How to correct for back-focus

[tlitody]

Another potential problem is the film holders. Some makes don't conform to the standard, namely Toyo when I tested some new ones that I bought. There was a significant difference to the fidelity holders.

[Larry H-L]

Try this. Take the back off of the camera. Get a good straight ruler and lay it on edge across the inside of the back. Now clamp a toothpick to the ruler so that the tip of the toothpick just touches the ground glass. Remove ruler temporarily. Then place an opened film holder into the back with a piece of junk film in the holder. Put ruler with toothpick again across the back. Toothpick should not move and should just barely touch film.

A fresnel inside complicates this, but the GG should be very close to the toothpick, just a tiny gap 1/3 the thickness of your fresnel.

[r.e.]

David,

Could I suggest that you phone Cooke directly to discuss the problem, if only to rule out some possibilities? I had some dealings with them a few years ago and found them responsive and helpful. It is a small company, and this is a new, and expensive, lens. I suspect that they'll be happy to speak with you. Like rdenny, I doubt very much that this has anything to do with how the lens focuses, but it can't hurt to speak to the horse's mouth.

You might also try identifying someone in your area who has a camera on which you could try the lens to see if you experience the same issue.

[David Solow]

But if it doesn't have a focus adjustment then it will likely be focussed to whatever it is resting on and not the two or three milimiters further away to compensate for the thickness of glass. Setting the diopter alone is not correct (I think) and the diopter should be set to whatever your last eye test said and not what puts the GG in focus (I think). I may be wrong but someone will correct me if so.

Actually, I misspoke. The adjustment is not a diopter. It is a focusing adjuster. I have the loupe focused right on the grain in the gg. I have used two different loupes: a Rodenstock 6x Aspheric and a Schneider 10x loupe. Both loupes focus incredibly well, especially the Rodenstock. Everything is dead on focused on the gg.

David

[David Solow]

Thanks, jp498. That pdf on the Cooke lens is full of great stuff. I'm not sure how the chapter on uncommon optical behavior relates exactly to my problem, but I just scanned it. I'll be sure to read the whole thing soon.

Thanks again,
David

[David Solow]

r.e.,

I have already spoken to the folks at ZGC who work on Cooke lenses here in the US, but I have not spoken with Cooke directly. I will give them a call soon.

Thanks.

[David Solow]

Another potential problem is the film holders. Some makes don't conform to the standard, namely Toyo when I tested some new ones that I bought. There was a significant difference to the fidelity holders.

I used about 20 Fidelity film holders, some used, some new. I get the same results with all of them.

[Frank Petronio]

It seems as though you've eliminated everything down to it being the ground glass out of position. You could try a different lens on your camera and try your Cooke out on a different camera to be absolutely sure, but any competent camera tech should be able to position the ground glass (not just Marflex). You can probably do it yourself in a couple cycles of film testing. All it requires is a screwdriver and a few layers of tape as shim material. It would be more reliable than "compensating after focus".

If B&H sold you a camera out of adjustment you should tongue lash them, but everyone makes mistakes and this is starting to sound like one.

[Dan Fromm]

David, I've read this thread from your first post to your last (#24, as I'm typing) and I've looked at what the maker and Reichman & Friends have to say about the LensAlign.

You've got the same result with 20 different film holders and you focus at shooting aperture. This suggests, as you've been saying, that the GG and the film plane's position aren't the same. The chances that 20 holders are off by the same amount in the same direction are low and focusing at shooting aperture eliminates focus shift on stopping down.

But you say that the focus error is a constant 2" (with what precision?). This bothers me a little. So does that you've shot at apertures as small as f/22 and still had the problem.

With a 229 mm lens, film-to-subject distance of 150" is roughly 17 focal lengths. At that distance at f/22 with a CoC of 0.025 mm, smaller than typically assumed for LF, DoF is a little more than 4". How can you tell where the plane of best focus is?

The constant 2" off bothers me for two reasons. First, the higher magnification, the greater depth of focus (not depth of field, depth of focus). So the closer you focus, the safer you should be. Secondly, a constant displacement between GG and film plane doesn't translate to a displacement between actual and intended planes of best focus that's the same at all magnifications.

So, like you I'm puzzled. I hope that you manage to get your camera and lens to Marflex and that they can solve the problem.

Good luck, please keep us posted,

Dan

[Dan Fromm]

If B&H sold you a camera out of adjustment you should tongue lash them, but everyone makes mistakes and this is starting to sound like one.

Frank, even a person with your qualities should understand that vendors like B&H receive boxes and ship boxes without testing their contents. B&H relies on manufacturers to provide quality control.