Lens Breathing on Maya Cameras

[nathan_thompson1]

Hello Vray Community,


We at Realtime UK are currently in the process of matching cameras between DCCs and renderers. Doing so we stumbled upon an issue with how Vray handles FOV calculations.
As the title suggests this is about lens breathing which Vray seems to have baked in without a way of turning it off.

Small recap: Lens breathing is the FOV change when you change the focal point. It is an artifact of real cameras because of how lenses shift internally. It is most pronounced when the focal point is very close to camera.

The problem is that not all DCCs or even all renderers have that feature.
On the 3Ds Max Physical Camera (which as I understand was developed together with Vray) there is an option called "lens breathing" that you can set to 0 to turn it off.
In Maya there doesn't seem to be an attribute that does that.

Now there is the option to specify the FOV instead of focal length + aperture.
However, at least in Maya, the internal focal length value is then adjusted to match the FOV at a given focal distance including lens breathing. Our tests have shown that this changes the DOF strength which does make sense from a "Realism" point of view but is quite annoying when you want to match renders.


Is there a way to turn off lens breathing on maya cameras?
It would make sense to at least have control over the amount, because afaik the amount of lens breathing depends on the actual lens being used. With some of them having less

If not is there maybe a way to fake the vray lens breathing in other software?


Any help or idea would be greatly appreciated

[matthew999]

I have also been burnt by this recently. Changing focal distance changes fov slightly and renders no longer line up. Would be great to have a tick box to turn this 'feature' off

[nikolay.kusht]

The V-Ray Physical camera doesn't have a "lens breathing" parameter, because the effect is turned off if "Specify FOV" is enabled.

Also, if "Specify FOV" is disabled, then the lens breathing is always 1.0 because this is what the thin lens equations give naturally.

In short, to have "lens breathing", just turn off the "Specify FOV" option.

[matthew999]

I think my problem is that the specify focus distance defaults to 200 or something. So I render with no dof and car is in good position. If I tick render with dof and change the focus distance to desired point then the position of car changes well it's position is the same but the fov changes. So I have to manually adjust the lens fov/focal to match with the non dof render. Is this expected behaviour. I guess a work around would be to specify the focus distance even if not rendering with dof. So if I choose to render with dof at a later date it doesn't change.

[nathan_thompson1]

The V-Ray Physical camera doesn't have a "lens breathing" parameter, because the effect is turned off if "Specify FOV" is enabled.

Also, if "Specify FOV" is disabled, then the lens breathing is always 1.0 because this is what the thin lens equations give naturally.

In short, to have "lens breathing", just turn off the "Specify FOV" option.

That is good to know. I wasn't aware of the fact that lens breathing is disabled when the FOV is specified (to be exact in can either be "From Maya Camera" or "Specify")
The reason why I thought it was still on is because the (greyed out) "Focal length (in mm)" field is changing still when changing the focal distance.
This would have an effect on the dof strength, which is what I observed when comparing vray renders between max and maya.

Please see the following image.
Those are 4 renders of the same scene next to each other.
FOV is set to fixed 60
fstop is 8
film width is 25.4

As can be clearly seen, the max camera with lens breathing turned to 0 has a stronger dof effect than all other cameras
hence my conclusion that vray has lens breathing baked in somehow.
However it is now entirely possible that the max physical camera is at fault here