Pressure Plate Pressure Difference between 120 and 220 Film Backs

[mtjade2007]

I looked at two Mamyia Pro SD film backs today, one of 120 and the other of 220 back. I had the slides off and pressed the pressure plates with my finger. I knew it was not going to be any precision in any way but I just could not feel any pressure difference. I opened up both backs and still I could not see anything different between the pressure plate mechanisms. I got the impression that the 120 pressure plate is supposed to be lighter in pressing against the 120 film with paper back. Is this really true?

The reason I ask is I am going to load 46mm film of 120 film length in 220 film style, meaning with leader and trailer paper attached to the film only. The reels I use will be simply the 120/220 reels with no modification. I will just use my left over 220 paper leader and trailer taped to the front and rear end of the 46mm film. Basically it is going to be a 46mm film of the length of 120 but with 220 paper leader and trailer. It will be used in the camera as a 120.

I don't think I will have a pressure plate issue in terms of the pressure against the film. I am doing this so that I can put my expired 46mm film stock to use to shoot panoramic pics. Any thoughts anyone?

 

[Deleted member 88956]

If I were to do something like this with all the trouble of taping lead/trailer to film stock, I'd rather use 120 backing paper at its full and use 46mm film within it, so full "120 style" to paraphrase yours.

 

[mtjade2007]

Thanks for the suggestion. I simply do not have many 120 paper backs collected. Rather, I have a large bag of 220 leader and trailer paper that I can use.

 

[Deleted member 88956]

I see. Well in that case all I would say is to give one a try and see what you get. Some shots with critical focus in mind would help establish pressure plate issues if any. I doubt you would see any. I don't have 220 SD back to compare, but 120/220 are typically in the thickness of the plate NOT pressure applied itself. In other words, pressure is in same ball park, so 120 film in 220 back gives some more resistance, and less if things are backwards. 120 in 220 was still an issue with motor driven systems (some claiming accelerated wear on gear), but I never saw that on Pentax 645 inserts.

 

[AgX]

The proper way of adapting a camera would be changing the height of lateral guide rails, respectively of changing protrusion of contact areas at the pressure plate to such guides.

 

[Vaughn]

Going by a similar feature in Rolleiflex cameras, the shift in the pressure plate moves the pressure plate slightly forward to compensate for the lack of backing paper with using the 35mm film in the adapter. That way, the 35mm film remains on the same plane (for critical focus) as when using 120 film with the added thickness of the backing paper.

I suppose the same issue applies to 120 vs 220 when using backs that take both. One has to make sure the film is on the same plane of focus with and without backing paper adding to their thickness.

 

[reddesert]

I don't know about those specific (RB67) backs. However, I can describe other backs I've looked at, including other Mamiya systems (220 inserts for the 645, and the switchable 120/220 pressure plate in the Mamiya Press backs). The difference is not in pressure = force. Rather, the 220 pressure plate is shaped so that it positions the film slightly farther forward, to compensate for the lack of backing paper. On the 645 and Press systems, the outer edge of the pressure plate is relieved (cut away) very slightly, so that when the outer edge of the plate rests on the outer film rails, the center of the plate is very slightly farther forward, by about 0.1 mm.

For the RB67 backs it may be a little different since the entire back is different between 120 and 220, but there's probably a similar relief somewhere.

It sounds like you want to use the 46mm film with leader and trailer but no backing paper in a 120 back (why not use it in a 220 back?) which means the film will be slightly farther back, so you'll be focused closer than the viewfinder says. However, the difference is almost negligible.

The difference in film position of 0.1 mm means that, for a circle of confusion of 0.05mm, the depth of focus just covers the focus offset at an aperture of f/2. For critical work, one would want to do better than just barely cover the offset. But for practical purposes, at an aperture of f/4 or slower, I don't think the focus offset will be noticeable. Since many medium format lenses are that slow anyway, I don't think it's a big deal. Making sure that the edges of the narrow film don't curl up might be more important.

 

[itsdoable]

There is a gap between the film pressure plate and the film rails, which is set to the thickness of the film and paper backing with 120. The gap for 220 film is less, just the thickness of the film.

I placed diagrams for the P645 120/220 system here in the other forum:
https://www.pentaxforums.com/forums...645n-120-film-220-magazine-4.html#post4584266
(Mods, if posting links to another forum is not kosher, let me know and I'll upload the pictures.
There was a lot of talk about how to convert 220 P645 film insert to 120.

