I have you to thank for learning this critical fact here on the forums. Once I got that, that set me on the path to figure out the rest.
-
What you can rely on is this: If you type in an angle for the vertical angle of view, you get exactly what you asked, no problem. For example, if you type 37.8° with the Zoom tool, you get the view of a 35mm lens. You can resize the window and the vertical part of the image holds while the horizontal part floats showing you either more or (in the case of the iPad) less of the horizontal field of view depending on the proportions of the window (iPads are generally 4:3 so it’s cut off).
-
If you type in a focal length, like 35mm, what SketchUp does is convert that to an angle of view and then enters that for you for the value. The bug is this: if you type 35mm, what SU comes up with is 54.4° which is the correct value for the horizontal angle of view, but that’s not what SU actually uses; it uses the vertical one. As a result, what SU reports for a focal length on horizontal images is always off by a factor of 1.5x because the aspect ratio of 35mm pictures is 3:2. The exception is a vertical image because the wider “horizontal” angle of view coincides with SU’s vertical angle of view.
During the party at 3D Basecamp, I caught @thomthom’s ear, and he seemed to be aware of this already. He pointed out if you use his Safe Frames plugin, then it will report the correct focal length.
Here for reference is my 3D Basecamp handout:
SketchUp AoV Ref Table v1,0.pdf (500 KB)
I keep a printout of this table on my desk for reference all the time.