Critical focus and 120 film bulge

[blopin]

Hi strangers, 

I recently ordered some ground glass and a 7x loupe to calibrate the infinity focus on my Rolleiflex. In the past I discerned rumours that 120 film does not always lay flat on the pressure plate, and may bow outwards slightly. 

By how far does the film curve outwards? I wish to compensate for this to achieve the sharpest possible image. All replies are welcome. 

[rodeo_joe1]

9 hours ago, Lucas Lee said:

By how far does the film curve outwards?

Some to none at all. 

It's film for goodness sake! It's flexible and has an emulsion thickness. 

If you want an absolutely flat, sub-micron thin image-plane, shoot digital. 

Edited January 25, 2023 by rodeo_joe1

[Moto-Uno]

F8 ?  Peter

[ed_farmer]

The key word is "pressure" . . . It's the pressure plate and it's designed to hold the film extremely well in something like you Rollie.  In cheaper cameras with worn metal or plastic parts, this could be a problem but the center of the film is almost always flat and in proper registration.

Every once in a while, I start to wonder how we ever took sharp images in the 70's and 80's . . . We were barely even aware that something like this could be an issue.

[Dustin McAmera]

You're talking about a problem that might occur, sometimes, and to a variable extent. If you offset your focus by a fixed amount based on some estimate of how big the problem might be, it will be out by a little every time the film sits correctly.

I have heard people talk about the roll fim backs for Graflex cameras, saying that some hold the film more reliably flat than others - different rollers or something. That's with a 2¼x3¼ inch frame, with more length for the problem to occur. Also a detachable back, with a removable insert, and with the film wrapping 'inside out' round that insert.

There is a back for 70mm film for the Rolleiflex 6006 SLR with a tiny vacuum pump to hold the film against the plate. I guess that wouldn't work on film with a backing paper.

Overall, I think if what you have is a Rolleiflex, it's probably good enough. People have taken quite good pictures with them, I understand! 😁

Edited January 26, 2023 by Dustin McAmera

[kmac]

13 hours ago, Lucas Lee said:

In the past I discerned rumours that 120 film does not always lay flat on the pressure plate, and may bow outwards slightly. 

By how far does the film curve outwards? I wish to compensate for this to achieve the sharpest possible image. All replies are welcome. 

Actually it's not a "pressure" plate as such, it's a "locating" plate. It's there to stop the film from moving too far away from the body of the camera (film plane). There's a gap of a few thou between the film and the body, and a similarly small gap between the film and the plate, and those gaps are to allow the film to travel freely between the two without getting damaged. That's common knowledge, we all know that, but generally we fail to check how much tension the film is under to allow it to miss both the body and plate. Theoretically the correct plane for the film is halfway between the two, or if the camera has rollers, along the top of the rollers. If the tension on the film is not strong enough, the longitudinal curve in the film will come into play, and therefore throw the focus out a bit. 

Longitudinal curve is inherent in roll films, they want to go back to being rolled up again, it's the camera's job to straighten them and keep them straight while the shutter button is being pressed. The secret is to have adequate tension on the film so that it doesn't slacken off after you've wound to the next frame. In a quality camera like the Rollei, all this should be sorted out, but if you still suspect the film is too loose, then it'll need tensioning more. It's up to you how you tension it, I can't tell you what to to with your Rollei, but the film must be tensioned at both ends, the supply end, and the take-up end, from frame 1 to frame 12 (or the last frame)

I use translucent grease proof paper for checking focus, and in the beginning, silly me used to tape it tightly across the film plane and I wondered why my images were always out of focus a bit. Of course it had to be two or three thou (the thickness of a piece of paper) off the film plane, or across the rollers, if there were rollers in the camera. With the aperture opened right up to f1.4, f2, or whatever the camera has, I poke the grease proof paper to see how far it will go before the focus goes blurry, and that would give me an idea of how far the the film would have to move forward or backward before it too would be out of focus. At the full opening of the aperture, it didn't take much of a poke to make the translucent paper blurry, but it usually depressed in further than a film could move. All I was doing was checking that the film was within the depth of field at the widest opening of the aperture, but the film must be straight to start with when you eventually load the camera. 

Using frosted glass, it must be across the rollers in your Rollei, with the frosted side in towards the camera, because as you know, the film emulsion runs over those rollers, but if the film is slack, it can be slightly off the rollers and give you false focus, so again, the film needs to be adequately tensioned, at both ends, but not tensioned so much that the film will get marked. Fresh film is best for testing focus after any adjustments, fresh film won't have the strong curl in it that expired film acquires over years of standing around doing nothing. 