Although the additional pressure from the film back may create more drag on the system when you use 120 paper backing in the 220 film back, that is usually not an issue. The main issue it film flatness. The gap, along with a slight curl on the film, helps keep the film flat, and resist buckling.

The gap is set by the outer rails in the case of the P645 (and many others). With Rolleiflex, the shift of the pressure plate causes the tabs on either side of the pressure plate to sit on lower/higher stops to adjust the plate height.

In practice, most people running 120 film in a 220 bask do not notice any issues.

 

[mtjade2007]

I looked at the backs more closely today. I think the pressure plate has nothing to do with where the focus plane is. The paper back of 120 films also has nothing to do with the focus plane either. In fact the pressure plate should not be called as pressure plate. It should be called as film back plate. It is designed to give no pressure to the film at all. I cut a 6 inch long film and put it in the back. After closing the back I removed the slide. I found I could move the film easily with just one finger pushing in the film's travel direction back and forth. I found that the film is actually hold up by the two pairs pf rollers in the back. When the back is closed the rollers will kind of squeeze the film. Since they are rollers so the film can still move freely. The rollers determine where the focus plane really is. I think I have the answer of my question. I can shoot 46mm film without worrying if the focus plane is shifted. The pressure plate does not affect focusing at all.

 

[MattKing]

The pressure plate does not affect focusing at all.

No, but it keeps the film in contact with the brails, which do affect focusing.
As posted above, the pressure is provided to keep the film flat and where it is supposed to be.
A very long time ago Mamiya America maintained a user forum for its medium format film camera users. They included a FAQ section which discussed using 120 film in 220 inserts/backs. They warned that while it worked, it could lead to premature wear of the 220 inserts/backs. Of course, in those days, a lot of 220 inserts/backs were used with 220 film, in high volume applications.

 

[mtjade2007]

I should have said the pressure plate does not apply pressure against the film. I tried with a piece of film+back paper in the magazine. I could move the film easily still. Yes, the plate is there to keep the film in focus. It keeps the film from curling toward the direction of the plate. The actual focusing plane is established by the rollers.The interesting part is on a 120 magazine if I put only film in it and check if there is any gap between the plate and the film. There is none. It demonstrates how precision there is built-in in the magazine. When I tested a strip of 46mm film in the 220 magazine there is also zero gap between the film and the plate. This tells me I can shoot 35 mm and 46mm film in it without worrying about focus shift issues. Film curling along the top and the bottom edges is still an issue though.

 

[Deleted member 88956]

I should have said the pressure plate does not apply pressure against the film. I tried with a piece of film+back paper in the magazine. I could move the film easily still. Yes, the plate is there to keep the film in focus. It keeps the film from curling toward the direction of the plate. The actual focusing plane is established by the rollers.The interesting part is on a 120 magazine if I put only film in it and check if there is any gap between the plate and the film. There is none. It demonstrates how precision there is built-in in the magazine. When I tested a strip of 46mm film in the 220 magazine there is also zero gap between the film and the plate. This tells me I can shoot 35 mm and 46mm film in it without worrying about focus shift issues. Film curling along the top and the bottom edges is still an issue though.

Pressure plate is called that for a reason. It presses film against film gate. Your test is not telling you this. Take a precision ground straight edge and put across rollers over the plate, you will see you need to press down the plate in order to meet the rollers. Besides, how else would film be kept flat it it had no provision to be pressed against film gate. Film gate has precision rails running along the length of film and that's where film is actually touching the gate at ALL times (not across the frame though, those rails are below that level ever so slightly).

The point here is that if you want to do what you want to do you need to run a test roll with focus test shots and see if its good enough. But there is no question 120 back is NOT designed to handle film without backing paper and meet original tolerances. But it may be enough to satisfy your idea of using it.

 

[AgX]

There were different concepts applied to hold film, of any kind, at the film gate.
The latest is to form a channel of fixed dimensions through which the film runs. But this is not applied at all latest models of cameras.

