The focal length of a camera's lens refers to the the magnification or telephoto power of the lens and is expressed in the millimeters of the lens, like 100mm, 300mm, etc.
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How precise do you want to get? Precision method... Calculate the diagonal of the camera's film image. Compare the lens focal length to that to get your answer. Example for 35mm camera with 50mm...
It depends on the camera. If it has detachable lenses, there should be a variety of zoom lenses available. Vivitar and Sigma are good manufacturers, and of course your camera manufacturer's offerings...
Basically, the focal length is the distance from the center of the lens to the focal point or the principal foci.
The calculations will depend on the curvature of the lens, the lens thickness, and...