[Radiance-general] Re: Getting the focal length of a Radiance camera

[Georg Mischler]

Greg Ward wrote:

> Hi Rich,
>
> Marcus Jacobs asked a similar question about focal lengths last month
> on this group (Oct. 19th, in fact).
>
> For a 35mm frame (which measures 36x24mm on the actual exposure), the
> equation for the horizontal and vertical view angles (in degrees) is:
>
> 	h = 2*180/PI*atan(36/2/f)
> 	v = 2*180/PI*atan(24/2/f)
>
> To derive the focal length from the view angle, you just invert these:
>
> 	f = 36/2/tan(PI/180/2*h)
> 	f = 24/2/tan(PI/180/2*v)
>
> As you can see, there are two formulas above, which means that you can
> derive the focal length from either the horizontal or the vertical view
> angles, but they won't necessarily agree.

Actually, the focal length is determined by the largest
dimension of the picture, the diagonal, which makes it
somewhat independent of the aspect ratio.
The normally used formula for 35mm film pictures
therefore goes like this:

  f = 43.27/2/tan(PI/180/2*h)


-schorsch

-- 
Georg Mischler  --  simulations developer  --  schorsch at schorsch com
+schorsch.com+  --  lighting design tools  --  http://www.schorsch.com/