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Photography Question 
Naomi 
 

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


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9/24/2002 8:08:50 PM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  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.


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9/25/2002 6:14:44 PM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  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).


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9/25/2002 6:17:09 PM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  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 ;).


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9/25/2002 7:24:42 PM

 
Naomi    Thanks for the info...
I will have to digest it for a little while...
N


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9/26/2002 12:11:47 PM

 
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