Camera Focal Lengths in TG & LW don't match

[PabloMack]

Our group is planning on using TG for generating backgrounds while using LW for our main animation app. We will composite renders from the two packages together with live action. I have discovered that the camera focal lengths in the two apps don't correspond. LW comes up with a default of 24mm which is really wide angle while TG defaults to 31.1769mm focal length. So when I copy the TG default focal length to the LW camera settings and render both, the two do not have the same magnification. The LW render is more zoomed in than the TG render at any given focal length. If I adjust the LW camera to about 27~28mm then I get about the same perspective. Do any of you have a formula that will convert accurately between the two systems?

For many shots slight differences won't matter. But if we need to pan the camera then the two are going to drift or crawl relative to each other if they are not set to the same effective magnification.

[bigben]

Can you specify a FoV in LW by degrees rather than focal length? I use this measurement when stitching images and it works with pixel precision using the camera settings from TG for positioning the images in the stitching software. The applications could be using different criteria to calculate the equivalent focal length (e.g. circle to enclose diagonal vs horizontal fov)

[Matt]

Do you have the same film aperture (or film back, or sensor size) settings in both TG and LW? The field of view is a function of not only the focal length but also the aperture.

Terragen's default film aperture is 36mm x 24mm. Often you will work with a camera that has a smaller aperture than that. A smaller aperture results in a smaller FOV (i.e. higher zoom).

So the first thing to do is match the film back (or film aperture, or sensor size) settings between both cameras, as well as the focal length. Then there's another thing to consider. Terragen's camera defaults to "Horizontal FOV". If the aspect ratio of the film aperture is the same as the aspect ratio for the image, then it makes no difference whether you choose horizontal FOV or vertical FOV. But if the aspect ratios are different, Terragen has to make a choice about which dimension to use for its calculations. To avoid this ambiguity, it's a good idea to set the aspect ratio for the aperture to be the same as that for the image. Since you're matching an existing Lightwave camera, that might not be possible, so just make it the same as the Lightwave camera, and if it still doesn't match, then you might need to switch to vertical FOV mode.

Matt

[cyphyr]

Pf course if you have the Pro or animation version you can simply export a .chan file for the camera.
Not sure if you can export .fbx from Lightwave and a .mov file is specific to Maya (in this case)
All in all much easier ...

[PabloMack]

Can you specify a FoV in LW by degrees rather than focal length?

Yes you can. I used horizontal FOV on both and set both to 60°. Framing for the renders looked pretty identical. So I will stop using Focal Length and use FOV from now on. Looks like this problem is solved. Thanks all.