[Sidefx-houdini-list] aperture focal length relations

Mario Marengo mario at axyzfx.com
Thu Jun 21 14:02:16 EDT 2007


On June 21, 2007 12:35 pm, Adrian Lee wrote:
> for any one following this I found a nice little webpage that works out the
> angle of view for you
>
> http://www.mat.uc.pt/~rps/photos/angles.html

That's very interesting. I wish I knew what he means by "width" and 
"length"... the sizes of the exposed frame on film ("filmback")? And what the 
heck do the row headings mean (135, 120, APS, large)?

Anyway, if I knew more about what all those meant, we could be well on our way 
to having a generic "real-world" camera setup in Houdini, because the one 
thing that is pretty obvious is how they're arriving at those AOV values:

AOVh = deg( 2 * atan( length / (2*focal) ) ) // horizontal aov
AOVv = deg( 2 * atan( width / (2*focal) ) ) // vertical aov
AOVd = deg( 2 * atan( sqrt(width^2+length^2) // diagonal aov

where 'length' and 'width' are the names they give to those format-related 
quantities I'm not sure about, and deg() just converts from radians to 
degrees and could be replaced with 180/PI.

From the above, and assuming that the 'focal' parameter is set to the desired 
focal length (e.g: 21 [mm]), then the aperture value can be made to depend 
only on those mystery 'width' and 'length' values:

apx 	= 2 * focal * tan(fovx/2)
	= 2 * focal * tan(AOVh/2)
	= 2 * focal * tan( deg( 2 * atan( length / (2*focal) ) ) )

Then all you'd need would be a drop down menu with all those formats and a 
focal length (which is usually the *one* known thing from a shoot) and we're 
done...



-- 
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Mario Marengo                                  mailto:mario at axyzfx.com
AXYZ ANIMATION                                 http://www.axyzfx.com
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