Medium format folder camera infinity adjustment / focus calibration

I am opening this thread because I have had a long story with my Ensign Selfix 820 folder camera, with the Ross Xpres 105/3.8 lens. And i have had many other folder cameras in the past and there's always the task of aligning the focus scale to the actual focus achieved on the film plane.

* a note on the film plane: Finding out the real film plane is a task in itself in many folder cameras, because, for example in the Agfa Record series, there are no "film rails" a la 35mm cameras, where the film contacts the rail. On the Agfa Record series, and also other folders like the Zeiss Nettar, the film rollers and the pressure plate set the film plane position, so for those cameras, IMO, it's best to use some Scotch "Magic" tape rolled onto two 120 spools, and then fitting it very taut into the camera, so the Scotch "magic" tape simulates film under tension. And then focus on that 'film plane'. Now, the Ensign Selfix, fortunately, is a different story, because it does have film guide rails, and you can fortunately put a ground glass over them.

The 105/3.8 is a four-element tessar-like lens, and at f3.8 it is a fast lens for a 6x9 machine. It is also a front-cell-focusing lens, like in the vast majority of MF folder cameras. Thus, the lens design is compromised in performance, due to the fast speed and due to the front cell focusing.

What happens with this? The problem is that the performance at close distances and at infinity will differ strongly. And there will be strong curvature of field at one or the other distances. I stumbled with this problem frontally with the Ross Xpres. But it's not the only lens that shows this problem and what I'll mention here will be applicable to other folder cameras.

A camera lens never focuses to a 100% flat image plane. Thus, if one calibrates the lens for maximum central sharpness, the edges will be a bit soft, and if one calibrates for best edge sharpness, the center will suffer in resolution. Of course, some lenses are better corrected for "flatness" than others (sometimes to detriment of other parameters). But the effect is that if you calibrate for center sharpness, edges will suffer, and viceversa. This can be readily seen in this link:

http://www.hevanet.com/cperez/MF_testing.html

See the resolution figures for the miscalibrated Rolleiflex with the 75/3.5 Xenar. Or the figures for the Kodak Special Six-20 at different focusing distances.

Now, what happened on the Ensign Selfix 820 is that when i calibrated the infinity focus for maximum central sharpness at wide open aperture, infinity-focused (landscape) pictures were sharp centrally, but the edges were terrible until stopped down to about f16... AND, normal distance (3m-8m) pictures were not having correct focus.

So I researched the subject. I've read on the 'net that a front-cell-focusing lens is usually optimized for best performance at "40x the focal length", so in the 105mm Xpres that would be about 4m. Thus, i reasoned that, in distances closest to 4m, the lens should perform good at both center and edge definition and the focus scale ring would be calibrated using such distance as a starting point. The lens has no mark for 4 meters, but it does have one for 5 meters.

What i did, then, was to calibrate the focus ring so wide-open central focus at 5m is correct for a subject located exactly 5m away. Good, because images at 3m and 2m also were being correctly focused on the ground glass. But what about the infinity focus mark?

Now here comes the SURPRISE: Afterwards, when setting such lens to the infinity position, the lens was NOT correctly focused for best wide-open sharpness in the center!! In fact it was focused "beyond" infinity by some margin. But, at such position, the "middle" or "near edge" zone of the image was correctly focused. When stopping down to about f8, the center became sharp again and the performance was uniform in the ground glass. Except for the extreme edges, which only became pin-sharp at about f16, but I can live with that.

So, what does this means? That doing the procedure of "aligning the focus ring's infinity mark with a sharp center image on the ground glass" is NOT always a good idea!!

Of course, i need to test the camera with film, since the film image plane position will not always be exactly where the groundglass was sitting, but it gives me a starting point for giving such machines a better calibration.

If it's a front cell focusing lens, one way to do this is by mounting the shutter w/ no spacers, find infinity by turning the front cell, reset your front bezel distance marker to infinity, then lock it down. Then get close distance by using thin spacers behind the shutter between it and the lens board. That's what I was taught, but have never actually had to do this. Unfortunately, there are other issues to consider, especially if someone has had the lens apart before you. Starting the helical on the wrong place will give you no end of problems. You might get infinity focus, but have the other distances wrong. Or it might focus fine up to 10 or 20 feet but never get infinity.

