I removed the bowed Beatte and we installed a new linhoff ground glass w/o fresnel (for the moment).
I decided to test the new rig.
I have a test setup using books with clear text each set up about 3 inches deeper than the previous. I focus carefully on the center book and shoot a polaroid (55).
The test was with a 150 f5.6 at about 10 feet away to target. Lens wide open, close in giving a fairly shallow dof.
it appears the camers is about 6 inches back focusing. What is acceptable?
Bob
What you put in sharp focus on the ground glass should be in sharp focus on the film. Nothing else is acceptable, at least to me.
Bruce Watson
Anything other than dead on accuracy is not acceptable. There are two many other variables that can effect sharp focus, why start off with a set up that wouldn't let you find it even if everything else went right.
A 6" discrepancy at 10 feet is definitely not acceptable.
If the Beattie had a fresnel on the lens side (it may have been a one-piece fresnel/focus screen unit), this could create such an error. What camera is it?
Technika 2000Originally Posted by David A. Goldfarb
Would the fact that I shot wide open impact your opinion?
bob
Not in the least.
Bruce Watson
It was the Beattie one piece that was bowed. That was the reason I replaced the GG with a Linhof glass GG.
I used the old mounting position, hoping that it was the original linhof setup. The adjusting screws were still painted over.
Oh well. F22 and be there!!
bob
A 5% focusing error of 6" at 10' translates into a huge focusing error close up.
The fact that it is back focusing suggests to me that the image forming part of your ground glass is too close to the lens. Is your camera is designed to have the ground portion of the glass set further away from the lens, e.g. if there was a fresnel in there?
If you know your error accurately, you can determine roughly how much shimming to add to bring the screen to the proper position. If it formerly had a fresnel, you need to accurately measure the thickness of the fresnel and shim the screen that amount. Then keep testing and adjust if necessary.
I'm not quite sure how the professional repairman setup the screen on a LF camera. For 35mm and medium format cameras its possible to collimate to the film plane but I don't see how one can collimate a 4x5 in this way.
It might be that your back was recalibrated for the Beattie, and then the screws were painted over. On the current Technika backs, if I'm not mistaken, the groundglass goes between the fresnel, if you have one, and the lens, with the matte side of the groundglass facing the lens. It's probably best to send it to Marflex and have it recalibrated.
Last edited by David A. Goldfarb; 8-Jun-2006 at 09:38.
Using the equations for focusing a lens of focal length f=150 mm, an object distance of 10 ft (3048 mm) corresponds to an image distance of 157.76 mm; for an object distance of 10 ft 6 in (3200.4 mm), the image distance is 157.38 mm. So the discrepancy is focus position is only 0.38 mm. (I'm not 100% sure which direction you meant by back focusing -- the magnitude of the focus error is about the same, 0.43 mm, if you meant the other direction.)
Depending on how sharp the screen image is, the brightness of the scene illumination, how well your darkcloth blocks out ambient light, the power of your loupe, your experience, etc., there are limits to how accuractely one can determine the position of best focus. Are you confident that you focused to 0.4 mm accuracy? You could test for a systematic error from the camera versus a random offset by the photographer by repeating the experiment and seeing whether you get the same result. You could try shining a bright light on the book that you are focusing on while focusing to improve your abilty to focus.
Last edited by Michael S. Briggs; 8-Jun-2006 at 09:43.
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