[Sidefx-houdini-list] aperture focal length relations

Adrian Lee dryrainstudios at gmail.com
Wed Jun 20 13:10:06 EDT 2007


thanks Mario. We have been playing with those equations most of the day with
the houdini defaults but we were using standard 35 mm film measurements and
not matching

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

i have created a unit box, moved it off the origin by 0.5
if I then translate the box 2.1 units in -Z and then move the aperture i am
getting a value of 10

if aperture is closer to gate size/sensor size the figure should be closer
to the "plate" which is a 1 inch square ccd 25.4mm?

ed


On 20/06/07, Mario Marengo <mario at axyzfx.com> wrote:
>
> On June 20, 2007 08:29 am, Adrian Lee wrote:
> > Hello back again and still trying to match move Houdini's cameras.
> >
> > Does anyone know the relationship of aperture to the focal length in
>
> Open up the on-line help and search for "world camera". Near the top of
> the
> results should be a document called "Match Houdini camera lenses to the
> real
> world". This has all the relevant information regarding how focal length
> and
> aperture are related in Houdini. Like Jeff, I also think Houdini's
> "aperture"
> is closer to "film aperture" -- geometrically: a 1-unit-wide object placed
> focal_length units in front of the camera will project to an aperture-wide
> object on the image plane (i.e: the entire with of the image on the
> "film").
>
> The page gives the basic relationships, from which you can derive anything
> you
> want. Here are some more relations (BTW: another misnomer in the following
> is
> "fov", or "field of view", which should really be "aov" or "angle of
> view"):
>
> fovx = 2 * atan(apy/(2*focal))
>
> fovy = 2 * atan(apx/(2*focal))
>
> apx = 2 * focal * tan(fovx/2)
> or
> apx = (aspect * apy * resx) / resy
>
> apy = (apx * resy) / (aspect * resx)
>
> ...
>
>
> HTH.
>
> --
> ......................................................................
> Mario Marengo                                  mailto:mario at axyzfx.com
> AXYZ ANIMATION                                 http://www.axyzfx.com
> ......................................................................
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