[blopin]

Thanks everyone, 

I guess common consensus is that even if the emulsion does not lie perfectly flat on the focal plane, the effects are negligible(?) and less so with a quality Rollei. 

4 hours ago, Dustin McAmera said:

You're talking about a problem that might occur, sometimes, and to a variable extent. If you offset your focus by a fixed amount based on some estimate of how big the problem might be, it will be out by a little every time the film sits correctly.

I have heard people talk about the roll fim backs for Graflex cameras, saying that some hold the film more reliably flat than others - different rollers or something. That's with a 2¼x3¼ inch frame, with more length for the problem to occur. Also a detachable back, with a removable insert, and with the film wrapping 'inside out' round that insert.

There is a back for 70mm film for the Rolleiflex 6006 SLR with a tiny vacuum pump to hold the film against the plate. I guess that wouldn't work on film with a backing paper.

Overall, I think if what you have is a Rolleiflex, it's probably good enough. People have taken quite good pictures with them, I understand! 😁

You make a good point with the variability of this theoretical issue. However, I’m sure there’s a zero point every time the film is freshly wound on, such that the film may jut slightly even under tension from the spools and pressure plate. After all, film curls like a watch mainspring in its natural state. 

2 hours ago, kmac said:

Actually it's not a "pressure" plate as such, it's a "locating" plate. It's there to stop the film from moving too far away from the body of the camera (film plane). There's a gap of a few thou between the film and the body, and a similarly small gap between the film and the plate, and those gaps are to allow the film to travel freely between the two without getting damaged. That's common knowledge, we all know that, but generally we fail to check how much tension the film is under to allow it to miss both the body and plate. Theoretically the correct plane for the film is halfway between the two, or if the camera has rollers, along the top of the rollers. If the tension on the film is not strong enough, the longitudinal curve in the film will come into play, and therefore throw the focus out a bit. 

Longitudinal curve is inherent in roll films, they want to go back to being rolled up again, it's the camera's job to straighten them and keep them straight while the shutter button is being pressed. The secret is to have adequate tension on the film so that it doesn't slacken off after you've wound to the next frame. In a quality camera like the Rollei, all this should be sorted out, but if you still suspect the film is too loose, then it'll need tensioning more. It's up to you how you tension it, I can't tell you what to to with your Rollei, but the film must be tensioned at both ends, the supply end, and the take-up end, from frame 1 to frame 12 (or the last frame)

I use translucent grease proof paper for checking focus, and in the beginning, silly me used to tape it tightly across the film plane and I wondered why my images were always out of focus a bit. Of course it had to be two or three thou (the thickness of a piece of paper) off the film plane, or across the rollers, if there were rollers in the camera. With the aperture opened right up to f1.4, f2, or whatever the camera has, I poke the grease proof paper to see how far it will go before the focus goes blurry, and that would give me an idea of how far the the film would have to move forward or backward before it too would be out of focus. At the full opening of the aperture, it didn't take much of a poke to make the translucent paper blurry, but it usually depressed in further than a film could move. All I was doing was checking that the film was within the depth of field at the widest opening of the aperture, but the film must be straight to start with when you eventually load the camera. 

Using frosted glass, it must be across the rollers in your Rollei, with the frosted side in towards the camera, because as you know, the film emulsion runs over those rollers, but if the film is slack, it can be slightly off the rollers and give you false focus, so again, the film needs to be adequately tensioned, at both ends, but not tensioned so much that the film will get marked. Fresh film is best for testing focus after any adjustments, fresh film won't have the strong curl in it that expired film acquires over years of standing around doing nothing. 

I did not know that the emulsion and backing paper does not touch the locating plate and guide rails. Is this true? When I close the Rolleiflex there is a spring-like force exerted, similar to that of film being sandwiched between the pressure plate and guide rails of a conventional 35mm camera. 

[rodeo_joe1]

 

5 hours ago, Lucas Lee said:

I did not know that the emulsion and backing paper does not touch the locating plate and guide rails. Is this true?

Sadly, yes, it's true and Kmac is right; the film plane location in most rollfilm cameras is a bit rubbish. 

Here's the inside of my old Yashica-mat - 

And a closer look at the film guide rails - 

The 4 outer 'pips' are slightly raised and mate with the 4 tabs on the pressure-plate to prevent it contacting the inner rails - leaving a thin channel for the film to 'float' in. 

