Ground glass vs. roll film discrepancy

[OAPOli]

I'm working on a Rapid Omega 6x7 and found the following statement in the repair manual:

Ground Glass Focusing Screen ​

​

The ground glass focusing screen is 0.2mm closer to the lens than the plane of the camera film gate . This is to allow for the surface characteristics of roll film. Although the mechanical distance from flange back to film gate is 66.8 mm , the real image projected by the lens is formed at a distance of 66.6mm.​

​

What are those surface characteristics? I suppose there is some bulge. Happy to hear your ideas.

 

[koraks]

What are those surface characteristics?

I think they refer to the thickness of the film, with a slight allowance for small variations. I'd consider it a suboptimal formulation the way they put it there.

 

[Dan Daniel]

Weird. It sounds as if the film itself is getting pushed forward into the film gate, ahead of the edges?

0.2mm is not a small amount for focus.

 

[Light Capture]

I found similar implied statement in one of the old Rolleiflex service manuals.
They had difference in calibration planes for that particular TLR. I don't remember which one was that.
There were also different setting on pressure plate for different types of these.
My method is to use piece of film and check the reflection with the collimator. That's not really available to everyone but digital camera with target can be used.
Not as fast as collimator but it will work if used carefully. Focal length of 2-3 times longer than the focal length of the calibrated lens is still recommended. This is also required to minimize impact of the collimator's own calibration on final result.
The best is to use known cheap lens and calibrate it to infinity and tape/glue the focusing ring (moon is the best target, something few km's away will work as well).
The biggest challenge with this method is that target in the camera needs to be lit in some way. I've used glass slide cover with clear frosted tape over it. Few dots with the marker will be a good target.
If using film with the camera's pressure plate same marker dots can be used but some kind of illumination is needed inside the camera, behind the lens to illuminate film surface. Small LR44 button battery with LED would work. It needs to be shielded so it doesn't shine into the camera.
Later calibration mirror fixture for Rolleiflex cameras is flat and the calibration is done at the same plane as film rails.

For all this to line up, body length needs to be correct, back surface to film rails distance needs to be correct and tested and lens needs to be collimated to specifications. Lens can be only properly collimated if the first two are correct or if the calibration fixture for collimator is available.

It all seems to depend on the design of the particular camera.
Hasselblad doesn't mention anything like that and Leica doesn't seem to mention that anywhere.

Another piece of information is an old paper by Carl Zeiss where they claim that film flatness is never perfect. If I recall the numbers correctly they would be in that 0.2mm range. Can't find it now. It was a scan of a very old document.
But this doesn't seem to be a general rule and should be checked with piece of film on each camera. With purpose made lens collimator it's a quick check. There is a number of DIY designs or used collimators out there. I made one some time ago with broken Nikon SLR with ground glass taped on film rails and permanently attached long lens. Small plastic 30-60x microscope can be used to look at it and is also recommended for focusing calibrations on focusing screens. They're usually sold for $15-25. Digital camera with macro lens can be also used for live view.

 

[ic-racer]

I had a Rolleiflex SLX that was notorious for this. Easily identified by putting the camera on "B" with no lens. The tip of a pencil touching the film clearly showed how much the film was bulging forward.

I will say the 6008 style film back of the later cameras eliminated the problem for me. Realize the SLX prototypes were shown back in 1974.

 

[sojournermike]

I’ve been calibrating focus on some M mount lenses using an appropriately sized piece of ground glass against the film rails and a macro lens and 20x magnified live view on my mirrorless camera to view the ground glass. I tend to calibrate the M at 1.5 to 2 metres and then everything else is OK as long as the focus ramp is correct.

More fun to be had with an f1.1 sonnar design lens that has pretty monumental focus shift through to F2.8 - at the moment I’ve left it calibrated at f1.1 and work on the basis that if I’m outside and focus on the closest thing I want sharp then everything will work out…

 

[ags2mikon]

If I remember correctly, I think that the Graflex XL manual had a similar value of 6-8 thousands of an inch offset too.

