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Anniversary Speed Focus Scale

 
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45PSS



Joined: 28 Sep 2001
Posts: 3212
Location: Mid Peninsula, Ca.

PostPosted: Fri Mar 09, 2012 5:20 am    Post subject: Anniversary Speed Focus Scale Reply with quote

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.
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essessem



Joined: 19 Feb 2003
Posts: 48
Location: California

PostPosted: Mon Mar 12, 2012 7:02 pm    Post subject: Anniversary Speed Focus Scale Reply with quote

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.
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William Hallett



Joined: 07 Jan 2012
Posts: 20

PostPosted: Tue Mar 13, 2012 11:54 pm    Post subject: Reply with quote

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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