You can see from the film spool width that the film+backing paper fits between those 4 outer raised guides.

The rollers either side of the film gate are lower than the film rails, and serve no part in tensioning or positioning the film, apart from preventing it from scraping on the rails and film gate. 

I measured the relative height of the guide rails with a digital depth gauge, as well as I could, and there's approximately 0.5mm difference in their height. 

The film-rail arrangement in my Mamiya 645 cameras is very similar, and again I measured a 0.5mm difference in height. I would be quite surprised if a Rollei TLR was much different. 

Then I measured the thickness of a strip of processed Ilford 100 Delta 120 film; it was 0.12 mm thick and its discarded backing paper measured 0.11 mm thick. However, unprocessed film is a little thicker than when it's processed, so if we say a total film+backing-paper thickness of 0.25 mm, we won't be far off. That leaves a gap of 0.25 mm (about 1/100th of an inch or 10 thou) for the film to float about in. Which is not very precise IMO.

But having said that, the Yashi-mat and my Mamiyas have always delivered sharp and in-focus negatives. As has every other make and model of rollfilm camera I've yet used. So the system obviously works, despite its apparent sloppiness. 

In short, I wouldn't worry too much about film bowing in the gate. It's obviously designed to be a loose fit. 

Incidentally and FWIW, a focussing screen can be easily made by sticking some Scotch 'Magic' tape to some flat glass or acrylic sheet. The finely-dimpled surface of the tape is perfect for a focussing surface and resembles that of etched glass. Also, glass microscope slides can be easily broken to size after scratching them with a glass cutter. It's a better option than trying to flatten a bit of tracing paper adequately. 

Edited January 26, 2023 by rodeo_joe1

[kmac]

4 hours ago, Lucas Lee said:

I did not know that the emulsion and backing paper does not touch the locating plate and guide rails. Is this true? When I close the Rolleiflex there is a spring-like force exerted, similar to that of film being sandwiched between the pressure plate and guide rails of a conventional 35mm camera. 

In a camera with rollers, the film emulsion rolls over the rollers. That's why the rollers must be in very good condition, the emulsion can get marked if there's any nicks in the rollers.

The "pressure" plate seats itself on the body of the camera, it doesn't seat itself on the film, it would act as a brake on the film if it did. If you were to measure the gap between the plate and the film plane, you'll find it's the thickness of the film and backing paper plus 0.004", give or take thou. The film with it's backing paper needs clearance so that it can transport freely along. The backing paper might rub on the plate, but the clearance the film needs is still there. With 220 film, a different plate is required because there's no backing paper except for a short leader and a trailer.

If your Rollei is in good condition and hasn't been messed with too often, the infinity focus is probably ok ... but you asked about the film bowing. The film can only bow a few thou, in between the film plane and the plate, the plate stops it from bowing any further. I hope that answers your question.

[Dustin McAmera]

Some of my old Ensigns (1930s folding cameras and boxes) have a cover on the red window, which also retracts the pressure plate a little while you wind the film. Thus the channel is nice and open while the film is moving, but when you close the cover the pressure plate is put firmly back on the film. If you started winding without opening the cover, you'd damage the film.

 

Ensign Carbine No. 6, for 120 film.

[blopin]

To kmac and rodeo, both these answers have satiated my curiosity. Thank you so much for the very ample write up! 

I found out prior to your responses that the film indeed “floats” between the pressure plate and guide rails at 450 microns of allowance for Rolleiflex, and I was researching a definitive answer for the thickness of the emulsion + backing paper until your replies. 450 nm of gap - 250 nm of film = 200 nm divided by 2 gives us 100 nm of lateral play on both sides, and a sheet of printer paper comes in at 100 nm. The silver/gelatin composites are usually the top layer of C41 stock making calculations easier for us, no need to factor in depth. Hence, only a sheet of paper is needed to act as a shim between the guide rails and ground glass. Correct me on this if I’m wrong. 

24 minutes ago, rodeo_joe1 said:

I measured the relative height of the guide rails with a digital depth gauge, as well as I could, and there's approximately 0.5mm difference in their height. 

Is this between the “pips” and the guide rails? 

26 minutes ago, rodeo_joe1 said:

The rollers either side of the film gate are lower than the film rails, and serve no part in tensioning or positioning the film, apart from preventing it from scraping on the rails and film gate. 

And don’t you mean that the rollers are higher than the guide rails? I’m confused as to what you meant, because the rest of your reply was under the pretence of the rollers positioning the film between guide rails and pressure plate. There also needs to be tension for that to happen. 

