Photography is first and foremost a record of light. You are alone behind the camera, doubling as artist and scientist, hoping that your light—and it is your light—will bring it all to life.
A correct exposure is one in which the brightness values of the scene are rendered accurately (top). Overexposure occurs when too much light hits the sensor, causing all of the brightness values in a scene to be rendered too bright, with whites losing detail (clipped) (bottom left) and underexposure occurs when insufficient light hits the sensor and everything is reproduced too dark, with blacks losing detail and becoming a solid mass (crushed) (bottom right).TTL metering averages the overall luminance in your frame to a middle gray reference. With a range of luminance values in the shot, this method can work well (top), but if your frame has very limited tones, like filling the frame with a black subject (bottom left) or with a white subject (bottom right) the auto iris will be “fooled” and attempt to render it middle gray.Exposing for middle gray in the sunny areas will underexpose the shaded areas (left, f/16), while exposing for middle gray in the shaded areas will overexpose the sunny areas (right, f/5.6). Note the effects of exposure change on the middle gray cards (which are both exactly the same shade of gray).A common situation shooting outdoors on a sunny day: exposing for the sunny side, at f/16, underexposes the subject in the shade; exposing for the shade, at f/8, overexposes the subject in the sunny side. A compromise exposure of f/11 can be used to show both subjects with some degree of detail. All options are viable if the narrative calls for it.On a properly exposed gray scale test chart, the pure white value (a) will read at 100 IRE on a WFM, the pure black chip (b) falls to 0 IRE, and middle gray (c) is at 50 IRE. Notice that the black and middle gray values are on the same vertical in the chart and therefore read on the same vertical on the WFM.Overexposure clips the brights, causing them to become pure white with no visual detail (top); conversely, underexposure will crush shadows, pushing them to 0 IRE where they lose all visual information and appear pure black. Notice what over- and underexposure has done to the middle gray chip (c).Monitors should always be carefully calibrated to color bars for luminance, contrast, hue, and saturation, providing an accurate rendition of the video signal from the camera. See the color insert.
