Question regarding lenses and distance...

[metsuyan]

This may sound like a totally unorthodox / unprofessional kind of question but I'm gonna risk it here anyway.

Is there any correlation that can be made between the focal distance (mm) of a lens and the distance that one may be from a human subject. I know it depends on the size of the subject and what part(s) of the subject one wishes to include.

To make a really confusing question just a bit less confusing... here's the type of question I'm looking to answer and perhaps find a formula for.

The human subject is 5' tall (sorry to all you metric people) and I'm 100 feet away from the subejct. What mm lens would I need to have the subject fill most of the frame?

If there is a formula for this in some way, that would make me happy. I know all the stuff about angles and degrees etc... This just seems more direct but then again I may just be a bit nuts ;)

Thanks in advance for anyone who attempts to answer this one. I tried asking this question to a "pro" at a famous camera shop and he got all pissy bc "it's not the way things are done." Whatever!

[PacAce]

Take a look at this thread. I think you'll find the answer you're looking for.

http://www.photography-on-the.net/forum/showthread.php?t=40259&highlight=dof+simulator

[photoguynorth]

Here goes ...
The focal length of the lens needed would be the distance to the subject times the height of the image (at the sensor) divided by the subject height (all units the same). In your question, that would mean 30480 mm (100 feet) times 24mm (approximate height of ful frame sensor) divided by 1524mm (5 feet), or 480mm. If you have an APS size sensor, replace 24mm with 15mm Then you need a 300mm lens. Now you know why it isn't done that way :)

[Marvinspu36]

If I understood your question right, what you are looking for is a Field of View calculator. Click here (http://www.dudak.baka.com/fovcalc.html) for one FOV calculator. See if this is what you are looking for.

Enter your distance to subject and vertical AND horizontal measurements, and then click on the Calculate Focal Length button.

Keep in mind that the focal length listed will be for a 35mm camera. Adjust by the Crop factor for your camera.

Hope this helps...