How to correct for back-focus

[David Solow]

Hi all,

I use a Linhof MT 3000 with a Cooke f/4.5 229mm lens. I have tested the focus four times from three different distances using the LensAlign testing setup. In all cases, the camera/lens combination back-focuses about 2". I can compensate, of course, by focusing 2" in front of the plane I want in focus. Is there anything else I can do?

Thanks,
David

[Greg Miller]

I'm assuming your are focusing on the ground glass, then developing the sheet, and seeing the actual point of focus on the film(?).

If so, then your ground glass is not positioned properly.

[taulen]

http://www.vintagelargeformat.com/2011/12/26/how-to-focus-a-vintage-soft-focus-lens/

Maybe this can help ?

[Frank Petronio]

If you don't want to send the camera and lens to a technician like Marflex or Precision Camera Works then you would likely add a very thin shim to move the ground glass further away from the lens.

[Bob Salomon]

It sounds like your gg is not lying in the film plane. Has someone installed a non-factory gg? Marflex is also in NC in Havelock. Currently he is under the weather but you should have him set the shims to factory specifications if they have been set improperly.

Also, is the grain side of your gg facing towards the lens? That is where it belongs, resting on the gg shims. Do you have a Fresnel and, if so, is it on top of the gg closest to your eye? That is also where a Fresnel belongs on a Master Technika.

[David Solow]

Greg,
Thanks for asking for clarification. You are correct in your assumption. I did focus on the ground glass using a Rodenstock 6x Aspheric Loupe. I developed the sheet and saw the actual point of focus on the film. I did this numerous times from three different distances. Each time, all of my results were the same.

Taulen,
Thanks for the link. I am not sure how this will work with my lens. I didn't see any halos, but I will look into this in more depth.

Frank,
Thanks for the suggestions. I will send my camera and lens to Marflex, but it will be a while. Right now, I can't afford to be without them. I have a lot of work to do with them over the next two months.

Thanks again everyone,
David

[jp]

It's a portrait lens. The OP has figured out the way Kodak suggests focusing their portrait lens. There is a different shape to the in-focus area in terms of where and how it goes in and out of focus.

If you stop it down after focusing, you also throw off the focus. Due to the designed aberration, axial rays focus in a different distance than the rays going through the center of the lens. This is the physics that provides the diffusion, mixing the soft and sharp. When you stop it down, you go from mostly axial rays to mostly central rays, altering the focus. Thus, focus at the aperture to be used.

[David Solow]

Jp498,

Thanks for your response.

I never stop down after focusing. I always focus at the aperture I'm shooting, which 90% of the time is wide open at F/4.5. For now, I have shot only in the studio and use a powerful tungsten fresnel in order to focus, which has no problem in providing a well lit image on the ground glass, even at f/22, which is the smallest aperture I've used.

thanks again,
David

[David Solow]

Bob,

Thanks for your response. I have left a message with Martin concerning my camera and lens. I understand he will not be available for while, which is fine. I'll continue to compensate until he returns to work. I won't be able to be without the camera or lens anyway until sometime in February. I'll send him both at that time.

As far as the gg is concerned. It is original to the camera. I bought the MT3000 a few months ago brand new from B&H. I have treated the camera with the utmost care and have not added anything to the camera. The lens is brand new as well. I am using the lens board you recommended, which was also bought new at B&H.

Thanks again,
David

[Frank Petronio]

A quick way to check whether it is the lens focusing technique or the ground glass positioning would be to shoot with a "normal" lens wide open using either your testing stuff or the simple newspaper on the table technique....

If it were the lens I would expect Clive or someone from Cooke to mention it requires a certain technique to focus properly.

[jp]

http://www.cameraeccentric.com/html/info/kodak_1.html

(for the kodak lens) describes focusing on the nose instead of the eye catchlights. Russ young told me there was something wrong in this manual, but got on a rabbit trail and didn't say what. I would suspect the cooke has similar focusing traits, but I don't have personal experience with that lens.

[rdenney]

http://www.cameraeccentric.com/html/info/kodak_1.html

(for the kodak lens) describes focusing on the nose instead of the eye catchlights. Russ young told me there was something wrong in this manual, but got on a rabbit trail and didn't say what. I would suspect the cooke has similar focusing traits, but I don't have personal experience with that lens.

If David is focusing at taking aperture, this doesn't make sense to me. It would seem to me that what one sees on the ground glass ought to exactly match what one sees on the film, warts and all. A focus shift when stopping down makes sense, but I don't see the possibility of a focus shift solely on the basis of the surface being focused on, assuming both are flat and in the same place. Was this lens intended for use on press cameras with rangefinder focusing? That may also explain the instructions.

