A previous thread about lens length: Lens length for rendering
The Rhino “lens length” is based on the focal length a lens used with standard 35mm film/sensor size of 24 mm x 36 mm which provides the same field of view. For view ports with other than a 3:2 aspect ratio I don’t know if the Rhino lens length is based on view port width, height, diagonal, square root of area or something else…
Equivalent full frame 35mm" is currently, the de-facto standard in photography for comparing focal lengths of different cameras with different sensor sizes. Point and shoot cameras are frequently advertised and even marked with the equivalent 35mm format focal length rather than the actual focal length. Technically, field of view angle would be better because it is independent of format/sensor size but it very rarely used.