 

[mtjade2007]

I looked at my Pro-SD backs again today and I realized that the pressure plate indeed applies some pressure against the film and the film in turn presses against the top and the bottom chrome rails. So the chrome rails indeed are there to set the focusing plane. However, the two pairs of reels at the leading and trailing edges of the film gate also play a same roll in establishing the focusing plane as well. The mechanism is not complicated but the precision of it is critical. It keeps the emulsion side of the film firmly and right on the focusing plane but does not squeeze the film hard and allow the film to travel through the film gate easily. Based on my observation, if 35mm or 46mm film is in there the film will firmly stay on the focusing plane as well except the top and bottom edges of the film. I agree I will need to physically shoot some film to find if there is or not focusing issues.

 

[mtjade2007]

There were different concepts applied to hold film, of any kind, at the film gate.
The latest is to form a channel of fixed dimensions through which the film runs. But this is not applied at all latest models of cameras.

I looked at a 35mm camera today. It does not have reels at the leading and trailing edges of the film gate. The two chrome rails at the top and the bottom of the film gate are for establishing the focusing plane indeed. However, the pressure plate is on much softer spring so it applies very slight pressure against the top and bottom of the film only. The pressure plate does help keeping the film flat and in focus.

 

[Deleted member 88956]

I looked at a 35mm camera today. It does not have reels at the leading and trailing edges of the film gate. The two chrome rails at the top and the bottom of the film gate are for establishing the focusing plane indeed. However, the pressure plate is on much softer spring so it applies very slight pressure against the top and bottom of the film only. The pressure plate does help keeping the film flat and in focus.

It does not matter what kind of pressure, it does apply pressure and that is how it functions. If pressure plate has any tiny nicks it will scratch back of the film too as film rides against it, no matter how slightly. Looking at Mamiay's film inserts for 645, they did what I stated earlier, 220 pressure plate is formed forward to compensate for lack of backing paper and most likely same is done in RB backs.

 

[AgX]

I looked at a 35mm camera today. It does not have reels [rollers] at the leading and trailing edges of the film gate. The two chrome rails at the top and the bottom of the film gate are for establishing the focusing plane indeed. However, the pressure plate is on much softer spring so it applies very slight pressure against the top and bottom of the film only. The pressure plate does help keeping the film flat and in focus.

No 35mm camera comes to my mind that got rollers at the film gate.


The typical latest kind of film guides at the film gate consist of:

-) a rail at each each side of the film gate (in film travelling orientation) that forms the focal plane
-) a higher rail at some distance at the outer alogside to this that forms a lateral guide for the film, but more important it forms the arrest for the pressure plate
-) at MF added by a roller at each side of the film gate, as a means to form the focal plane in transverse orientation

Thus a channel is formed for the film (with or without backing paper). The height of this channel should be about the thickness of the film resp. film+backing paper.
This height and the gaps between the lower and higher rails are intended to yield best average position of the film at the focal plane.

 

[BrianShaw]

No 35mm camera comews to my mind that got rollers at the film gate.

Nikon F3 has a roller on the body side at film cassette end. I think on the other side there is another roller is on the film back.

See steps 9 and 10 of the film loading procedure:
https://cdn-10.nikon-cdn.com/pdf/manuals/archive/F3.pdf

 

[mtjade2007]

I also looked at my p67-ii. There are rollers on the leading and trailing side of the film gate while chrome rails are on the top and the bottom. It is quite obvious that the chrome rails set the focusing plane no doubt. The pressure plate pressure is compensated for 220 at the pressure plate by moving it left or right a small step. I put a strip of 46mm film in the camera and fired the shutter setting to B and removed the lens to observe into the film gate. I found the film is resting flat and firm on the pressure plate. I don't feel there will be focus shift issue at all but again only actual shooting with the film will prove it.

Thanks to everyone's input to my inquiry. Every input of yours provides me hint to look into the Pro-SD's film back for more accurate observation that leads to correct conclusion. One ironic conclusion that I can make is the top and the bottom chrome rails are where the focusing plane is. It is so obvious. I guess I undermined the simplicity of it initially. I should have looked at my P67-ii's mechanism first. Its design is much cleaner and simple.

 

[mtjade2007]

-) a rail at each each side of the film gate (in film travelling orientation) that forms the focal plane

I think rails should be used on the leading and trailing edges of the film gate to reduce friction and prevent scratching of the film. Probably most 35mm cameras do not have rails nor rollers there because the film area is much smaller. The pressure plate alone is enough to flatten the film.
.

 

[AgX]

Yes, at MF such rollers were already introduced with box-type cameras.