Focal Length Calculator?

[perdendosi]

Does anyone know of a good formula/chart/website/calculator that compares focal length, distance from subject and size of subject? What I mean is some sort of calculator where I could ask -- "If I'm X feet/meters away from my subject, and I'm using a Y (35mm equivalent) focal length, then a subjectZ big would fill the frame." Are there any "default rules" that I don't know about?



Thanks

~kjk

[KCan]

Camera lens is d feet/meters away from subject
Focal length is f mm (35mm equivalent)

Then the 24x36 film plane will capture a max of [ 24 mm x (d/f ) ] of the subject height , and [ 36 mm x (d/f ) ] of the subject wide .

Said another way, a subject with a size S , at a distance of d from the lens with a focal f, will produce an image with a size = S x f/d on the film plane .
If you use the real focal length value , then it will be independent of the fact that's a film plane or a CCD, or what ever capture / viewing surface.

Example:
with a 200 mm focal ( 35mm equivalent) lens,
a 100 mm (4 inch) size bird
at a distance of d= 10 000 mm (10 meters )
will fill in (or produce an image of) :
100 x 200/10 000 = 2 mm on the (24 x 36 mm) film plane

Of course, this is a simple optics consideration. It apply by the book with a pinhole camera for example. In reality, the virtual optic center of a 200 mm zoom lens is away from the lens itself ; this make the above (simple) formula true only if the subject are relatively far from the lens (or the camera).

[KCan]

FYI:

http://www.infodotinc.com/photography1/ph20916.htm

http://www.infodotinc.com/photography1/ph20924.htm