From: Michael Healy (mjhealy@kcnet.com)
Date: 01/11/03-12:55:06 PM Z
I posted this inquiry to Pure Silver this AM, but thought there might be
some stray wonks here that don't find their way over to that neck of the
woods.
I am working with Alan Greene's "Primitive Photography" in designing a 7x17
folding camera. Trying to, anyhow. I find myself stuck on his discussion of
the way to determine the angle of view of a lens. Is there someone here who
might be able to lend me a
bit of expertise on this? I know this is going to sound a bit esoteric. It's
just that my focal length determines all sorts of other measures that figure
in the conversion of his 10x12 blueprint to 7x17.
I will quote Greene at some length (all from pp. 88-89). He is using
Clarence Woodman's table. "Finding the angle of view for a given len and
format combination is very easy," he says. "The following method,
established by Clarence Woodman, is recommended:
1. Determine the diagonal of the format... For example, the diagonal of a
210mm x 270mm rectangle is 342mm.
2. Determine the quotient of the lens and format being used by dividing the
lens focal length into the diagonal. For example, with a 250mm focal length
lens used with the 342mm format diagonal, 342 is divided by 25 to arrive at
0.8047.
3. Taking the quotient from step 2, refer to Table 3.1 to determine the
angle of view for the lens. For example, using the quotient 0.8047 from step
2, a 400mm lens used with a 210 x 270mm format negative is found to have an
angle of view measuring approximately 44 degrees. <end quote>
I have two problems with this. By my calculations, it would seem that Greene
means to divide diag 342mm by foc 250mm, **not** 25. IE, it looks to me like
the book has a typo. However, 342 divided by 250 is not 0.0847, either, it's
1.368, which (in Woodman) doesn't come to an angle of 44 degrees, but an
angle of 68 degrees. This (I think) sounds plausible in this example, since
a 250mm lens on 210x270 (8.5x10.5???) would be somewhat wide (wouldn't it?).
But I do NOT see how his arithmatic comes even close to reaching 0.8047 (an
angle of 44 degrees).
My next puzzler is how Greene manages to get from step 2 to step 3. Suddenly
he's on another example, a long lens; but instead of recalculating, he's
applying the 250mm lens's (miscalculated?) angle of view to a 400mm lens. I
don't see how this follows from his contention that the angle of view is
"FOR a given lens and format combination".
Anyone able to shed light on this? Am I just missing something because it's
the size of a barndoor? Oh, one more thing, case anyone on this list should
happen -- ahem -- to possess the largest library of photographic materials
in the history of the common man... Greene's Table 3.1 is titled "Clarence
Woodman's Table for determining the angle of view (found by dividing the
diagonal of the format by the focal length of the lens)". It is said to have
been adapted from Tho[ma]s Bolas and George E. Brown, "The Lens: a practical
guide to the choice, use, and testing of photographic objectives" (London:
Dawbarn and Ward, 1902), 28. I've been unsuccessful in finding Woodman on
the internet, or I might have tried independently to verify Greene's method.
Mike
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