Most basic exposure meters will analyze the scene, look at the brightest and darkest area’s and convert that information into what is called “Average” gray for the scene. If we take a photograph of either a white or black piece of paper they will both be rendered a medium gray tone in our print.
Through experience we learn to overcome this phenomenon of metering characteristics and we make adjustments to the exposure to compensate for it. What we have to remember is that the meter has darkened the white areas and lightened the black areas to make them average gray. In our discussion we will assume that all other shades, colors and tones in the photograph are neutral and have no effect on the exposure.
To make the whites brighter we must increase the exposure by 1 or 2 stops
To make the blacks darker we must decrease the exposure by 1 or 2 stops.
We can dial in the changes on modern cameras by using the +/- EV compensation in the automatic mode, or change the shutter speed or aperture if using manual mode. Most modern cameras will allow you to change the increments by 1/3rd of a stop allowing for precise exposure changes. Arriving at the correct exposure is a matter of trial and error in most cases. This technique is called “Bracketing”.
For black & white film users and for slide film users this technique is a valuable tool to achieve a correct exposure. Today’s digital cameras offer us a quick and convenient way to see the results immediately after we take the photograph and know if we have the shot or not. Regardless of which medium you choose to use, today’s meters have come a long way towards achieving a correct exposure when coupled with internal computers and the stored reference shots provided by the camera manufacturer, which in turn frees us to concentrate on the photograph.