Arca Swiss 6x9 rollfilm back for Arca Swiss Field Camera

[JPlomley]

Cheers Gordon. What about positioning of the film plane? Once focus is achieved on the GG and I switch to the roll film back, do I need to apply a focus shift, or is the film plane spot on?

[Oren Grad]

Cheers Gordon. What about positioning of the film plane? Once focus is achieved on the GG and I switch to the roll film back, do I need to apply a focus shift, or is the film plane spot on?

There used to be a page on the medium-format megasite that had information on this, but it seems to be down now, so I'll paste in a relevant chunk here. The last number on each line is the film plane register for the respective model of roll holder:

Model - Type - Formats - Film - Depth
Cambo slide-in 6x4.5 120/220 4.95mm
Cambo slide-in 6x7, 6x9 120/220 4.95mm
Cambo slide-in 6x12 120/220 4.95mm
Horseman clip-on 6x7,6x9 120 4.95mm
Horseman clip-on 6x7, 6x9 220 4.95mm
Horseman clip-on 6x12 120 4.95mm
Linhof S-Rollex clip-on 6x7,6x9 120 4.85mm
Linhof Rapid Rollex slide-in 6x7 120 4.85mm
Linhof Techno-Rollex clip-on 6x12 120/220 4.85mm
Sinar Zoom slide-in 6x4.5 to 6x12 120/220 4.85mm
Sinar standard slide-in 6x7, 6x9 120/220 4.85mm
Toyo clip-on 6x7,6x9 120 5.05mm
Wista clip-on 6x7, 6x9 120 5.10mm
Wista Type DX slide-in 6x7, 6x9 120 5.10mm

The ANSI spec for depth to film surface for 4x5 holders is 0.197", which is equivalent to 5.0038mm. You can decide whether the theoretical discrepancy between this and the various numbers for the different roll holders is enough to worry about. I suspect that in virtually all roll holders, unevenness in the surface of the film itself contributes as much or more slop. Over the years I've owned various roll holder models from Cambo, Horseman, Linhof, Sinar and Toyo, and have never seen any systematic focus problem with any of them in my typical usage, which is with distant subjects and middling to small apertures.

[Bob Salomon]

Linhof Techno-Rollex clip-on 6x12 120/220 4.85mm

The Techno Rollex is 120 only. You can buy a conversion cassette to hold 220 film. But out of the box it is 120.

The Super Rollex 67 back is available as a 120 back or as a 220 back or you can buy a spare insert to convert from one to the other.

[Gordon Moat]

Cheers Gordon. What about positioning of the film plane? Once focus is achieved on the GG and I switch to the roll film back, do I need to apply a focus shift, or is the film plane spot on?

You could measure it with a dial caliper, which is basically what I did when I got mine. I found it matches my Fuji Quickload and Kodak Readyload set-ups, as well as the ground glass focus distance. Besides, it has to be accurate enough to do this:



This is a crop of a 56x72 frame. Selective focus was done by using tilts, and the talent was instructed to stay along a line parallel to the camera. The full shot shows a bit more of the selective focus, though due to being vertical orientation does not show as much detail.

Ciao!

Gordon Moat Photography

[Oren Grad]

So there are two questions here.

The first is whether there's a systematic offset between the standard for 4x5 film holders and the standard(s) for roll holders. Per the numbers I posted above, the answer to that is evidently yes, though you'd have to figure out whether the possible discrepancy matters relative to the depth of focus and required focus accuracy typical of your applications.

The other question is whether your 4x5 camera is built accurately to ANSI standard. That can't be taken for granted. So if you really do have ultra-critical requirements, you'd better measure and/or test everything - though measuring to this level, even with a good depth micrometer, is tricky when the pieces flex under pressure, as film holders and camera backs do.

Again, though, calculate the depth of focus you're going to need for your intended applications, and think about how critically you focus in the field, and how rigid your setup is, before you lose too much sleep over it.

[Emmanuel BIGLER]

Hello from France and happpy New Year to all.
My 0,02 euro to the discussion, regarding tolerances on film plane positioning.
The classical model for depth-of-focus suggests that the depth of focus can be computed as

DoFoc = plus or minus N.c

where N is the numerical aperture (effective numerical aperture in macro) and c the required diametre of the circle of confusion.

If we demand c = 50 microns on rollfilm like for classical DoF tables in 6x6 format, which is very strict for 6x9, @f/8, N=8, the tolerance is plus or minus 400 microns.
Taking into account that most LF users will actually stop down to a minimum of f/11, adding to this the fact that we sometimes work in the close-up range (at 1:1 magnification ratio, the effective numerical aperture N is doubled with respect to what is engraved on the ring, hence the tolerance is doubled) the question of accurately positining the film plane has to be revisited in a more relaxed way. Take it easy !

Sure, people using a 2.8-100mm planar (e.g. for the baby linhof) at full aperture, however, do have to take care about where their film plane is located, hence the tight tolerance in Linhof rollfilm backs, remember the good old days where people used a Linhof-selected, 135mm f 3.5 planar at full aperture on a daily basis

And about bulging effects of the backing paper in 120 rollfim, remember that Chris Perez has measured 96 line pairs per millimetre @f/8 with a freshly recalibrated Rolleiflex TLR on 120 rollfilm.
So in any case if bulging effects occur, and they are probably bigger in 6x9 than in 6x6, remember the figure of plus or minus 400 microns at f/8 !! And even if we demand c= 25 microns, like in 35 mm photography, we are left with a comfortable range of plus or minus 200 microns @f/8.

