Up to today, I had always used the multiple of 0.83 times the diagonal field of view to attempt to know what the horizontal field of view was. My calculations were based upon the Pythagorium theorem, which is a about the sum of squares of the two sides is equal to the square of the hypotenuse.
Wrong...this is not the way to do it. It involves a calculation using tangents, angles, etc. This came to light when I was looking at the Zeiss Distagon T* 2,8/15 and noted the diagonal FOV is 110°, the horizontal FOV is 100°, and the vertical FOV is 76°. And, these are not the numbers I got using the method I had been using.
I emailed Zeiss. I think I have gained such respect for the customer service I may have to get this lens. Within a few hours, I had a very nice explanation of how this is calculated, along with nice diagrams. Unfortunately I cannot figure out how to draw the image, but the formula is
tan (alpha/2) =18mm/15mm and alpha comes to 100.4° for the full frame 35mm sensor. Note, alpha is the angle, 18mm is half of the horizontal plane, and 15mm is the focal length of the lens. So, if anyone wants to do this calculation, which I like as Nikon does not seem to publish the horizontal FOV, the easy way is via a link and I am placing this here;http://www.isotton.com/misc/lens-angle-calculator/
I think some of us may find this of interest, especially for the architectural and landscape shooters. And, Zeiss has gained so much respect from me...well...