FOV on VRay physical Cam doesn't make sense to me

[mts]

Hey there,
I got a question concerning the Vray physical Cam. I have been working on some still images now where I had to insert 3d graphics into a background plate using mental ray.
Now I would like to switch to VRay using the physical Cam and I am surprised that the vray cam's FOV does not match the former maya cam's Angle of View although the settings are the same (DSLR with a full frame sensor of 36mmx24mm):

Maya Cam for rendering with mental ray:
Focal Length 16.2
Camera Aperture 1.417 0.945

This results in an Angle of View of 96.02 which should be right according to the formula for calculating the angle of view: 2* arctan (film width /(2*focal length))
(provided that the value maya states is the horizontal angle of view)

Vray Physical Cam:
VRay Camera Physical Focal Length: 16.2
Vray Camera Physical Film Width: 36

This results in an VRay Camera Physical FOV of 73.821 and I can clearly see that the rendered images differ.

Can anyone tell me where I am doing something wrong?
Thanks a lot!

[vlado]

What units is your scene in? (I.e. what would be 36mm in your scene?)

Best regards,
Vlado

[mts]

Thanks, Vlado - my scene units are set to cm. When I set my physical Cam's film gate to 3.6 instead of 36 and the focal length to 1.62 instead of 16.2 the result at least looks better and the FOV is 94.196.
Anyhow, it's still not the same - can you tell me why that is?

Furthermore I am wondering why it says "Film gate (in mm)" and "Focal length (in mm)" in my physical Cam's Attribute Editor when I actually have to not use milimeters but my scene units....??

Regards,
Matthias

[vlado]

Hm, no idea what's wrong in your case, but it seems to work fine here:

(*) Film gate of 36 mm and focal length of 16.2 mm give a FOV of 96.021 degrees with the V-Ray physical camera. Keep in mind that the formula that you quoted is only valid if the camera is focused at infinity; to simulate this I entered a very high value like 20000 for the focus distance.

(*) Focal length of 16.2 mm and Camera aperture of 1.417 inches give FOV of 96.03 (keep in mind that if you change the Focal Length, the Angle of View does not update to reflect the new value until you deselect and select the camera again, at least in Maya 2011).

Best regards,
Vlado