Focusing problems with the fresnel groundglass on the Chamonix 45N-1?

I have a stick with slots to hold playing cards as targets. It was intended for use in teaching use of swings and tilts, but should be useful in diagnosing focus problems also. You would just place the stick at a moderate distance from the camera, focus on the middle card, then see which card is the sharpest on a negative or print.

I was a little baffled with this thread. Two people with the same camera having identical problems with a particular focal length lens. I would have thought that the Chamonix being a new camera the position of the gg/fresnel combination would have been tested. So I thought I would test my own Chamonix, and using mm ruler verify any small differences if they existed.

For a target I used a window mat on a mat board that would provide a bright line (reflecting the overhead light), and a dark line that would be a shadow from the overhead light. Both lines would be parallel to each other, and the target would also test flatness of the gg (i.e. there should be no change in focus of the parallel lines).

I checked three lenses, my 58mm Schneider XL, 90mm Linhof Scneider Angulon f5.6, and my Apo Sirornar S 135mm. All were tested with either the fresnel in the proper position, with it on top of gg, and no fresnel - just the Chamonix gg. I used a Scneider 6X loupe to check for focus.

With the 58mm lens the focal shift +0.5 mm with the fresnel in the proper place. With the 135mm lens the focal shift was +1 mm with the fresnel in the proper place. This should not make a big difference in depth of field unless shooting wide open I would think. However, with the 90mm lens the focal shift was +3.5mm with the fresnel in the proper place. There was no focal shift when the fresnel was placed on top of the gg. I have tried 2 other gg that I have, and made the same observation.

One possible explanation is the focal length of the fresnel is affecting the focus of 90mm lenses. I do not have another fresnel to test. But I think hereafter I will keep my fresnel on top.

Mike

Mike,

Thanks for adding such a thorough test. I did not measure it in terms of mm on the camera. But did note the false focal point at about 25 feet (fresnel in place between gg & lens). This then focused about 30% closer with the fresnel removed (about 17 feet).

Hugo is aware of this and has informed the owner. It would be great if Chamonix had some sort of modification to the backs going forward.

Regardless of the optical focus shift that may be happening here, doesn't the Chamonix 4x5 GG position physically change with removing the Fresnel? I'm not certain that it does (though most others with the GG to the back and the Fresnel in front do). If the Fresnel shims the GG-- that is, if the depth of the shoulder cut which the edges of the glass rest upon is correct only with the Fresnel-- simply removing it introduces the a problem of focus inaccuracy from having the the film holder and the GG now occupying different planes. So you'll need to shim it back to where it belongs.

For that you'll need either a depth gauge on a granite surfacing block, or a dial indicator that measures to .001" on a sturdy stand. Any decent machine shop will have the surfacing block. A decent dial indicator won't be much more than $50 these days. Any reasonably flat scrupulously clean surface like a polished marble countertop or formica will be probably be close enough for this task if doing it yourself.

The ANSI standard depth for a modern 4x5 film holder is .197". So, ideally, the GG should be set at this depth minus the depth of the film base you're using (typically .005 to .007"). Anywhere within +/- .007" is within the acceptable margin of the ANSI standard.

Ivan,

The fresnel is recessed; if you remove the fresnel the gg is sitting on a separate support so the depth is unchanged.

Mike

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Originally Posted by Ivan J. Eberle

If the Fresnel shims the GG-- that is, if the depth of the shoulder cut which the edges of the glass rest upon is correct only with the Fresnel-- simply removing it introduces the a problem of focus inaccuracy from having the the film holder and the GG now occupying different planes. So you'll need to shim it back to where it belongs.

And where it belongs without the Fresnel is different than where it belongs with the Fresnel, so your shims will actually need to be thinner than the Fresnel screen. If the Fresnel is .030" thick, a starting place for shim thickness would be around .020". Film tests at wide apertures with an appropriate step target are advised before pronouncing such a modification as ready to use.

For that you'll need either a depth gauge on a granite surfacing block, or a dial indicator that measures to .001" on a sturdy stand. Any decent machine shop will have the surfacing block. A decent dial indicator won't be much more than $50 these days. Any reasonably flat scrupulously clean surface like a polished marble countertop or formica will be probably be close enough for this task if doing it yourself.

Smooth and flat are two different things. This is not a measuring/machining job for someone without the correct tools and the knowhow to use them. In any case, you will not be machining anything, just adding shims. If you remove the Fresnel, measure it's thickness with a micrometer and buy some mylar shim stock in thickness equal to 2/3 the thickness of the Fresnel. You may have to stack some thinner shims to create the correct total amount of thickness. This is a starting point. Cut and sandwich the shims between the GG and the mounting pads and do some film tests.

The ANSI standard depth for a modern 4x5 film holder is .197". So, ideally, the GG should be set at this depth minus the depth of the film base you're using (typically .005 to .007"). Anywhere within +/- .007" is within the acceptable margin of the ANSI standard.

The ground glass needs to be right on the money and not just anywhere within the +/- .007" range specified for the film holder. Film holders are made of plastic and thin pieces of aluminum and will vary all over the place. If the ground glass is set to a position merely within, but near the extremes of the specified range of depth for a film holder, there is a good chance more than half your film holders may render unsatisfactory results. If you can't correct a GG positioning error with shim stock, send the camera back to the factory and have them make it right. You really don't want to be removing material from the pads of any camera.

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Originally Posted by Michael Rosenberg

Ivan,

The fresnel is recessed; if you remove the fresnel the gg is sitting on a separate support so the depth is unchanged.