[kmac]

9 minutes ago, Dustin McAmera said:

Some of my old Ensigns (1930s folding cameras and boxes) have a cover on the red window, which also retracts the pressure plate a little while you wind the film. Thus the channel is nice and open while the film is moving, but when you close the cover the pressure plate is put firmly back on the film. If you started winding without opening the cover, you'd damage the film.

 

The Koni Omega Rapid is much the same except the plate is actuated by the shutter release button, the plate is always standing back from the film, but the moment a shot is about to be taken, the plate moves into position up against the film to flatten it ... genius because the plate is miles away from the film when the film being transported along to the next frame.

[kmac]

50 minutes ago, Lucas Lee said:

Hence, only a sheet of paper is needed to act as a shim between the guide rails and ground glass. Correct me on this if I’m wrong.

You're not wrong, a 0.002"- 0.003" thick strip of paper on each rail will place the ground glass where it needs to be. But remember this is for very shallow depth of field stuff with the aperture as wide open as it can be. Otherwise, at f8 or even f5.6, it doesn't really matter all that much, but I would use the paper spacer anyway.

[rodeo_joe1]

2 hours ago, Lucas Lee said:

450 nm of gap - 250 nm of film = 200 nm divided by 2 gives us 100 nm of lateral play on both sides, and a sheet of printer paper comes in at 100 nm.

Ermm, that's out by a factor of 1000! There are 1000 nanometres in a micron (= 1/1000th of a mm, 10^-6 metres or 1 um). A nanometre is 10^-9 metres) - blue light has a wavelength of 450 nm for comparison. 

So, the Rollei's 450 micron spacing is actually 0.45 mm in more understandable units, and pretty close to the 0.5 mm that I measured on the Yashica-mat and Mamiya 645. That still leaves between 1/5th and 1/4 of a millimetre slop between film-rails and 'pressure' plate. 

However, as Kmac already said, the film wants to stay curved as it comes off the spool. Therefore any bowing will tend to be back toward the restraining plate with a TLR film-path. Not outward toward the lens. The film has no incentive at all to float exactly halfway between rails and plate. 

This same concave emulsion-side bowing can be seen in processed film when fitted to a glassless negative carrier in an enlarger. 

In view of which, I'd suggest that any camera focussing check be done at between 0.2 and 0.25 mm (that's common millimetres!) away from the lower guide rails. E.G. by packing a ground-glass or other focussing screen with two thicknesses of 80gm/sq-m copier paper, which I've just measured as being 0.11 mm thick. Cheaper copier paper will be correspondingly thinner at about 0.1 mm.

Edited January 26, 2023 by rodeo_joe1

[rodeo_joe1]

2 hours ago, Lucas Lee said:

Is this between the “pips” and the guide rails?

Yes. 

2 hours ago, Lucas Lee said:

And don’t you mean that the rollers are higher than the guide rails?

No. On the Yashica-mat both the rollers are definitely below the level of the rails, as can be seen in my 2nd picture.

[John Seaman]

2 hours ago, Dustin McAmera said:

Some of my old Ensigns (1930s folding cameras and boxes) have a cover on the red window, which also retracts the pressure plate a little while you wind the film

That's pretty clever of Ensign. I've never come across yjat system though.

[ed_farmer]

By the way . . . Back to the original issue . . .. I once bought a used Mamiya RB body very cheap because the owner said that it "wouldn't focus to infinity".  After I got it home and played with it for about an hour, I discovered that the ground glass was installed in the frame upside down.  Disassembled, reassembled and then shot hundreds of weddings with that body!  I've owned for over 25 years and it's now worth just about what I paid for it.

[James G. Dainis]

Glass plate cameras would seem to be the answer to film bulge.  No fancy-shmancy digital camera needed. I believe that up into the 1930s they were much more in use than one would expect.  Old mystery books that I read often have the bad guy breaking the glass plate negatives that a detective had taken.  The photos taken of the Cottingley fairies were taken by children using a glass plate camera in 1917.

https://en.wikipedia.org/wiki/Cottingley_Fairies

[John Seaman]

It may be worth mentioning that Rollei made plate backs for their TLR's, which presumably did not suffer from lack of film flatness, suggesting that this was not allowed for in the focus setting.

Edited January 26, 2023 by John Seaman

[rodeo_joe1]

3 hours ago, James G. Dainis said:

Glass plate cameras would seem to be the answer to film bulge.  No fancy-shmancy digital camera needed. I believe that up into the 1930s they were much more in use than one would expect.

