----internet says:----
A "normal" lens produces a field of view that is similar to human vision. The diagonal of the image plane is typically used as the reference for field of view determination. A "normal" lens has a focal length that is approximately equal to the diagonal measurement of the image plane. When the focal length is equal to the diagonal measurement it produces a diagonal field of view of approximately 53° which is similar to human vision.
---------------------

But when we shoot widescreen then we can't use the diagonal of the image plane.

so how much of the diagonal image plane of a widescreen has to be the focal length to produce a field of view that is similar to human vision?

Why not use this calculator (http://www.panoguide.com/reference/lens_calculator.html) and run some numbers. As an example, the DV953 has a full wide focal length of 2.85 mm and a frame size of 18 x 24 (6 mm diagonal). An anamorphic lens is ~.67x or it produces a focal length of .67X2.85 = 1.91 mm. Run those numbers and see what you get. I believe the diagonal of the image plane stays at 6 mm because of the horizontal squeeze of the camera.

hmm with dry math, if I didn't get something wrong, I got that the focal length should be
sqrt(255/337) or about 0.8697 of the diagonal of the image plane of the widescreen


.. nice calculator! =) I'm checking out right now

I was looking for general solution....
but I guess for GS400 it won't matter much, and if there's minimal to no cropping the result will be more or less the given specs..

using the calc, I got that the GS400 has 1.8 x 2.6 mm active video matrix for TELE or this should translate to 1.8x 3.2 (assuming no or insignificant cropping) for WIDE.

this means that while widescreen recording we have range
from: 30.5 vertical and 51.73 horizontal FOV (about 60 FOV diagonal, and 60x0.8697 = about 52-something (using the new found formula) which is about "normal" / and checking the TELE.. the diagonal is 52.8)
to: 2.6 vertical and 4.63 horizontal degrees FOV