Originally Posted by

**Dan Fromm**
Russ, stop and think for a minute.

A lens projects a cone of rays. The relationship between the cone's angle, the lens' focal length and the diameter of the circle covered (the circle is the intersection of the cone with a plane perpendicular to the lens' axis and one focal length behind the lens' rear node) is, in pidgin Excel: circle's diameter = focal length*2*TAN(RADIANS(angle/2)). We say that a lens covers a format when the circle covered is no smaller than the format's diagonal.

The cone's angle is the angle mentioned in lens makers' propaganda. So is the circle covered. Got that?

Now, LF lenses typically cover circles larger than the formats they're used on. Otherwise, no decentering movements.

An 8x10 sheet of film's diagonal is approximately 305 mm. Agree or disagree?

Rewrite the magic formula I gave above, putting the angle on the left of the equal sign and everything else on the right. In pidgin Excel, the angle =2*DEGREES(ATAN((circle's diameter/2)/focal length))

Now set the circle's diameter to 305 mm and the focal length to 210 mm. That gets the angle the film sees. 72 degrees. Do the same with focal length 250 mm. 62.8 degrees.

If you were right, lenses with the same coverage and different focal lengths would produce the same image with the same camera setup. Do you really believe this? If you do, I have a bridge that you should buy.

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