Naomi |
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mm vs feet
What is the relationship of mm to feet...
In other words, if I have a 35-300mm lens and I am 15 ft away from something what mm would be closest to fill the frame?????
Am I making sense?Thanks,
Naomi
September 25, 2002
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Jon Close |
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It depends on the size of the object that is 15 ft. away. A large truck may fill the frame at the 35mm zoom setting, but even 300mm will not be telephoto enough to fill the frame with a robin. You need to use the angle of view provided by the lens. A 35mm lens provides an angle of view of about 60 degrees. Using trigonometry, tangent of 60 deg = 1.7321, so at a distance of 15' the frame will be filled by an object that is 26' (15' x 1.7321) long. A 300mm lens provides an angle of view of about 8 degrees. tan 8 deg = 0.1405. At a distance of 15' the frame will be filled by an object that is 2' long (15' x 0.1405). To determine the lens needed for a specific subject you'd work with the inverse tangent (TAN-1). An object 5' tall at 15' away gives a ratio of 0.3333 (size/distance = 5/15 = 0.3333). The inverse tangent of 0.3333 is 18 degrees, which is about the field of view given by a 135mm lens.
September 25, 2002
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Jon Close |
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I forgot to add in the first part that the tangent x degree gives the ratio of height (or size) divided by length (or distance).
September 25, 2002
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Jon Close |
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D'oh! D'oh! D'oh!
As soon as I logged off I knew I made this much more complicated than needed. Besides, angle of view is usually given as the diagonal when you can find it at all.The 35mm film frame size is 24mm x 36mm.
The ratio of image size to focal length is the same as the ratio of subject size to distance. So you use the relation: S/D = 24/f if you want to fill the frame top of bottom, or S/D = 36/f if you want to fill the frame side to side.
For example if you have a subject that is 5' tall and 15' away, you solve 5/15 = 36/f for the focal length. f = 108mm. Sorry about that tangent and angle of view stuff ;).
September 25, 2002
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Naomi |
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Thanks for the info...
I will have to digest it for a little while...
N
September 26, 2002
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