Posted by Bob Stoddard on October 19, 2000 at 14:07:29:
In Reply to: Re: 80mm W.F. Ektar posted by Les on October 16, 2000 at 21:26:27:
: : Thanks, Les. I wonder if the data isn't really
: for a small stop, even though it's stated to be
: for maximum aperture. An image circle of 152mm
: represents about 87 degrees of angular coverage,
: larger than any of the other WF Ektars... RKS
: Today lens companies give the angle of coverage of
: the image circle. Back then kodak gave the angle
: of coverage as two: 55°x68° (vertical x horizontal).
: If the image circle covers this rectangle, then the
: diagonal of the rectangle must be angle of coverage
: of the image circle. So A squared + Bsquared=c
: squared
: That's how I came up with 87°. Now if you draw an
: x with the lens at the vertex then the top and
: bottom angle is 87°. It's 80mm from the vertex to
: the film plane. If you draw a perpendicular line
: from film plane to the lens you will end up with a
: right triangle with a 43.5° angle at the top (the
: line bisected the angle of coverage) and one
: known side of 80mm.
:
: Machinery's Handbook says you can find the opposite dide (or half the image circle diameter) with the formula b= tanB*80mm. for the diameter multiply by 2.
: Now exactly what Kodak meant by 'maximum aperture'
: does have me confused. Why would anybody want to
: know this info at wide open?
Les, I calculated the size of the rectangular field which corresponds to 55 x 68 degree coverage with an 80mm lens-- it is 86.3 by 107.9 millimeters, with a diagonal length of 136.3mm, just about 80 degrees of coverage. Even so, I think the lens would have to be stopped down considerably to achieve good performance near the edges. RKS