[Bf-committers] Blender "lens" - Historic questions?

Robert Wenzlaff (AB8TD) rwenzlaff at soylent-green.com
Sun Nov 6 02:09:54 CET 2005

What you are missing is that the Blender camera is based on a 35mm movie 
camera, not a 35mm SLR (still) camera.

While 35mm movie film is the same physical film format as 35mm still film*, 
the film goes through a movie camera sideways.  This mean for a 35mm movie 
camera the WIDTH of the film, including sprockets, is 35mm.  The fact that 
the sprokets come out of the width, not the height probably has something to 
do with the different ratios used.

This is why a Normal lens on a SLR camera is 50mm, and the Normal lens on a 
movie camera (as Blender shows) is 35mm.

*There's an outfit in Seatle, WA that loads 35mm SLR cans with movie film and 
sells it really cheap to photo buffs.  They sell it cheap because since it's 
movie film you can't get it processed at the local deptartment store, and 
have to send it back them to do it (and the price of processing includes more 
film so it's a never ending cycle). 

On Saturday 05 November 2005 09:10, Jonathan Merritt wrote:
> Hi Everyone,
> For basic renderings (without "parts" parameters or anything else
> complicated), I've been able to establish the following relationships
> between the "Lens" parameter (lens), horizontal FOV (FOVx) for the
> Blender camera, and the image width (w) and height (h):
>     lens = (32/2) / ( tan( FOVx / 2 ) ),  for w > h
>     lens = (32/2) * (w/h) / ( tan( FOVx / 2 ) ),  for w < h
> Now, I've done some research on 35mm film.  My reading tells me that
> 35mm is the vertical height of the film, /including/ sprockets.  The
> actual exposed area of 35mm film is supposedly 36mm x 24mm.  Using this
> fact, I get the following expression for FOVx in terms of focal length
> (L) for 35mm film:
>     FOVx = 2 * atan( (36/2) / L )
> Notice the factor of (36/2)?  That looks similar to the factor of (32/2)
> doesn't it?  In fact, if these factors were equal then the Blender
> "lens" parameter would correspond exactly to the lens of a 35mm camera.
> Now, the big question is: does anyone know why Blender uses a factor of
> (32/2) and not (36/2)? :-)  I'm very interested to get to the bottom of
> this, since information on both film sizes and Blender's lens parameter
> both seem to be pieces of closely-guarded black magic! :-)
> Jonathan Merritt.
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  Catapultam habeo. Nisi pecuniam omnem mihi 
  dabis, ad caput tuum saxum immane mittam.
Robert Wenzlaff    rwenzlaff at soylent-green.com

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