[Bf-committers] Blender "lens" - Historic questions?

[Jonathan Merritt]

Hi Everyone,

For basic renderings (without "parts" parameters or anything else 
complicated), I've been able to establish the following relationships 
between the "Lens" parameter (lens), horizontal FOV (FOVx) for the 
Blender camera, and the image width (w) and height (h):
    lens = (32/2) / ( tan( FOVx / 2 ) ),  for w > h
    lens = (32/2) * (w/h) / ( tan( FOVx / 2 ) ),  for w < h

Now, I've done some research on 35mm film.  My reading tells me that 
35mm is the vertical height of the film, /including/ sprockets.  The 
actual exposed area of 35mm film is supposedly 36mm x 24mm.  Using this 
fact, I get the following expression for FOVx in terms of focal length 
(L) for 35mm film:
    FOVx = 2 * atan( (36/2) / L )

Notice the factor of (36/2)?  That looks similar to the factor of (32/2) 
doesn't it?  In fact, if these factors were equal then the Blender 
"lens" parameter would correspond exactly to the lens of a 35mm camera.

Now, the big question is: does anyone know why Blender uses a factor of 
(32/2) and not (36/2)? :-)  I'm very interested to get to the bottom of 
this, since information on both film sizes and Blender's lens parameter 
both seem to be pieces of closely-guarded black magic! :-)

Jonathan Merritt.