[Sidefx-houdini-list] aperture focal length relations

Mario Marengo mario at axyzfx.com
Wed Jun 20 14:10:00 EDT 2007


On June 20, 2007 01:10 pm, Adrian Lee wrote:
> trying the aperture thing out and with the values of:
>
> resx 1024
> resy 1024
> ratio 1
> houdini units 1 = 10mm
> and a given focal length of a film lens of 21mm

A 21mm prime lens (and 35mm film) has a horizontal angle of view of about 
81.47 deg. (based on interpolation between 20mm and 24mm Canon lenses). So, 
assuming fovx=81.47 is somewhat in the neighborhood, then you can set 
Houdini's camera as follows:

focal = 21
aperture = 2*ch(focal)*tan(81.47/2) = 36.1704

Now create an object with a unit grid on the XY plane and parent it to the 
camera. Translate it by -21 units in Z, and scale it by 36.1704 (the 
aperture). It should fill your 1024x1024:1 frame exactly.
However, whether this setup matches your plate depends on how accurate the 
(assumed) horizontal aov of 81.47 deg. is and on whether you're looking at a 
full-aperture image or at some kind of cropping.... things definitely can get 
messy with all this stuff (and I'm certainly no expert on any of it).

HTH.

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