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

Wed, 12 Nov 2003 18:00:25 -0000

Ah!
Thanks Greg, I thought the question was familiar when I asked it!
Rich

____________________________
Richard Gillibrand
Department of Computer Science,
University of Bristol,
Tel: 0117 9545256
____________________________

-----Original Message-----
From: Greg Ward [mailto:[email protected]] 
Sent: 12 November 2003 16:45
To: [email protected]
Cc: R Gillibrand
Subject: Re: Getting the focal length of a Radiance camera

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.  This is obvious because any 
given Radiance view may not have the correct image aspect ratio to 
match 35mm (or any other format) film.  You have what is known in math 
as an "overdetermined problem."  You can choose to ignore this and 
simply average the results from these two formulas, or pick the larger 
or smaller of the two, depending on whether you wanted to fill the 
frame completely or keep the image with black borders (i.e., 
"letterbox" format).

-Greg

> From: R Gillibrand <[email protected]>
> Date: Wed Nov 12, 2003  4:10:34  AM US/Pacific
>
> Hi all,
>
> Hopefully this is a simple question:
>
> Is it possible to get the focal length of a camera view from within 
> Radiance?
>
> I know that the view angle specification makes this unnecessary as an 
> input parameter but was wondering if it is possible to reconstruct the

> physical camera properties from only a Radiance view description.
>
> Cheers
>
> Rich