A 2" focus error at still life and portrait distances seems like a large error to me.

Rick "what am I missing?" Denney

[David Solow]

I asked Guy at ZGC who are the folks who Cooke to repair their lenses. He said that to his knowledge, there is no special focusing technique to use with this lens and he is unaware of any problems with back-focusing or front-focusing.

The lens is not intended to be used on a press camera with rangefinder focusing. The Cooke Portrait PS945 f/4.5 229mm lens is a replica of the Pinkham & Smith Visual Quality Series IV Soft Focus Lens. It is made for portrait photography on a 4x5 camera. For more info, go to: http://cookeoptics.com/cooke.nsf/products/largeformat.html

As far as distances are concerned, I did tests at 52", 75" and 150" from the front of the lens to the LensAlign testing set-up. The lens, gg and testing surface were all in perfect alignment. The film from all distances showed roughly the same amount of back-focus. The same held true with both still lifes and portraits, which I shot in the 52" to 75" range. Most shots were taken at the 75" end of the range.

Thanks for everyone's responses,
David

[tlitody]

Your loupe should be focussed on the grain in the ground glass. Check it is focussed properly.

If your loupe does not have a focus ring then look at the GG from both sides. If the grain of the GG is sharp when looking at it from the ground side, then your loupe is no good as it needs to be sharp when looking at it from the plain glass side.

And if it has diopter adjustment, make sure its set correctly for your eye.

[rdenney]

The lens is not intended to be used on a press camera with rangefinder focusing.

Oh, yes, I knew that. But someone had referenced a Kodak soft-focus lens that provided special instructions for front focusing, and I wondered if that Kodak lens and its instructions were aimed at a rangefinder-focused press camera.

Rick "thinking GG focusing ought to always give you what you see" Denney

[David Solow]

My Rodenstock 6X Aspheric Loupe has a diopter adjustment. When the grain of the ground glass was in focus, I gaff-taped the adjustment ring, so it wouldn't move. I made sure the grain of the gg was still in focus.

I'm not sure why the Kodak lens thing keeps being brought up. I have never had one and know nothing about them. I am using a Cooke lens, made in England. The Cooke lens is a soft-focus lens, so perhaps that is why the Kodak soft focus was mentioned, but there are a lot of soft focus lenses out there.

Rick,
The plane of focus on the gg is crystal clear, but when I get my film back, the plane that is in focus is about 2" behind what was clearly in focus on the gg.

Thanks everyone for their suggestions. Keep them coming. User error is usually the first thing to look for. I just can't seem to find mine, yet. if I (with all of your help) haven't figured it out by the time I get back in February, I will test the camera with both 210mm and 75mm lenses to see if the problem is consistent or not. If I find that it is consistent. I will send my camera and lenses to Marflex.

David

[David Solow]

By the way, I have tried these tests using 20 different film holders, and the results are consistent.

David

[tlitody]

My Rodenstock 6X Aspheric Loupe has a diopter adjustment. When the grain of the ground glass was in focus, I gaff-taped the adjustment ring, so it wouldn't move. I made sure the grain of the gg was still in focus.


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.

[E. von Hoegh]

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.

If the GG is in crisp focus it doesn't matter.(I know)

It seems that his GG is in the wrong plane. I wouldn't expect this on a brand new $8000 camera.

[jp]

I'm not sure why the Kodak lens thing keeps being brought up. I have never had one and know nothing about them. I am using a Cooke lens, made in England. The Cooke lens is a soft-focus lens, so perhaps that is why the Kodak soft focus was mentioned, but there are a lot of soft focus lenses out there.
David

I mentioned it because of how soft focus lenses can have unusual focusing techniques.

This lens interests me, but it's beyond my present budget, having spent too much $ filling my freezer with film.

http://cookeoptics.com/cooke.nsf/downloads/Cooke_PS945_v1.4.pdf
page 7 says under the section "uncommon optical behavior" that the focus needs adjusting in the way you seem to describe.

[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.

[Frank Petronio]

Yeah but it hard to curse in German!

They may think it's a love song.

[r.e.]

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.

I read that you spoke with ZGC. They have a good rep, but if you can't quickly eliminate the lens as the issue (by which I mean a fault, not some focusing peculiarity), I'd suggest that you call Cooke directly. (Note that ZGC is in the motion picture business and that Cooke principally makes motion picture lenses.) Personally, I would test the lens on someone else's camera, and if the problem persisted, I would call Cooke. I saw from another thread that you are doing an ICP course mid-January. If you are already in New York, I'd be happy to lend you an Arca-Swiss 4x5 to do some additional test shots.