It's important to put the same shims on the back of the shutter, as these were placed there at the factory. Why? Usually because of manufacturing tolerances. Often lenses were sourced from outside the camera maker's company, and were collimated at the lens maker's. Infinity has been set and locked down by the grub screws on the outside of the lens, and the camera maker would use shims to get infinity because each camera would be a little different than the other. It's impossible to make them all exactly "correct" w/o spending a fortune, so the shims make things fast and inexpensive.

In truth, the only problem I have ever had w/ a front cell focusing folder was getting the helical started at the *^!&!* place after a disassembly! Sometimes the little marks get obliterated, so you have to resort to trying the thing at different starting points. Just one notch off can give you that scenario where you get infinity, but close focus is off (or unobtainable). Or vice versa. I have found that if you screw the front cell all the way in until it bottoms out, then back it out a little, that is where you should be checking infinity focus w/ a loupe on your old camera focus screen that is taped on the back of the film rails. If its pretty close, and you used the same shims that came off the back of the shutter, then you can fine tune the infinity focus and all the distances will be fine once you lock the front cell into place by means of the grub screws. If you do this and can't get infinity, you started the threads in the wrong place.

It's also important to make sure you have the GG or old focus screen taped to the right place on the film rails, as often the horizontal rails are raised at the top and the bottom to help guide the film along. The actual film plane is the wide vertical metal areas on either side of the bellows opening.

Interesting post, for me, at this point in time, because I have to re-set the focus on a Super Ikonta A, and I noted that:
- the rollers are not co-planar (possibly the sheet metal that supports the ends has been bent out of alignment at some point)
- they are not in the plane of the "guide rails" (higher)
- the "guide rails" cannot be effective because the pressure plate is resting on some outer rails (beyond the width of the film) that a raised by decidedly more than the thickness of film+paper. The pressure plate may also be pressing against the rollers.
So, your italicized section answers some of my questions. And a good reason for advancing the film just before taking a picture, and with the bellows already open.
Another question: Is the distance always measured from the film plane, like for 35mm cameras?

@momus: Puzzled by your procedure for front cell focusing lens. I'd say that if you shim the whole lens assembly, you lose the infinity setting that you achieved previously by re-setting the distance bezel...

Just a note for this great thread - the scotch magic tape is kind of a pain to deal with - the thinner frosted mylar is really great for this kind of work.

Maybe a bit less light than the tape (there are usually 2 thicknesses for sale and the thin one isn't much different than the tape), but heck, you can cut a strip of it the same width as film if you need to put it across the rollers. There are plenty of darkroom uses for the stuff as well.

It's pretty easy to maximize the focus experience too… grab a big cardboard box, cut a hole in the bottom, go in a dark room and stick the camera on a little travel tripod, put 'er in the box facing the hole, and aim a spotlight or a desk lap or whatever you have at your focus tester. For infinity, if you have a dark cloth (or a black towel) you can use that with the cardboard box trick outdoors or through a window. It's nice to really have the faux-ground-glass image pop.

bernard_L said:

- the rollers are not co-planar ..

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I checked my Zeiss Ercona II. The rails are some half millimeter higher than the rollers. The camera is almost mint, so I'm quite shure it was made that way. (When I finish the film in my Ercona I, I'll check if that's alike.)

Maybe this is to allow for a certain stiffness in the film/backing?

But my Nettar (518) has the rollers higher than the rails, like my prewar folders (Icarette, Voigtländer) that don't have pressure plates.

momus said:

If it's a front cell focusing lens, one way to do this is by mounting the shutter w/ no spacers, find infinity by turning the front cell, reset your front bezel distance marker to infinity, then lock it down. Then get close distance by using thin spacers behind the shutter between it and the lens board. That's what I was taught, but have never actually had to do this. Unfortunately, there are other issues to consider, especially if someone has had the lens apart before you. Starting the helical on the wrong place will give you no end of problems. You might get infinity focus, but have the other distances wrong. Or it might focus fine up to 10 or 20 feet but never get infinity.

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Sorry but the above statement doesn't feel completely right to me.

Changing the baseboard-to-shutter distance will change focus, as well as moving the front cell. The correct way of calibration is mounting the shutter with whatever spacer rings were fitted at the factory (you are correct in the assessment that it's important to leave those factory rings in place). And then performing the calibration by moving the front cell focus. No reason to mess with the baseboard-to-shutter distance.

