Quote Originally Posted by Gary892
First off, thank you for the quick replys.
I don't mean to beat a dead horse but, how can one determine if the 450mm Nikkor M will cover a 12x20 by just looking at the lens? I am sure there is a tech sheet that would come with the lens but when trying to buy one and looking at them the tech sheet is not always available. I did some more research and found that to cover a 12x20 neg I would need a 180 degree coverage at minumum.
Any other thoughts.
Since you need a 590mm circle of illumination for 12X20 you determine the minimum focal length required to cover 12x20 using the following formula. To use this formula you must know the angle of coverage of the lens.
FL = (590 / 2) * (1 / tan(coverage_angle / 2))*
which is derived from the simple relationship:
tan(coverage_angle / 2) = (image_circle / 2) / FL

Lens specifications are mostly useless in that they are not made with contact printing in mind and often don't indicate as much coverage as the lens will really provide. A case in point would be the 450mm Nikkor M you mentioned. As I recall the angle of coverage given for this lens is about 65 degrees and if you do the calculations above based on 65 degrees the coverage indicated will not be nearly enough for 12X20, but it is a well known fact that the Nikkor 450mm covers 12X20 with many inches of movement.

The only modern lens of 210mm that will cover 12X20 is the 210 SSXL and its angle of coverage is something around 120 degrees.

Sandy King