 

[ic-racer]

Some more information here from Zeiss:

Zeiss Camera Lens News No. 10... (Film Bulge)

I used to have a copy or link to Zeiss Camera Lens News No. 10, but can't find it any longer. Anyone have this PDF? I'm interested in a hard copy of the information that is paraphrased below: (in Camera Lens News No. 10) the bulge increases with time after winding medium format film: it's...

www.photrio.com

 

[OAPOli]

So a 0.2mm bulge on the centre seems to be somewhat accepted. One adjusts the body length to 0.05mm while there is 4 times the variation in film height! Fortunately there is some depth of focus, which goes up with f-number and magnification.

@Light Capture using a target on a piece of film to collimate lenses is a good idea. I once tried it but the results were a bit different... so I dismissed the method.

Now how can one make a piece of glass with 0.2mm recesses...

 

[sojournermike]

So a 0.2mm bulge on the centre seems to be somewhat accepted. One adjusts the body length to 0.05mm while there is 4 times the variation in film height! Fortunately there is some depth of focus, which goes up with f-number and magnification.

@Light Capture using a target on a piece of film to collimate lenses is a good idea. I once tried it but the results were a bit different... so I dismissed the method.

Now how can one make a piece of glass with 0.2mm recesses...

It’s tempting to suggest a Dremel…

Perhaps more pertinently, maybe wind before exposing rather than afterwards. 0.2mm is quite a lot for a wide angle.

 

[nosmok]

The Kodak Medalist 620 camera also had a different focal distance for its plate and sheet film backs-- but the backs themselves had IIRC a tab of some sort that would bring the RF into alignment for these accessories (I have one of these backs, and a Medalist, but I don't remember ever using the combo).

 

[Dan Daniel]

Now how can one make a piece of glass with 0.2mm recesses...

Mmill down the edges of a plastic focus screen?

 

[snusmumriken]

Coincidentally (or perhaps not?), Harold Merklinger discusses the consequence of a 0.2mm shift in the film plane in his book The Ins and Outs of Focus. It varies with focal length of the lens. Since the book is freely available on the internet, I guess I may quote it here:

<<
Well, let’s suppose we are photographing a person indoors. He is seated in a high backed rocker about 10 feet from our camera. About three feet behind him is a window. Through the window we can see part of the house next door and beyond that the mountains. We use a variety of lenses from 300 mm to 20 mm in focal length. Unknown to us a bit of film is stuck on the film pressure plate of the camera and so the film is actually .2 mm closer to the lens than it is supposed to be. What objects will be in focus? Using the basic lens formula, Equation (1), we obtain the answers shown in the table below. f is the focal length of our lens, Dis the distance from the lens to the object actually in focus. The subject’s eyes are at a distance about 3.000 meters (about 10 feet) from our lens.

From this table we find a number of interesting results. With the 300 mm lens, we focused on the bridge of the subject’s nose, but his eyes are in perfect focus. With the 200 mm lens we focused on his eyes, but his ears are perfect. With the 100 mm lens we can see every grain in the wooden back of his rocker. With the 50 mm lens the window is perfect. With the 28, the house next door is just beautiful. With the 25, those mountains aren’t bad at all. With the 20, nothing, absolutely nothing is in focus. We have a soft focus effect throughout!

What we see here is that long lenses are actually quite tolerant of errors at the film plane. Short focal length lenses, on the other hand, require absolute precision!
>>

Merklinger is talking about 35mm here, but earlier he discusses the effect of format, concluding that although the geometry of larger formats means they are more tolerant of errors at the film plane, they have a larger image area for film curvature to manifest, and this will have the greater impact. Hence cut film, vaccuum film holders, or even glass plates for critical work; also very small apertures.