[David Solow]

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

Thanks for replying, Dan. The 2" is an estimate. For the LensAlign tests. I shot from the three different distances with the lens wide open at f/4.5. While the DOF changed. It looks like the numbers that are in focus are always about 2" behind the "0". I shoot portraits almost exclusively at f/4.5 but have shot at least 30 or 40 at smaller apertures, as small as f/22. All of these images are back focused. I have also shot still lifes as small as f/22, which also resulted in back-focused images. For the portraits shot at f/4.5, people's eyes are not in clear focus, but their hair around the outside of their heads are dead-on in focus. I have never shot at f/22 at 150". The 150" distance was for the LensAlign test at f/4.5. I do plan on shooting at that distance in the future to include the whole body, but I haven't done that yet. It will be February before I can get the camera and lens into Marflex. I will call Cooke and will report back, once everything is resolved.

David

[Paul Fitzgerald]

David,

"The plane of focus on the gg is crystal clear, but when I get my film back, the plane that is in focus is about 2" behind what was clearly in focus on the gg."

So the film is closer to the lens than the GG. Is there anything impeding the GG frame from seating fully down against the camera back? Can the GG frame be removed to check like a graflok back?

Good luck with it, must be a PITA.

[tlitody]

I wouldn't touch the adjustment of the GG unless you are 100% sure that is where the problem is. Large Format photographers have a high propensity for trying to fix things that ain't broke. Possibly an adjustment of your loupe focussing will solve the problem for you. Another test or two with different loupe focussing should verify that for you.

But if you do decide to adjust the GG make sure you measure its depth very accurately before you change anything so that you can put it back where it was before you moved it if necessary. A tooth pick isn't accurate enough. An aluminium bar with some holes drilled in it and a depth micrometer is what is needed. The bar needs to be consistent thickness.

The Linhof backs have four sunken screws on the outside of the back which will be covered with some glue like stuff. You can use a watchmakers screwdriver to adjust the GG depth without taking the glass out or using replacement shims. Those screws are what the shims on the glass side rest on and turning them adjusts the height. Be careful and have very accurate measuring technique to get the depth and plane correct. Without that very accurate measuring of all four corners and center of glass depth you are just wasting your time. You need to measure to the point where the micrometer touches the glass and not to the point where it clicks because the glass will move in its mount before the micrometer clicks. It isn't easy to get right.

The camera must be under warranty so why not get it checked under that warranty?

[Dan Fromm]

David, thanks for the reply. Have you tried the camera with a different lens and got the same result?

[David Solow]

Thanks, Tlitody for responding. Both the lens and the camera are new and under warranty, so I'm not touching anything. I'll send both the camera and lens in to be tested and corrected.

Dan,
I haven't tried the camera with a different lens yet. I'll be away from the studio until February. I'll test the camera with a couple of different lenses then, before I send it in.

Thanks everyone.
David

[Larry H-L]

Hmmm... Do you wear progressive bi-focals? Wonder if they could interact with your loupe?

Is there another person nearby who could focus the camera for you as a test?

[rdenney]

Here's what bothers me: 2 inches is a big error for a 150" focus distance. At this magnification, it one will have to move the ground glass a lot to correct that error. I think it's likely to require more adjustment than the adjustment screws provide. Anybody want to run that calculation? How much would the ground glass have to be moved to change the focus plane two inches at 150" distance with a 9" lens? (Similar triangles suggests about an eighth of an inch, but this seems so significant that I'm sure I'm mistaken.)

Two inches at 30 feet (360 inches) would be much less of an error.

Blaming the lens also bothers me. I can't think of any reason whatsoever why a lens should make a different image on a ground glass than on a piece of film, if both are in the same position. I can see where a lens might have a weird depth-of-field pattern that would favor a slight bias one way or the other to provide a best compromise focus for portraits (as with the Kodak lens example cited previously), and I can see a focus shift from stopping down. I can't see a lens at taking aperture making a different image on film than it does on the ground glass.

Is the camera equipped with a plain ground glass? With my Maxwell screen, which has the focus surface towards the lens and the extremely fine Fresnel on the back side (protected by cover glass), it's tempting to focus the loupe on the Fresnel pattern and not on the actual focus surface. A high-power loupe will be more critical in this regard. But you say that you are seeing the focus plane sharply.

I think I would look for a simple, systematic problem related to something unexpected and unrelated to what must be a proven work process for you, like there being a chunk of schmutz or other whatever in the alignment channel for the film holder, preventing it from seating fully.

