A Nice Surprise and a Question

MikeK · Jan 16, 2008

My Surprise - Last night I found front and rear elements of a 210mm f9 Computar inside the bellows of an old process camera I picked up last year on Craig's List.

They a little dirty but cleaned up and look to be in pretty good condition. The elements screwed straight in to a Copal No.1 shutter. Put it up on my 8x10 - covers with room to spare - I mean lots of room to spare. Did a Google search and find it should be useable on my B&J 11x14 - so a nice surprise

My question - my Copal No. 1 has f-stops from f4.5 through f64 - so my question will these f-stops after f11 be true for this lens or do I need to recompute the series?

Thanks

Mike

RobC · Jan 16, 2008

Chances are they are wrong unless you got real lucky.

The aperture marks are focal length specific so you need to know the focal length of the lense and use aperture marks for that focal length.

Put a flat mirror over the front of the lense with mirrored surface towards the lense. Then put a point source light against ground glass and focus the reflected point on the ground glass. That should give you infinity focus for the lense which will be at its focal length. Then measure the distance from lensboard(?) to ground glass surface which will be the focal length.

? I forget where the actual point to measure from is, but its close to the lens board.

Then you need aperture marks for that focal length. If its a copal shutter you should be able to order one or someone may know how to calculate the spacing.

If you are sure its a 210mm lense then just order fstop marks for that focal length.

mmcclellan · Jan 17, 2008

Measuring should done from the "optical center" of the lens.

It might be easier just to test the lens with a known film speed, a known shutter speed, and a good meter and figure out what "f/11" really is and then with that new known value, run a couple of tests to see if the other apertures hold true, relatively speaking.

Just a thought.

PHOTOTONE · Jan 17, 2008

Are you sure that 210mm f9 Computar will cover 11x14???? I know the 260mm will.

GeorgesGiralt · Jan 17, 2008

"Are you sure that 210mm f9 Computar will cover 11x14???? I know the 260mm will."
Well there is a difference from circle illuminated and good resolution circle ...
Going back to the F-stop calculation :
You've to know the exact focal length (assume it is 210 mm, it's not rocket science), and compute the diameter of the opening at f:9 (so to make it obvious 210 mm /9 =23.33 mm ). Then you've to measure the diameter of the iris as seen from the front cell. Here is the method I was told is perfect :
First, place the camera on a sturdy tripod, and focus with the lens on infinity. (a subject above 50 times the focal or more). Lock everything in place.
Next, replace the ground glass with a black cardboard with a pinhole at the center.
And place on the front lens cap a piece of enlarging paper (obviously, you're in the darkroom to make this...)
Put a light bulb at the pinhole and expose the paper in the lens cap. Ensure your settings are still good.
Develop the paper. The diameter of the black mark is the exact opening of the F-stop. In 3 or 4 try, you should be good.
If you make the measurement wide open and for 2 or 3 more openings, you should have a correct scale on a cardboard...
I wonder if you will have a correct scale for this lens by asking Copal ? The spacing from one stop to another would be, I think, proportional but the starting point is dependent on lens construction. So it could be close enough for Government work, but not exact.

MikeK · Jan 17, 2008

GeorgesGiralt said:
"Are you sure that 210mm f9 Computar will cover 11x14???? I know the 260mm will."
Well there is a difference from circle illuminated and good resolution circle ...
Going back to the F-stop calculation :
You've to know the exact focal length (assume it is 210 mm, it's not rocket science), and compute the diameter of the opening at f:9 (so to make it obvious 210 mm /9 =23.33 mm ). Then you've to measure the diameter of the iris as seen from the front cell. Here is the method I was told is perfect :
First, place the camera on a sturdy tripod, and focus with the lens on infinity. (a subject above 50 times the focal or more). Lock everything in place.
Next, replace the ground glass with a black cardboard with a pinhole at the center.
And place on the front lens cap a piece of enlarging paper (obviously, you're in the darkroom to make this...)
Put a light bulb at the pinhole and expose the paper in the lens cap. Ensure your settings are still good.
Develop the paper. The diameter of the black mark is the exact opening of the F-stop. In 3 or 4 try, you should be good.
If you make the measurement wide open and for 2 or 3 more openings, you should have a correct scale on a cardboard...
I wonder if you will have a correct scale for this lens by asking Copal ? The spacing from one stop to another would be, I think, proportional but the starting point is dependent on lens construction. So it could be close enough for Government work, but not exact.

The source of my information about the coverage was our famous Sandy King I found a number of threads about this lens as well as some of the longer focal lengths. I am looking forward to testing this on my 11x14 this coming weekend. If it does indeed provide decent coverage I will send it off to the Grimes organization and have them put a new scale on the shutter.

Thanks for all the feedback

Mike

A Nice Surprise and a Question

MikeK

RobC

mmcclellan

PHOTOTONE

GeorgesGiralt

MikeK

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