 

[Dan Fromm]

Hmm. All this theorizing is great fun but so far no one has addressed the real question, which is empicical. Does it matter?

OP, I suspect that you have the camera that started this discussion. If you do, please shoot some film with it, carefully focused using the presumably well-calibrated rangefinder and bracket the focused distance by tiny increments. Repeat at a number of distances. Report.

 

[BrianShaw]

Well… that makes sense but wouldn’t be as much fun as overthinking!

 

[reddesert]

Coincidentally (or perhaps not?), Harold Merklinger discusses the consequence of a 0.2mm shift in the film plane in his book The Ins and Outs of Focus. It varies with focal length of the lens. Since the book is freely available on the internet, I guess I may quote it here:

<<
Well, let’s suppose we are photographing a person indoors. He is seated in a high backed rocker about 10 feet from our camera. About three feet behind him is a window. Through the window we can see part of the house next door and beyond that the mountains. We use a variety of lenses from 300 mm to 20 mm in focal length. Unknown to us a bit of film is stuck on the film pressure plate of the camera and so the film is actually .2 mm closer to the lens than it is supposed to be. What objects will be in focus? Using the basic lens formula, Equation (1), we obtain the answers shown in the table below. f is the focal length of our lens, Dis the distance from the lens to the object actually in focus. The subject’s eyes are at a distance about 3.000 meters (about 10 feet) from our lens.
View attachment 367847
From this table we find a number of interesting results. With the 300 mm lens, we focused on the bridge of the subject’s nose, but his eyes are in perfect focus. With the 200 mm lens we focused on his eyes, but his ears are perfect. With the 100 mm lens we can see every grain in the wooden back of his rocker. With the 50 mm lens the window is perfect. With the 28, the house next door is just beautiful. With the 25, those mountains aren’t bad at all. With the 20, nothing, absolutely nothing is in focus. We have a soft focus effect throughout!

What we see here is that long lenses are actually quite tolerant of errors at the film plane. Short focal length lenses, on the other hand, require absolute precision!
>>

Merklinger is talking about 35mm here, but earlier he discusses the effect of format, concluding that although the geometry of larger formats means they are more tolerant of errors at the film plane, they have a larger image area for film curvature to manifest, and this will have the greater impact. Hence cut film, vaccuum film holders, or even glass plates for critical work; also very small apertures.

In my opinion this discussion is flawed. It neglects the depth of field in the subject space at the actual taking aperture in use, which is greater for wide angle lenses at a given f-number. The depth of focus at the film plane, to first order, depends on the f-number in use, and not on the focal length. Because the f-number defines the taper of the cone of light toward/away from the point of best focus at the ideal film plane. If you have a 0.2mm focus offset, but an acceptable circle of confusion of 0.03mm, then the f-number that allows depth of focus to cover the error is 0.2/0.03 = f/6.7. This is true whether you are using a 200mm lens (where the focus offset at the subject is small, but the depth-of-field is small), or a 20mm lens (where the focus offset is past infinity, but the depth-of-field is large in the subject space).

Pictorially, the effects of a film plane offset might still be worse for wide angle lenses because one is trying to get the entire picture sharp from near to far, as opposed to tele lenses where one accepts or wants some blur of the subject away from the focus point. But this text gives a misleading impression.

I agree with Dan that the proof is empirical, at the small-offset level one has to test the system on film before knowing whether to get agitated or modify the camera over a nominal 0.2mm offset.

 

[Mark Crabtree]

In my opinion this discussion is flawed. It neglects the depth of field in the subject space at the actual taking aperture in use, which is greater for wide angle lenses at a given f-number. The depth of focus at the film plane, to first order, depends on the f-number in use, and not on the focal length. Because the f-number defines the taper of the cone of light toward/away from the point of best focus at the ideal film plane. If you have a 0.2mm focus offset, but an acceptable circle of confusion of 0.03mm, then the f-number that allows depth of focus to cover the error is 0.2/0.03 = f/6.7. This is true whether you are using a 200mm lens (where the focus offset at the subject is small, but the depth-of-field is small), or a 20mm lens (where the focus offset is past infinity, but the depth-of-field is large in the subject space).

