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45PSS
Joined: 28 Sep 2001 Posts: 4081 Location: Mid Peninsula, Ca.
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Posted: Fri Mar 09, 2012 5:20 am Post subject: Anniversary Speed Focus Scale |
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What focal length lens is a focus scale, ID # 24784, for?
Scale is on a late 1942 Anniversary Speed Graphic (all black), original lens long gone. Closest focus distance on scale is 9 feet. _________________ The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU. |
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essessem
Joined: 19 Feb 2003 Posts: 48 Location: California
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Posted: Mon Mar 12, 2012 7:02 pm Post subject: Anniversary Speed Focus Scale |
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Given that my Super Speed with 203mm Ektar only registers down to 8' on the focus scale, my guess is that it's for a 210mm or so. _________________ Speak softly and carry a big camera. |
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William Hallett
Joined: 07 Jan 2012 Posts: 93
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Posted: Tue Mar 13, 2012 11:54 pm Post subject: |
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You can work it out using basic optics. The distance between the infinity mark and (say) the 9 ft mark on the scale is the difference between the image distance xI for (say) 9 ft and the image distance at infinity, which is the focal length f:
distance on scale = xI - f
The image distance is given by the lensmaker's equation
1/xI = 1/f - 1/xO
where xO is the object distance (9 ft in this example). These distances are measured to the lens; strictly speaking, the object distance is measured to the film plane, so the object distance that corresponds to the numbers on the scale is
xO' = xO + xI
which is very nearly the same as
xO' = xO + f
The last equation allows a quick and simple solution for the scale distance, whereas the previous one requires solving a quadratic.
In theory, the lensmaker's equation is for a simple thin lens only; in practice, I have made up a number of focussing scales for different lenses this way and found them very accurate when tested. |
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