Now the issue is : what level of confidence can we put in the conventional model of DoFoc, based on geometrical optics ?
Recently with the help of friends who are lucky enough as professional photographers to have a full range of digital view camera lenses and a digital medium format back (to the best of my knowledge, silicon is not prone to bulging, at least in digital backs) we conducted a comprehensive range of manual focusing tests with a monorail view camera by recording many images at different positions of the focusing knob. Call it : bracketing with focus or whatever. The series of test is performed much quicker with a digital back than with film of course.
The conclusion is simple : when all lenses are stopped down to the same numerical aperture, namely we used f/11, the tolerance on film positioning, measured by reading the sharpness of test targets, is the same for all focal lengths stopped down at f//11, exactly as predicted by the geometrical model and the approx tolerance we derived from our tests matches perfectly the classical value of plus or minus N.c, no mystery.
There was no digital processing/edge enhancement of the image files, this is important since the test is definitely biased as soon as some edge enhancements of any other digital sharpening are applied before reading the test targets.

[JPlomley]

Oren/Gordon,

Thanks for the valuable information.

Again, though, calculate the depth of focus you're going to need for your intended applications, and think about how critically you focus in the field, and how rigid your setup is, before you lose too much sleep over it.

My application is landscape photography and therefore I tend to be stopped down mainly between f/22 -f/45. I focus very critically in the field using the focus spread technique, referencing the mm scale on the monorail. My set-up is extremely rigid since I added an "extension bracket" (poor nomenclature on the part of Arca Swiss) which nests the optical rail. This bracket than slides into a Z1 ballhead which sits on a Gitzo 1321 Leveling Base, itself secured to a 5-series Aluminum Gitzo pod (total weight of pod + levelling base + ballhead = 11 lb). I realize this is overkill and will likely pick up a carbon fibre pod this spring. The current tripod was purchased almost 15 years ago before carbon fibre came on the scene. At the time I was shooting wildlife with 300/2.8 and 600/4 lenses on Wimberly heads, so just got used to carry a heavy support system.

The other question is whether your 4x5 camera is built accurately to ANSI standard

Well, it is an Arca Swiss, so I would hope for some semblance of precision given their reputation. Perhaps not on par with a Linhof though.

[JPlomley]

The conclusion is simple : when all lenses are stopped down to the same numerical aperture, namely we used f/11, the tolerance on film positioning, measured by reading the sharpness of test targets, is the same for all focal lengths stopped down at f//11, exactly as predicted by the geometrical model and the approx tolerance we derived from our tests matches perfectly the classical value of plus or minus N.c, no mystery.

Emmanuel, nice tests. Hard to argue with practical results. You now have me wondering if I should purchase a few dedicated digital lenses for the 6x9 back.

[Emmanuel BIGLER]

My application is landscape photography and therefore I tend to be stopped down mainly between f/22 -f/45.

... hence the expected DoFoc even with a very strict circle of confusion of 25 microns will be
plus or minus 550 microns@f/22
plus or minus.... well @f/45.... diffraction definitely occurs so the geometrical model, for sure, is no longer valid.... but the image is gradually blurred by diffraction ; OK, roughly, expect someting like plus or minus 1 millimetre !
Even a non-ANSI standard home-made camera can achive this level of tolerance

[Emmanuel BIGLER]

You now have me wondering if I should purchase a few dedicated digital lenses for the 6x9 back.
During the test I have played with some Rodenstock "digital" beasts, including the recent, impressive and (somewhat) extravagant 28mm HR lens (OFF-TOPIC !! does not cover 6x9!!) but not enough to make serious conclusions about the use of those dream lenses on film.
Frankly, so far with an Arca Swiss camera (6x9 plus the 4x5 kit) I'm very happy with "film" lenses.
Including, for example, the classical 75mm 6.8 grandagon-N, a discontinued "film" lens designed for 4x5", so compact and light, so sharp, perfect for 6x9.
Taking into account the excellence of availables "film" lenses in the range of focal lengths from 35mm to 135mm, I am not sure that the purchase of the digital series actually make sense for film use, even if for example, the MTF performance of the last 70mm Rodenstock lens seams, at least on paper, incredible.
The real concern, if you want to scan your images, is that you'll have to use an expensive professional scanner in order to extract all the quality of what those classical film lenses are capable ; it will be even more difficult with "digital" lenses !

This question of scanning medium format images can be considered as a major drawback of using 6x9 instead of 4x5". With a 4x5" image you can use an "amateur" scanner and get more than acceptable results (depending of course on the size of the final print !).
The smaller the image, even if you use the best film and the best lenses, the better you have to scan them if you insist on processing them digitally (may the error is there stay analog+enlarger+wet darkroom !)
So in a sense, 4x5" and above can be consider as amateur formats of the digital age, in the sense : film formats that the amateur can decently scan with an amateur-grade scanner (e.g. one of the good flatbeds available today).