Modify frame count on M645/M645 1000s 120 insert?

Years ago, I got a Zeiss Ikon 532/16 Super Ikonta B, and found it was designed to put only 11 frames on a roll of 120. A 6x6 camera usually gets 12. I could see why; starting the count with a red window on the 6x9 framing track (or so it seemed, now I think it was just uncertainty about film length in a format that didn't actually support 6x6 when the camera was designed).

Now I'm heading toward buying a Mamiya 645 1000s and find that they only get 15 frames on a 120 roll -- despite my having three red window cameras and a 6x4.5 back for my RB67 that all get 16.

There shouldn't be a problem with the film start -- the inserts are more compact than the film inserts for the 6x4.5 RB67 back. Further, the 645 back for the RB67 was putting 16 on a roll before the M645 came out, so we know Mamiya could count that high. Is there a good, sensible reason for this limitation? More importantly, is there a known mod for the inserts that will let them put 16 frames on a roll like everyone else seems to manage with 6x4.5?

I think that it is probably due to how "crooked" the film path is - which of course helps keep the inserts themselves very compact.
FWIW, the frames start and end at appropriate places on the film - there is neither too much nor too little unused film at either end - and the spacing and positioning matches exactly the alternate frame numbering (41 - 55) on the opposite side rebate included on Kodak films. The between frame spacing is good, and the 15 frames fit nicely in a three vertical channel Printfile page.
The match between the Kodak numbering and the Mamiya 645 framing makes me think that it may be at least a de facto standard, if not an ISO standard.

Okay, this begs the question of why Kodak put numbers 41-55 on the film rebate, as opposed to 1-16 on the other side (which never has 1-8, 1-10, or 1-12, only 1-16).

Kodak puts 1-12 on one side of the rebate - spaced to match 6x6 cameras - and 41-55 on the other side - spaced to match 15 exposure 6x4.5 cameras and of some use with 6x9 cameras.
An example that shows the numbers on a 6x6 strip:

I don't know that I have a scanned example with rebate numbers fron a roll of 6x4.5.

And here is an example with 6x7 negatives - that don't match the spacing at all!

The M645 inserts are mechanically simple and just have a gear that the body turns to advance the film, and a small gear that the film sensing wheel turns to advance the frame counter inside the body. To modify the gearing, you would have to modify the body. The only trick I can think of would be to pad out the film sensing wheel so that it turns more rapidly per length of film advanced.

The difference between 220 inserts and 120 inserts is just a little wedge to tell the body it's a 220 insert, and a small offset in the pressure plate. So you can mod one into the other, with a small error in the film location (probably covered by depth of focus if you shoot slower than f/4 or so). 220 inserts are relatively cheap if you want to try modifying things. I would just try it as designed first, though.

reddesert said:

The only trick I can think of would be to pad out the film sensing wheel so that it turns more rapidly per length of film advanced.

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That's the wrong direction; increase the diameter of the driving wheel, and you'll increase the amount of film needed to advance the counter one frame. I've done exactly that with the friction roller in an Adapt-A-Roll 620 that was leaving too little (almost no) space between 6x9 frames.

Counting is in the body, eh? So it puts what, 31 on a 220 roll?

I'll certainly shoot a roll or two as designed before I even consider any modification. Don't change the corn bread recipe until you've baked it the way Grandma told you.,,

In this case, however, it's no the gearing that needs to be modified -- it would be the counter wheel. In the Zeiss 532/16, there's a brass wheel with notches in the rim, and I just cut another notch and slightly modified how I start the film last warning mark instead of number 1, to make sure the last frame stays on the film). For the M645, I'd have to slightly shorten the start advance (to reduce the blank film at the start), cut another notch in the counter cam/wheel (to get a stop at the 16th frame) and possibly (after testing) reduce the friction wheel diameter by a hair to narrow spacing and be sure the 16 frames fit the film length.

Likely not worth the effort, since the counter is inside the body...

Donald Qualls said:

Counting is in the body, eh? So it puts what, 31 on a 220 roll?

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30.
The counter is in the backs for the cameras that have interchangeable backs, and in the body for the cameras that don't.\
If you make changes to add an extra frame, it would probably mean that the 16th frame wouldn't stop automatically after advance.
It might work okay if you modified the frame spacing on a 220 back, but then you would have to manually advance the last 14 frames of no film.

if it is any consolation,. the Pentax 645 is the same way, only 15 shots.

my guess is that in both cases they designed the spacing to be a bit long as they were measuring off the backing paper, and losing one frame is a lesser evil than Overlaping frames.

Donald Qualls said:

Okay, this begs the question of why Kodak put numbers 41-55 on the film rebate, as opposed to 1-16 on the other side (which never has 1-8, 1-10, or 1-12, only 1-16).

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My guess: 41-55 starts the count high enough that it could never be confused with the 1-12 (or 1-24 for 220) on the other side. One wouldn't want to allow a situation where someone says "print frame 8" and then there are two different frames with an 8. Even if it's theoretically clear which count matches the size of the images, you'd be relying on everyone to do that measurement correctly.

You're right that squeezing more frames in would need the frame counter to turn faster, so reducing the size of the sensing wheel, not enlarging it. That makes it a little more difficult. You could in principle do this by fiddling with the wheel on the 220 insert, which is cheap now. A 220 insert has a little wedge on the side held on by two screws that signals 220 to the camera. Remove the wedge and it can be loaded like a 120 insert, albeit with the film too close to the lens by a backing-paper thickness. However, to get the legendary 16th shot, you'd want to leave it as 220. Then you'd have to keep firing blank shots to wind on the tail of the paper.

reddesert said:

My guess: 41-55 starts the count high enough that it could never be confused with the 1-12 (or 1-24 for 220) on the other side.

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It would even avoid confusion with the 1-36+ count on 135 film.
I know I use it regularly.

I asked about the 41-55 numbers because I haven't used a Kodak film in ages. I've got an unopened Pro-pack of Tri-X 120 on hand, an exposed roll and one in camera of pre-discontinuance Ektachrome 100 and the rest of that Pro-pack -- and recently processed a roll of BW400CN that was in a camera for around twelve years. I didn't look at the edge markings on the BW400CN that closely.

The .EDU Ultra that comprises most of my 120 consumption has 1-16 on one edge, and rebranded stock name on the other. Nothing more. A recent Fuji (years old) I recall having only one number track as well (though I could be remembering wrong).