Glass plates? Maybe, maybe not. 

I stupidly bought an old quarter-plate Thornton-Pickard SLR camera back in the mid 1960s, as an 'investment'. So how did that work out? - It's barely worth what I paid for it now, leaving inflation totally aside. (The camera itself dates to about 1920 BTW.)

Anyway, it came with both glass plate and cut-film holders, and I could fairly readily get glass plates for it up until around the mid 1970s. However, the glass plates cost 2 or 3 times as much as cut film and could only be dish developed in the dark. Whereas cut film could be developed in a daylight tank and handled without risk of cuts from sharp edges. 

About 10 times the number of film negatives could be stored in the same volume as the glass plates, and with far less risk of damage to either negatives or owner. 

The GG focussing screen of the camera needed to be adjusted between using glass plates and sheet film, because the register was slightly different. 

As far as image sharpness was concerned, there was no discernible difference. So it's easy to see why glass plates fell out of favour and became obsolete. 

WRT 'fancy-shmancy' digital cameras: I firmly believe that my old 12 megapixel original Canon 5D and Nikon D700 gave perceptually the same, or better, image quality than I got from using 6x6 or 645 medium format film. Those older digital cameras today can be got in excellent condition for far less than the now inflated price of a medium format SLR in good order. 

Edited January 26, 2023 by rodeo_joe1

[sal_santamaura]

On 1/25/2023 at 5:28 AM, Lucas Lee said:

Hi strangers, 

I recently ordered some ground glass and a 7x loupe to calibrate the infinity focus on my Rolleiflex. In the past I discerned rumours that 120 film does not always lay flat on the pressure plate, and may bow outwards slightly. 

By how far does the film curve outwards? I wish to compensate for this to achieve the sharpest possible image. All replies are welcome. 

In answer to your question:  the distance depends on a number of factors, especially diameter of reverse-curl feed rollers.  I won't relitigate this matter, with so many dismissing it out of hand despite evidence.  Rather, I refer you to a 22 year old thread in which my posts covered everything in detail:

 

[blopin]

15 hours ago, kmac said:

But remember this is for very shallow depth of field stuff with the aperture as wide open as it can be. Otherwise, at f8 or even f5.6, it doesn't really matter all that much, but I would use the paper spacer anyway.

If I paid for f/2.8 I’ll use it at f/2.8 😉 

14 hours ago, rodeo_joe1 said:

Ermm, that's out by a factor of 1000! There are 1000 nanometres in a micron (= 1/1000th of a mm, 10^-6 metres or 1 um). A nanometre is 10^-9 metres) - blue light has a wavelength of 450 nm for comparison. 

Oops I used the wrong unit notation for micrometers. It should be μm, but the same principle stands. 

13 hours ago, rodeo_joe1 said:

Yes. 

No. On the Yashica-mat both the rollers are definitely below the level of the rails, as can be seen in my 2nd picture.

I could have sworn the Rolleiflex rollers were above the guide rails, it even depicts it in a cross section diagram of the user manual. But this could be idiosyncratic to Rollei. I don’t have the camera with me now as I’m only home on weekends, I’ll be sure to check then. 

Edited January 27, 2023 by Lucas Lee

[AlanKlein]

I wonder how much is off focus due to the groundglass being off possibly due to the mirror in an SLR?  Or any camera for that matter including large format where the film  holder mechanism is not holding the film holder exactly in the right place? Will older cameras have this problem more often?

Edited January 27, 2023 by AlanKlein

[blopin]

20 hours ago, rodeo_joe1 said:

No. On the Yashica-mat both the rollers are definitely below the level of the rails, as can be seen in my 2nd picture.

Good grief, same for the Rolleiflex. My guess is that when the film runs off the rollers it continues to curve slightly instead of having a perfectly straight tangent. Like when you bend a piece of paper over the edge of a table and it curves at the corner instead of assuming a right angle. That is my only theory as to how the film floats between the guide rails and pressure plate. Will this mean that the film will rest against the pressure plate given time to “relax”? 
 

 

[conrad_hoffman]

Using a ground glass seems like a perfectly logical way to test and adjust. It's better than nothing, but the way it's done correctly is to put the camera under an autocollimator and view the image as it projects to the film. It's then trivial to set the infinity focus or check various other points. Works with small and medium format; never tried it with large format, but that seems less of a problem due to slow lenses and larger scale of everything. Sometimes you can find these on eBay- https://www.fluidr.com/photos/29504544@N08/sets/72157629450378049