Pictorially, the effects of a film plane offset might still be worse for wide angle lenses because one is trying to get the entire picture sharp from near to far, as opposed to tele lenses where one accepts or wants some blur of the subject away from the focus point. But this text gives a misleading impression.

I agree with Dan that the proof is empirical, at the small-offset level one has to test the system on film before knowing whether to get agitated or modify the camera over a nominal 0.2mm offset.

reddesert, thank you for that explanation. I've thought and said something similar for years based on my personal experience with misfocusing medium format cameras but never did the math.

On a different note, there was mention early in the thread about Rolleiflex gg back. I tried, but could not find a reference to the relative focus point/back focus of the ground glass back. I compared to ground glass at the machined film plane in the camera and decided it was the same, but maybe I wasn't precise enough. My impression looking at the Rollei back is that the bare gg is pressed against that surface. There still could be a difference is in the recommended method to adjust the focus screen, but I've never seen that mentioned (I'll check my Nat Cam book), and it seems like it would be hard to implement for a widely popular camera with lots of people working on them in their day.

Overall this is an issue I've been aware of with rollfilm cameras. Because of the pressure plate, it is obvious that film and only move closer to the lens, which would move the focused point farther away (farther from the camera). I don't believe 35mm cameras suffer much from this, though if the film remains in the film gate for an extended time, them maybe it would be an issue.

 

[ic-racer]

I don't believe 35mm cameras suffer much from this, though if the film remains in the film gate for an extended time, them maybe it would be an issue.

I had a Rolleiflex SL2000F (35mm camera) that would bulge the film in the middle with the original Tmax film in the mid 1980s.

 

[BrianShaw]

So a 0.2mm bulge on the centre seems to be somewhat accepted. One adjusts the body length to 0.05mm while there is 4 times the variation in film height! Fortunately there is some depth of focus, which goes up with f-number and magnification.

@Light Capture using a target on a piece of film to collimate lenses is a good idea. I once tried it but the results were a bit different... so I dismissed the method.

Now how can one make a piece of glass with 0.2mm recesses...

Intended to be an innocent non-offensive question: what makes anyone think that Simmons/Omega either got it wrong or didn’t know what they were doing? Wasn’t that a professional camera with years of professional experience?

 

[OAPOli]

@BrianShaw I don't think Omega got it wrong, since the issue is corroborated by a few other manufacturers and specific equipment was made to address it. So it's past theorizing. Now does it matter pictorially at reasonable enlargements? I'll be testing it out for sure.

 

[BrianShaw]

Enlargement may be where any potential effect might be seen. Depending on what “reasonable” is… not too sure that enlargement was within the primary original use-case as a press camera. I could be wrong, though.

Looking forward to your empirical findings!

 

[BrianShaw]

Another ignorant question: isn’t that the version that didn’t clamp the film down to film plane until after it was wound? I seem to recall that was for both scratch-reduction and film flatness. I don’t have first-hand experience with them, just recollections, so please correct me if I’m in error.

 

[OAPOli]

Yes in that camera the pressure plate retracts when the film is transported, then returns.

 

[Mark J]

Older folding cameras suffer from greater problems in this way. I ran foul of this when using an Ensign Autorange 6 x 9 cm many years ago. The rollers and film gate are not as sophisiticated as modern MF cameras, and shots at f/5.6 and 8 would be a bit hit & miss. The worst thing to do was wind on the camera after a shot was taken, then close it up, come back later and open and deploy the camera. Something about opening the thing up probably caused suction and pulled the film further away from the platten. You had to have discipline to only wind on just before you were going to fire the shutter, then the film would be tight.

 

[BrianShaw]

Discipline and a good memory!