Rick "Occam's Razor and all that" Denney

[tlitody]

I would take that loupe in its taped fix position and put a tranny on a light box, place on top of tranny touching it and see if its in focus cos if it is then then loupe is not focussed where it should be.
Either the GG is in the wrong place or its a Loupe focussing problem. I don's see how it could be anything else.
I rekon if the film holders were not seating properly you would get some film fogging which would be fairly obvious.

[rdenney]

I would take that loupe in its taped fix position and put a tranny on a light box, place on top of tranny touching it and see if its in focus cos if it is then then loupe is not focussed where it should be.
Either the GG is in the wrong place or its a Loupe focussing problem. I don's see how it could be anything else.
I rekon if the film holders were not seating properly you would get some film fogging which would be fairly obvious.

Agreed, except that David said he was able to see the focus plane sharply using his loupe. If the loupe was focused on the back side of the ground glass and not on the ground side, he would never see a completely sharp image. David seems like he knows how to use his loupe on a camera, and has plenty of experience doing it correctly, which is why I was looking for a different problem.

But even if his loupe was slightly out of focus, if he adjusted the camera focus to provide the sharpest possible image, it should still be in focus. We are not focusing on an aerial image, where our eyes, glasses, and other intervening optics affect the apparent focus. We are focusing on an image projected onto ground glass, which is what it is whether our loupe is focused on the correct surface or not.

I was wrong (as expected) about the potential error of the ground glass. The simple gaussian lens formula is 1/object distance + 1/focus distance = 1/focal length. This formula should be close enough to examine the potential error. With that equation, a focus error of 2" at 150" object distance will change the focus distance by a bit over 0.008 inches. Not nearly as large an error as it seemed to me at first.

Rick "back to sending it to Marflex" Denney

[tlitody]

Well Linhof have been assembling and testing these things for a long time. That doesn't mean they don't make mistakes but I rekon it would be very rare which is why I suspect a Loupe focussing problem. I may of course be wrong. I have no way of telling and am just suggesting some simple tests to possibly negate the need to send the camera for checking.
I suspect those rodenstock loupes are optimised to look at film on a light table and not at a ground glass but I could be wrong about that too.

[rdenney]

I suspect those rodenstock loupes are optimised to look at film on a light table and not at a ground glass but I could be wrong about that too.

No, you're probably right about that, though my better loupes are focusable. But it won't matter. An out-of-focus loupe will keep you from evaluating how sharp your lens is or whether you have tilts set correctly, but it won't keep you from focusing as well as possible--in focus will still yield the sharpest image seen through the loupe. It just won't appear in the loupe as sharp as what is actually on the ground glass.

Rick "this should be as hard as we are making it" Denney

[Frank Petronio]

Everything points to the GG being off.

[Bob Salomon]

I suspect those rodenstock loupes are optimised to look at film on a light table and not at a ground glass but I could be wrong about that too.

The Rodenstock loupes, all three of them - 4x, 3x and 6x - are designed for use on any surface, print, slide, groundglass, coin, tree bud, etc. - they either have a sliding skirt (4x) with a focusing eyepiece or a reversible skirt (3 and 6x) with a focusing eyepiece. All three use asperhics so the user can have both the center and edges in sharp focus at the same time and to keep lines from bending from edge to edge, all have long eye relief to reduce eye strain and all are multi coated to deliver maximum contrast and color. The 6x has 3 lenses of which 2 are aspheric and the eyepiece is adjustable from +1.0 to -2.0 D.

So the problem isn't the loupe as David stated that he had the eyepiece properly adjusted and taped down.

[E. von Hoegh]

Everything points to the GG being off.

Yup.

But let's see if we can keep the debate going until the camera comes back from Marflex. Perhaps start a pool....:)

[tlitody]

You're on. 0.008in or 0.2mm is easily the amount the film can move forward or backward in the film holder. Sinar used to make some metal 4x5 film holders which tensioned the film so you could be sure where the film plane really was. They also made 8x10 holders with a tacky rubber back to achieve the same thing.
So if one of the top pro camera makers rekoned that was necessary then it shows that the level of accuracy for largeformat film plane placement is really not accurate enough for using very wide apertures without exact film sheet placement.
In short, it's quite possibly loose film causing the problem.
Now since it is claimed that the error is consistent you could argue that it must be the GG out of position. But then I might argue that since no one has asked exactly how the tests were done and whether the camera was pointing up, down or horizontal so that you might have a clue as to whether the film was falling forwards or backwards in the holder, then it isn't exactly a scientitific test and the GG might be in exactly the right plane according to the commonly used standard depth. We really don't know.

[E. von Hoegh]

Well, it takes about 12 seconds with a depth mike to check the GG position......

What did Sherlock say? "Eliminate the impossible, and whatever is left, however improbable, is the answer".

