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[Dave]

Graflexers, this is optical geekery, so stop reading now if that stuff turns you off. It may be a solution in search of a problem, but I wanted to bring it out for discussion.

Most of us who have Graphics have got a rangefinder coupled to a lens, and to the focusing scale on the bed. Some people use a second focusing scale when they exchange lenses: they read the subject distance off the main focusing scale that is coupled to the rangefinder, and then transfer this reading to the second scale. This means they're using their Kalart as an uncoupled rangefinder for that second lens, which is often a good approximation.

In my case, I have been using two lenses recently: my usual 105 Symmar and my 'new' 150 Symmar. The 105 is coupled to my rangefinder and my focusing scale. I don't have a scale for the 150, and I suppose I could just make one out of paper and glue it on. Instead I asked: if my rangefinder has my subject in focus, and I read a distance d1 off the (105mm-coupled) focusing scale, what distance d2 will be indicated on the scale when my 150 is in focus?

This sounds more complex than it is, so I'll give you an example. Suppose I have the 150 on my Century, on its 150mm infinity stops. I use the rangefinder to sight in on a subject, and the scale says the subject is 5m (d1) away. What reading d2 should I extend the scale to, in order that I have the right bellows extension for the 150mm lens?

It turns out I should rack it out to read 2.48m on the scale. In other words, with a 150, I have to adjust the scale to half the 'true' measured distance. (The actual ratio is x0.496.) You can show the argument using standard optical formulas, which I won't bother with unless anybody wants me to post it.

What surprised me is that this ratio is nearly constant over a broad range of subject distances:
100m: 0.490
30m: 0.491
10m: 0.493
3m: 0.501
Within the range of error of scale focusing, I should be OK dialing in 0.50x the measured distance, at least for small apertures and normal subject distances. I have to concede I haven't tested it on film yet, but it looks reasonable on the ground glass.

Does this ratio have a name? I've just been thinking of it as the 'scale focusing factor' in my notes, but something like this was probably well known to the old Graphic photographers.

You can do similar calculations for any coupled focusing scale and any uncoupled lens. Using the same focusing scale, this ratio is 0.35 for a 180mm lens, and 2.6 for a 65mm lens. Incidentally, I suppose this number is somehow related to the geometry of interchangeable focusing cams.

[ This Message was edited by: Dave on 2004-06-03 18:45 ]

[t.r.sanford]

Your method is interesting. For your purpose of using a rangefinder adjusted for a 105mm. lens to determine distances for a 150mm. lens, it looks as though it will work very well. For best results, one needs a distance scale in which multiples occur frequently. Your scale appears to be marked in meters, and may have this characteristic.

Using the typical U.S. scale, in feet, I calculate the intervals to be

F.L.: (105 mm.) (150 mm.)
Distance Travel Travel
4 feet = 11.05 mm. 25.17 mm.
6 feet = 6.84 mm. 14.86 mm.
8 feet = 4.96 mm. 10.57 mm.
10 feet = 3.89 mm. 8.21 mm.
15 feet = 2.53 mm. 5.27 mm.
25 feet = 1.49 mm. 3.08 mm.
50 feet = 0.73 mm. 1.51 mm.
100 feet = 0.36 mm. 0.75 mm.

One would expect the relationships between successive intervals to be the same for a lens of any focal length.

This has led me to speculate about the possibility of devising a sort of cam-cutting machine that would use a very large, accurately-made pattern, driven by a leadscrew. It would travel in contact with one end of a pantograph, the other end of which would push a "Dremel Moto-Tool" (or something of that sort) against the cam-sized workpiece. The workpiece would be driven by a geared leadscrew, with the gearing (and perhaps the pitch) changed to accommodate the specific focal length.

I venture to guess that a 150mm. lens on a 2¼x3¼ camera produces a very pleasing perspective. I always wanted a 65mm. auto diaphragm lens for my "Exakta" outfit, but unfortunately, nobody ever made one!

[Dave]

Thank you, t.r. After I saw no replies for a while, I wondered if I had posted something too arcane for anyone to be interested in.

My focusing scale is in feet, like yours. In the original post, I converted the subject distances to meters to make the numbers consistent. What might be useful sometimes would be an ordinary millimeter vernier on the bed, like on the mechanical stage of a microscope, but that would clutter things up a bit.

You're right that the 150 is a nice focal length on the small camera-- it's a nice perspective that seems 'right' to me personally. The downside is it needs a lot of bellows extension. I know I could simply enlarge the image from the 105 by an extra 50%, but I find it easier to compose in the camera.

[Dave]

OK, one final note on this obscure point, just in case this is ever of use to anyone.

You can show that for great distances, this factor is equal to the square of fS/fL, where fS is the focal length that applies to the focusing scale, and fL is the focal length of the lens.

The bottom line is if you have any rangefinder-coupled focusing scale on your Graphic, you can use this method to focus any lens.