Camera (very) Fundamentals

Version 2.1, page 4, © 2002, 2006 by Dale Cotton, all rights reserved

Chapter Four: Light Meters and Light Measurement

Fig 11. Photocell light meter

Digital: The light metering concepts on this page apply equally to digital and film.

Ready for the Mark IV? Because there's still an operational problem with the Mark III that's crying out for attention - how do we know how long an exposure time to use? Time to get electrical.

Going the nearest electronics parts store we buy a little gadget called a photocell that creates an electric circuit when light shines on it. The stronger the light, the stronger the current. We attach this to a little meter that has a pointer that moves from left to right as the current from the photocell gets stronger - something like a thermometer of light. By trial and error we can pencil in marks on the meter face that shows what shutter open time (shutter speed) to use to get a correct exposure for 100 ISO film.

So if the meter points at the half way mark and we find that a shutter speed of 1/60th of a second gives us a correctly exposed frame of film, we'd make a mark at the half way point on the meter dial and label it 1/60th. (Notice that using different apertures would complicate things, so we'll stick to a single aperture for all our tests.) We then do trials on the one-quarter brightness point on the meter dial and find that 1/120th of a second works for it, and so on.

Your SLR has a meter that is already calibrated and can directly control the shutter speed without your having to do that by hand.

Useful fact: Most modern cameras have a setting labelled A for aperture priority. This means that you manually set which aperture (f/stop) you want to use, and the camera controls the shutter based on the light meter reading. This is probably the mode that the majority of outdoor photographers use most often. It gives you control of depth of field with a minimum of hassle.

Useful fact: A similar setting labelled S for shutter priority does just the opposite. You set the shutter speed you want and the camera controls the aperture. This is useful when you want to freeze action, such as in sports.

Actually, sometimes you may want to blur action. If you are photographing a race car in motion, for example, stopping the action makes the car look as though its parked. So you use shutter priority set to about 1/60 and pan the camera to follow the car. If your hand is steady enough this gives you a sharp picture of the car but the tires and background are motion-blurred.