Programmed Automatic Exposure in Canon EOS Digital Cameras

Short Description
Many modern cameras, both film and digital, offer (usually as their basic mode of operation) a “programmed automatic exposure” mode. In this mode the camera, after measuring the luminance of the scene, sets both aperture and shutter speed with no further intervention on the part of the photographer. This article discusses the details of this operation as found in the Canon EOS 10D, 20D, 300D (Digital Rebel), and 350D (Digital Rebel XT) digital single-lens reflex (SLR) cameras. It also discusses the related matters of exposure compensation (exposure bias) and program shift, tools that allow the photographer to “tweak” the programmed automatic exposure control mode to deal with the special needs of a particular shot.

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Content
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As with most modern cameras (simple or complex, film or digital), the Canon EOS series of digital single lens reflex (SLR) cameras offer a “programmed automatic exposure” exposure metering mode (mode “P”, actually designated “program AE”). In this mode the camera, after measuring the luminance of the scene, sets both aperture and shutter speed with no further intervention on the part of the photographer. The algorithm followed by the camera in balancing shutter speed and aperture, so as to attain the overall exposure determined as appropriate by the metering system, is guided by a set of curves often called “program lines”. We will shortly look at a graphical presentation of those curves and the algorithm through which they are used as it applies to the Canon EOS 10D, 20D, 300D (Digital Rebel), and 350D (Digital Rebel XT) cameras. The general principles doubtless apply to various other Canon EOS digital SLR cameras.
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