuming that the camera is calculating the exposure correctly. As you'll begin to realize as you learn more, your camera does make mistakes, and often. They are getting better every year, but there are lots of scenes that will completely confuse modern sensors. Strong backlighting, snow, white on a black background or black on a white background will more often than not confuse your camera and produce images that are grey and muddy, too dark, or too bright.

So hopefully the simple idea of a faucet and a predetermined amount of water that needs to end up in the sink will help to understand exposure.


Anonymous said...

You might want to spell "faucet" correctly.

Alan Bailward said...

Whoops! Corrected, thanks.... wow, that was really bad! :)