When you assign a lens to a Rhino viewport using ViewportProperties, the resulting image you see is what that lens looks like on a standard SLR 36mm x 24mm film gate camera. If the viewport is not in a standard 1.5:1 aspect ratio, the minimum dimension is always 24mm. Calculate the major dimension by multiplying the minimum by the aspect ratio.
Lens length is the distance from the focal node to the film plane. So with a 50mm lens, the focal node of the lens is, you guessed it, 50mm from the film plane. So with our standard Rhino viewport (and a SLR camera) the resulting view angle in plan view would be 39.6 degrees:
Now, if we pop that same 50mm lens onto a motion picture camera, the film plane size changes to 24mm (this format is called Super35):
So although you are keeping the same lens length, by changing your film size, you change your field of view.
There is now a script to do the heavy lifting for you: http://en.wiki.mcneel.com/content/upload/files/Cam_Lens_090120.zip
For the nitty gritty of what the script is doing, read below:
The first thing you need to do is to figure out the aspect ratio of your viewport. The easiest way to do this is to use the command ViewportProperties with a “-” in front of it. From there you can select the Size option. The width divided by the height will be your aspect ratio, so a 2400 wide by 1000 high viewport would be 2.4:1, or for a viewport 300 wide by 500 tall, the aspect ratio will be 1:1.67
Now that you know your aspect ratio, you can figure out the film size that Rhino is showing you. Using our 1:1.67 aspect ratio, we know that the minimum aspect dimension in a Rhino viewport is always going to be 24mm, so the film size Rhino is showing me in a 1:1.67 viewport is 24mm by (24 * 1.67), or 24mm x 40mm.
So if you were trying to see what your view angle would be with a camera with a 3“ x 5” film size (76mm x 127mm), you need to figure out what your crop multiplier is, the difference between Rhino's film plane size and the film plane size that you want. The film size I want is 127mm. Rhino is showing me 40mm. 40mm/127mm = 0.315. This is your crop multiplier. So if you wanted to see what a 150mm lens on a 3“ x 5” camera looks like in Rhino, you have to specify a 47.25mm lens in Rhino's ViewportProperties.
Another example: You want to know what a 21mm lens looks like on a Super35 camera in 2.40:1 aspect.
Call ViewportProperties, make your viewport 720 x 300 (2.4:1). Again, we know that the minimum Rhino aspect film size will be 24mm. So with my minimum dimension at 24mm, my maximum = 2.4 * 24mm = 57.6mm. To get your crop angle, it's (what Rhino gives you)/(what you want) Super35 film width is 24mm, so 57.6mm/24mm = 2.4. So now I know my crop multiplier, so if I want to see what a 21mm looks like, I have to put in a (21mm * 2.4) = 50.4mm lens.