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Below are two photographs showing the effect of lens size. Both are pictures of the Straits of Gibraltar (separating Spain from Morocco), taken by astronauts on the space shuttle. The photo on the left was taken with a 40mm lens on the camera; the entire picture is about 230 miles across. The photo on the right was taken with a 250 mm lens on the same camera; this picture is about 37 miles across. 

[[image]]  [[image]]

If you know the altitude of the shuttle (its height above the Earth) and a couple of things about the camera, you can calculate how much of the ground will be photographed in a KidSat picture.

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The two triangles are similar triangles, which means that that the following ratios are equal:

altitude/ground distance = focal length/size of detector

07/22/96     Page 4.7