[Sidefx-houdini-list] aperture focal length relations

Rangi Sutton rangi at kanuka.com.au
Wed Jun 20 22:41:58 EDT 2007

Mario Marengo wrote:
> 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).
If you're no expert then I'm afraid the rest of us are total ignoramus. 
I think you've added a page to more than a few wikis around the world 
just then... ta.


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