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On my camera, between the mode dial and the flash, there is a white theta symbol. I've also seen it on review sites in pictures of other cameras, often in different locations but always near the pop-up flash. Why is it there, is it useful, and if so, how?

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This indicates the plane that the image sensor lies in, so you can use to calculate the exact distance between your subject and your "film".

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    Or in other words, there the surface of your sensor is relative to the outside of the camera body for the reasons davr has given – JamWheel Feb 19 '11 at 12:09
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    This mark is used, by the way, for calculating mostly in macrophotography. The exact distance between the subject and the film plane/sensor is used to calculate the reproduction ratio (the relative size of the subject on the sensor) and exposure compensation. (The aperture you set on the lens is relative to the focal length of the lens, which is the length of the light path when the lens is focused at infinity. At macro distances for most lenses, the effective length of the lens is longer, so the effective aperture is smaller. If you are not metering through the lens, you need to compensate.) – user2719 Feb 19 '11 at 16:20
  • @Stan Rogers thanks for that great explanation, I was wondering who needed that precision when measuring the distance to a subject but never even thought of macro. – gerikson Feb 19 '11 at 17:49

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