From: Gordon J. Holtslander (holtsg@duke.usask.ca)
Date: 01/12/03-10:15:05 PM Z
Have you tried to measure any of this empirically?
Can you set up some kind of camera obscura to let you measure the image
cicle? If you were to mount the lens so that it would project onto a
vertical card on some sort of stand in a dim room - you could move the
card until an image was focused - and measure its postition, and then
measure the diameter.
A friend of mine did this to determine the image circle of a very large
format lens. He covered a window with a curtain and a card with a circle
cut to accept the lens - turned his living room into a camera obscura.
Took the measurements with this set up.
Gord
On Sat, 11 Jan 2003, Michael Healy wrote:
> 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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Gordon J. Holtslander Dept. of Biology
holtsg@duke.usask.ca 112 Science Place
http://duke.usask.ca/~holtsg University of Saskatchewan
Tel (306) 966-4433 Saskatoon, Saskatchewan
Fax (306) 966-4461 Canada S7N 5E2
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