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what does the focal distance actually mean?
scott wrote: When you see a 200mm lens, what is the 200mm actually a measure of? I assume it's something to do with how much "zoom" there is, but surely measuring the angle that is shown by the image would be better? Eg a wide-angle lens would be a 90 degree field of view but a telephoto one would be maybe 5 or 10 degrees. Is it possible to work out the field of view in degrees from the focal length? Focal length is exactly what it sounds like -- the distance from the lens to focal (film or sensor) plane. So yes, a basic 200mm lens would be 20cm from the film when focused at infinity. Of course, we want things compact so more sophisticated optics are used to compress the physical length of the lens. Zoom lenses are more sophisticated again, but the focal lengths still refer to a simple lens. MJ |
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