For the front cell threads I've found on my time repairing folders, the amount of millimeters that the front cell move with respect to turning angle will be the same no matter where you start the thread, so starting the thread in the "wrong" place should NOT give you the problem of "getting infinity focus and have the other distances wrong."

On the other hand, the problem i have stated on the original post (involving curvature of field) WILL give you the impression of "achieving correct infinity focus and have the other distances wrong", if it is not taken into account.

bernard_L said:

Another question: Is the distance always measured from the film plane, like for 35mm cameras?

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That's an interesting question. In theory it should, but I recall there are cameras (can't remember if they were folders) where it's measured from the lens front.

In any case, my recommendation is to use the "40X the focal length" rule, so for a 100mm lens the preferred calibration distance would be 4m; and thus the difference between measuring the distance from the lens front versus the film plane is too small to worry.

SalveSlog said:

I checked my Zeiss Ercona II. The rails are some half millimeter higher than the rollers. The camera is almost mint, so I'm quite shure it was made that way. (When I finish the film in my Ercona I, I'll check if that's alike.)

Maybe this is to allow for a certain stiffness in the film/backing?

But my Nettar (518) has the rollers higher than the rails, like my prewar folders (Icarette, Voigtländer) that don't have pressure plates.

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Nettars (i.e. 517/16) are like this. The film is expected to go against the pressure plate, not against the rails. In fact it doesn't really has "rails".

The 105mm f3.8 Xpres is a reasonably good lens but cell focussing is a compromise, it's best stopped down. I have 3 although one is marked 107mm f3.8 Ensar and un-coated and judging by the SN a prototype (but it's the same lens).

Cell focussing Tessar type lenses aren't ass sharp at wider apertures as fixed spaced equivalents, they'll be better at a certain distance, that will have been decided by the lens designer.

The 105mm Xpres on my 820 is nowhere near as sharp at wider apertures compared to my 77.5mm f3.5 Xpres on my Micrord, this lens is actually better than my 75mm f3.5 Opton Tessar on an Automat.

I doubt the 105mm f3.8 Xpres is optimised for Infinity focus, I can't remember using a "normal" focal length lens on any camera focussed at Infinity, I'd guess a cell focussing lens is optimised for group shots 15 to 20 feet, very approx 5-7 meters.

Ian

Ian Grant said:

Cell focussing Tessar type lenses aren't ass sharp at wider apertures as fixed spaced equivalents

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Ass sharp lenses are really only useful when doing nude photography, otherwise regularly-sharp lenses are just fine.

Ian Grant said:

The 105mm f3.8 Xpres is a reasonably good lens but cell focussing is a compromise, it's best stopped down. I have 3 although one is marked 107mm f3.8 Ensar and un-coated and judging by the SN a prototype (but it's the same lens).

Cell focussing Tessar type lenses aren't ass sharp at wider apertures as fixed spaced equivalents, they'll be better at a certain distance, that will have been decided by the lens designer.

The 105mm Xpres on my 820 is nowhere near as sharp at wider apertures compared to my 77.5mm f3.5 Xpres on my Micrord, this lens is actually better than my 75mm f3.5 Opton Tessar on an Automat.

I doubt the 105mm f3.8 Xpres is optimised for Infinity focus, I can't remember using a "normal" focal length lens on any camera focussed at Infinity, I'd guess a cell focussing lens is optimised for group shots 15 to 20 feet, very approx 5-7 meters.

Ian

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So you were the one with the rare f3.8 ensar!! I saw the pictures on the internet.

From my experience I can safely say that the 105/3.8 Xpres is *not* optimized for infinity focus. When focusing at the center, for infinity, edges really go bad unless stopped down to f16 or so. But when calibrating it so the center is focusing slightly beyond infinity, apparently good overall performance can be obtained at reasonable (f8) apertures. I need to test with film, though.

tape, velum, n papers used as ground glass are part of the problem. they buckle n never sit perfectly flat on the film plane.

why add variables when a small piece of gg will cost almost nothing from a glasier.

paul ron said:

tape, velum, n papers used as ground glass are part of the problem. they buckle n never sit perfectly flat on the film plane.

why add variables when a small piece of gg will cost almost nothing from a glasier.

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Hi paul,

Note that I'm using a ground glass. Not tape.