Dptips :. Digital Photography Tips, Tricks, and Techniques

Camera exposure systems are calibrated on the assumption that the overall brightness of the scene will average out to the “18%” or mid-gray found on a gray card. In other words, if you looked at the entire scene in black and white, all the white highlights, midtones, shadows and blacks when mixed together would average out to the same tone as on a gray card. This is what the camera assumes it is seeing when you half-press and get a shutter speed/aperture combination.

Of course, the brightness of most real life scenes doesn’t conveniently average out to mid gray, so it’s usually necessary to dial in some exposure compensation to correct for the difference between the camera’s expectations and the actual scene. This is most obvious in the case of beach or snow scenes (which need +1 or 2 stops compensation, otherwise the camera would render them as mid-gray) or predominantly black or dark objects which would otherwise end up being over-exposed.

The second use, as mentioned above, is to set “white balance”. This essentially sets the correct “mix” of red, green and blue, rather than overall brightness. This is because all sources of “white” illumination have a slightly different colour cast. If we assume sunlight as being our reference for “true” white, then shots taken on a cloudy day will take on a bluish cast and those taken under tungsten lighting indoors will look reddish. So a gray (or white) card, photographed under the same lighting conditions as other shots to be taken, gives us a convenient reference for white balance as well as for correct exposure.Tomas