Mike

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In cameras where the Fresnel is actually suspended in front of the GG as opposed to being sandwiched between the GG and mounting pads (Horseman 45FA is another example, although they do it by attaching the Fresnel with clips to the GG), you still have to move the position of the GG to get it in the correct position for use without a Fresnel. In the Horseman, the designer had the presence of mind to include shims between the GG and the mounting pads as part of the design so that if you wanted to remove the Fresnel, you could simply remove the shims as well to move the GG forward to the correct postion. In an instance where the GG is already sitting on pads without shims and where the Fresnel is sitting on a separate set of recessed pads, removing the Fresnel will necessitate machining down the pads on which the GG is sitting. My advice... don't go there.

Robert, point well taken that the amount that the GG moves is not necessarily the thickness of the Fresnel lens as a shim. But the only ANSI standard that I've yet found is the median for the film holders, have yet to see a number for the distance from the gate to the GG. I've assumed it's either .190" or .192", the correct number being somewhat variable based upon the thickness of the specific emulsion in use (at least with filmholders like the standard 2-up darkslide having a film slot and no pressure plates).

Second is, how much difference does a couple of thousandths actually make at the film plane in practical use-- under what specific circumstances is it going to show up? I'd certainly expect it's much more critical with an Aero Ektar f/2.5 used wide open than with the typical f/5.6 or slower view camera lens stopped down to a taking aperture of at least f/11 (more likely f/22 for adequate DOF) for landscape. Can anybody point to a chart or give a formula for calculating depth of critical focus at the film plane for a given focal length, focusing distance, and aperture?

Realistically, can wooden field cameras or holders be produced in volume to within a repeatable tolerance of .007"? Agreed that stacked errors of say, a wooden back that's on the far side of the tolerance, with some swelling from humidity, coupled with a moisture-swelled wooden holder, could really begin to add up. It's not too hard to see why mid to late 20th Century technical designs began to be manufactured from more dimensionally stable materials like aluminum as soon as became practicable.

Also mentioned in another thread recently and still begging a satisfactory answer: Why are Fresnels ever put inside the optical path in the first place, if putting them on the outside (on top of the GG) achieves the same brightening effect and allows easier interchange for different focal length lenses?

Quote:

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Originally Posted by Ivan J. Eberle

Robert, point well taken that the amount that the GG moves is not necessarily the thickness of the Fresnel lens as a shim. But the only ANSI standard that I've yet found is the median for the film holders, have yet to see a number for the distance from the gate to the GG. I've assumed it's either .190" or .192", the correct number being somewhat variable based upon the thickness of the specific emulsion in use (at least with filmholders like the standard 2-up darkslide having a film slot and no pressure plates).

While compensating for average film thickness is certainly a legitimate factor when aligning gg placement, I would still be very cautious about using the same +/-.007" tolerance for error as is used in the ANSI spec for (4x5) film holders. Suppose gg placement is .007" shallow and you insert a film holder that is .007" deep. Both technically meet the +/- .007" ANSI spec, but combined as a system, your film is now .014" away from the gg position. This, I assure you will be noticeable.


Second is, how much difference does a couple of thousandths actually make at the film plane in practical use-- under what specific circumstances is it going to show up? I'd certainly expect it's much more critical with an Aero Ektar f/2.5 used wide open than with the typical f/5.6 or slower view camera lens stopped down to a taking aperture of at least f/11 (more likely f/22 for adequate DOF) for landscape. Can anybody point to a chart or give a formula for calculating depth of critical focus at the film plane for a given focal length, focusing distance, and aperture?

In a word, plenty. A lot will depend on what focal length of lens you are using. Shorter focal lengths with converging rays at a greater angle to each other than with long lenses, have a shallower depth of focus requirement to achieve satisfactory sharpness. While closing the aperture down to smaller f stops will increase depth of field, it will not fully compensate for poor gg/film plane coincidence and I have the negatives to prove it. A "couple of thousandths" beyond the range of tolerance will throw you into noticeable softness.


Realistically, can wooden field cameras or holders be produced in volume to within a repeatable tolerance of .007"? Agreed that stacked errors of say, a wooden back that's on the far side of the tolerance, with some swelling from humidity, coupled with a moisture-swelled wooden holder, could really begin to add up. It's not too hard to see why mid to late 20th Century technical designs began to be manufactured from more dimensionally stable materials like aluminum as soon as became practicable.

While a bit more of a challenge, by using aged hardwoods, properly sawn and sealed and precision milling equipment, the expert makers can achieve surprisingly accurate results. I've never personally compared the dimensional stability of wood to aluminum, but I can tell you with a fair degree of certainty that aluminum is not the most dimensionally stable material when it comes to changes in temperature. This I have measured.


Also mentioned in another thread recently and still begging a satisfactory answer: Why are Fresnels ever put inside the optical path in the first place, if putting them on the outside (on top of the GG) achieves the same brightening effect and allows easier interchange for different focal length lenses?

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This may have to do with protecting the ridged side of the Fresnel from scratching. These screens are made out of plastic.

With all the ways focusing errors can be generated (operator error when focusing, camera adjustments getting accidentally bumped, film buckling, film holders being a bit warped), the last thing you want to do is permanently stack the cards against ever getting a sharp negative by having your ground glass in the wrong place.

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Originally Posted by joshdaskew

As for the Maxwell screen, I have heard this recommended by a few people. Is this something that I would have to find to adjust or would I have to send him my camera for him to do.

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This is a great question. Has someone tried a Maxwell screen on the Chamonix yet, and if so, how did it fare with the 90mm lens focusing issue? Did you need to shim it or make any adjustments to calibrate the focal plane?


Edit: Oh well, neuroticism got the best of me so I broke down and just ordered the Maxwell Ultra Brilliant screen. I'm also likely sending it in to have the focal plane calibrated. I'll let you know how it goes.