[tlitody]

Well, it takes about 12 seconds with a depth mike to check the GG position......

What did Sherlock say? "Eliminate the impossible, and whatever is left, however improbable, is the answer".

If you have one then yes. And also to check the film holder with a sheet of film in it to make the comparison. As has already been pointed out by someone.

[Bob Salomon]

You're on. 0.008in or 0.2mm is easily the amount the film can move forward or backward in the film holder. Sinar used to make some metal 4x5 film holders which tensioned the film so you could be sure where the film plane really was. They also made 8x10 holders with a tacky rubber back to achieve the same thing.
So if one of the top pro camera makers rekoned that was necessary then it shows that the level of accuracy for largeformat film plane placement is really not accurate enough for using very wide apertures without exact film sheet placement.
In short, it's quite possibly loose film causing the problem.
Now since it is claimed that the error is consistent you could argue that it must be the GG out of position. But then I might argue that since no one has asked exactly how the tests were done and whether the camera was pointing up, down or horizontal so that you might have a clue as to whether the film was falling forwards or backwards in the holder, then it isn't exactly a scientitific test and the GG might be in exactly the right plane according to the commonly used standard depth. We really don't know.

Linhof also made vacuum film holders as did Hoffman. But the sag in film is well within the depth of focus of the lens. Linhof rather quickly discontinued their 45 vacuum film holders as impractical in practice. But the did continue to manufacture their vacuum 126mm (5") aerial roll film back for many years.

Before Kodak came out with their instant film they surveyed the major camera manufacturers to see what the actual gg positioning and film plane position was in the various cameras. After they completed the survey they contacted us to let us know the results of the survey. What they discovered was that only two of the manufacturers that were contacted had set the film plane and gg position to 0. Neither Linhof nor Sinar were one of those two. Camera manufacturers know film sags as do lens manufacturers. That is one of the major differences between digital and analog view cameras and analog and digital view camera lenses. Digital sensors have no sag and digital sensors have no depth like film emulsions do.

[tlitody]

Linhof also made vacuum film holders as did Hoffman. But the sag in film is well within the depth of focus of the lens. Linhof rather quickly discontinued their 45 vacuum film holders as impractical in practice. But the did continue to manufacture their vacuum 126mm (5") aerial roll film back for many years.

Before Kodak came out with their instant film they surveyed the major camera manufacturers to see what the actual gg positioning and film plane position was in the various cameras. After they completed the survey they contacted us to let us know the results of the survey. What they discovered was that only two of the manufacturers that were contacted had set the film plane and gg position to 0. Neither Linhof nor Sinar were one of those two. Camera manufacturers know film sags as do lens manufacturers. That is one of the major differences between digital and analog view cameras and analog and digital view camera lenses. Digital sensors have no sag and digital sensors have no depth like film emulsions do.

The Contax RTS III had a vacuum film pressure plate too.
I'm not sure that the depth of focus and depth of field are sufficient when using wide apertures to stop a noticeable shift in the focus plane if the film is out of place. The argument seems to be that moving the GG 0.2mm will fix it (assuming that figure is correct). Well if that 0.2mm move in GG will fix it then a 0.2mm displaced sheet of film is enough to cause the problem which means film moving in holder has to be considered as one possible problem.

[Brian C. Miller]

I read throught the whole thread, and there's something that everybody missed.


I use a Linhof MT 3000 with a Cooke f/4.5 229mm lens. I have tested the focus four times from three different distances using the LensAlign testing setup. In all cases, the camera/lens combination back-focuses about 2". I can compensate, of course, by focusing 2" in front of the plane I want in focus. Is there anything else I can do?

Yeah, check it with a different lens.

All of this is centering around the Cooke being a soft-focus lens. How about checking the camera with a sharp-focus lens? Any lens will do for that. Is the issue really with the lens, or is it really with the camera? You can pick up a Calumet lens for cheap. I can send you something that works to try to nail the problem down.

My Super Graphic came with a different ground glass, which threw off the focus. After I realized that was the problem, I ordered a replacement fresnel/gg combo from
Brightscreen and shimmed it into position with some film. (stable, thin, and it works) Since you have a brand new camera, you can have this done for you by the factory. But first, just test whether a different lens works for you. It should be a very quick test.

[David Solow]

Thanks, Brian.

When I get back to North Carolina in February, I will check the camera with another lens. I have a 210mm Rodenstock that should do nicely.

David

[David Solow]

To everyone who has contributed to this thread,

I have started a new thread, "Back-focus Images", that show my LensAlign tests as well as a shot of a friend that is representative of all of the shots I have done with this camera and lens.

David