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Old Nov 7, 2006, 3:37 PM   #1
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I know mm is millimeters and I know the general difference between macro and telephoto, and I know crop factors/image sensors change the resultant image. But can someone give a comprehensive description of what the lens measurements are actually measuring? ie., what does the 35mm measure in a 35mm lens?
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Old Nov 7, 2006, 6:48 PM   #2
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It is a measurement of the focal length of a lens. Approximately equal to the distance between the film plane, or sensor, to the optical centre of the lens, when the lens is focussed at infinity.

Try a google search for FOCAL LENGTH There is plenty of information.
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Old Nov 7, 2006, 7:06 PM   #3
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When we refer to a 35mm camera we are referring to the width of the film stock. There are two rows of sprocket holes down each edge which limit the picture taking width to 24mm. Length wise the camera will expose 36mm along the long axis. This 24mm by 36mm size is what defines full frame DSLRs.

If you are referring to a 35mm lens you will have to define what type of camera you are attaching it to. It could be a moderate wide angle for a 35mm camera, a normal lens for one of the APS sized sensor DLSRs such as the Nikon D70s or a very wide angle for a medium format camera such a a Hasselblad.

To classify any lens as normal, telephoto or wide angle you have to know what the normal lens focal length is. This is usually the length of the diagonal of the picture taking area. As the focal length of the lens gets shorter then this you get more and more of the scene recorded. As it gets longer you get less of the scene recoded, giving a telescopic effect. A normal lens will show approximately the same view as the human eye sees.

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