[Radiance-general] Focal Length

[Marcus Jacobs]

Dear Group

I have a question concerning the simulation of varous cameras in Radiance.

>From the book, "Rendering with Radiance", in order to simulate a particular 
focal length on 35mm photography, the following formulas should be emplyed:

h35 = 114.6*atan(17/f)
v35 = 114.6*atan(11.5/f)

where h35 and v35 are the horizonal and vertical view angle respectively (in 
the book, they are actually switched but I think this is an error).

Earlier this year, someone sent a question to this group about the same 
subject and Greg responded with the following:

"Focal length is specified with the -vh and -vv options of rpict, which
also controls the image aspect ratio.  Knowing one from the other
requires a specific image (film) size and format.  For 35mm film, the
formula is:

focal_length = 35/2 / tan(vv_angle/2)

Where vv_angle is the vertical view angle for a landscape picture.  The
horizontal angle is then given by:

hh_angle = 2 * atan(vv_angle/2 * 3/2)

To check your calcs, a 50mm lens in 35mm photography is equivalent to a
Radiance view specification of:

-vh 49.6 -vv 35.3"

What I have found is that these are different formulas that produce 
different results. Could someone tell me which is correct? Also, could 
someone give a formula(s) for producing views in other film formats (i.e. 6 
x 7, 6 x 4.5, etc.).

Thanks

Marcus

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