Focus accuracy - what's acceptable

[Bob McCarthy]

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

[Bruce Watson]

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.

[Kevin Crisp]

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.

[David A. Goldfarb]

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?

[Bob McCarthy]

A 6" discrepancy at 10 feet is definitely not acceptable.

What camera is it?

Technika 2000

Would the fact that I shot wide open impact your opinion?

bob

[Bruce Watson]

Technika 2000

Would the fact that I shot wide open impact your opinion?

bob

Not in the least.

[Mike Kovacs]

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.

[Michael S. Briggs]

......

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.

Your right, collimating through a lens wouldn't make sense for virtually all LF cameras since the position of the lens isn't fixed with respect to the back.

We went over the configuation of the back of Bob's camera on the previous thread, http://www.largeformatphotography.in...ad.php?t=17727. The screen rests against metal strips, the position of which are adjusted by turning screws. I'm not a professional repairman, but I'd do the adjustment by using a depth micrometer to measure the depth and adjusting the screws until it was correct. The depth should match the depth of filmholders, less the thickness of sheetfilm. One can easily make the measurement to sufficient accuracy.

[Bob McCarthy]

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.

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

[David A. Goldfarb]

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

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.