[asahijock]

kaseki, I am unsure if I understood your very full explanation but your first sentence suggests that both pressure plates are from an engineering aspect, the same thickness If I have understood this correctly and your explanation simply give substance to this then I am still scratching my head about the additional strain on the motor that results from a conversion from 220 to 120?

So how do we reconcile this with the experiences of rfom11 and lightproof?

Thanks

pentaxuser

[kaseki]

kaseki, I am unsure if I understood your very full explanation but your first sentence suggests that both pressure plates are from an engineering aspect, the same thickness If I have understood this correctly and your explanation simply give substance to this then I am still scratching my head about the additional strain on the motor that results from a conversion from 220 to 120?

So how do we reconcile this with the experiences of rfom11 and lightproof?

Thanks

pentaxuser

I believe that the goal is that the film emulsion be in the correct position relative to the lens flange and thus to the focal plane stop rails (say, a 120 film thickness forward of the rails), and thus for 220 film lacking this backing paper, the pressure plate area where the film is has to be closer to the lens by the 120 backing paper thickness. This requires the 220 pressure plate rail interface to be cut down by that amount.

Diving a bit deeper into the guessing pool because I don't want to open my "N" right now or even my 120 backs all loaded with film: If there is significant added motor load from using 120 film in a modified 220 back (but without the added metal tape at the rail edges), then I would ascribe it to the roller interfaces with the sides of the focal plane stop that are transverse to the rails. When the 220 back pushes the 120 film deeper into the camera cavity, drag at the roller locations could (!) increase. The likelihood of this would, I think, become apparent if I were to look into my camera back and see the details of the focal plane stop in the direction of film movement (transverse to the rails we have been addressing).

Last, by allowing an untaped 220 back to push the 120 film emulsion into a position too far forward, lenses have to be slightly defocused from their intended focal length to focus on the film, which will thereby put the viewscreen slightly out of focus. And vice versa. The more I think about this the more essential I see the tape solution to be.

kas

[itsdoable]

Perhaps this diagram will shed some light...

50 rolls may not be enough to be able to conclude ... asahijock

How many rolls may be enough to conclude? Sold off my MF film gear last year including 645N and a half dozen converted 220 backs. Not once did I have an issue. Okay, one time batteries died and I didn't have spares ...oh, and the other time I ran out of gaffers tape so except for that, wait, forgot my ex-wife called and demanded to know why there weren't exactly 16 exposures even though she had sacrificed and saved for a real 120 back .....


How many rolls?


Disclaimer: I'm trying to imagine anyone here shooting 50 rolls in less than multiple decades. If I'm wrong, I'd love to see the photographs. Really!

[kaseki]

Perhaps this diagram will shed some light...

Thank you for those drawings. The dual rails are new information for me.

If these two images are supplemented by a mental third image of 120 film in a 220 back, one can see that to the extent the springs push the 220 pressure plate down to the pressure plate rails, the film will be "crushed" by the film rails. Drag will predictably increase and the film plane may not reach the focal plane. And we haven't yet examined the details of the film-plate geometry at the roller sides of the pressure plate while this condition exists.

[asahijock]

How many rolls may be enough to conclude?

How many rolls?


Disclaimer: I'm trying to imagine anyone here shooting 50 rolls in less than multiple decades. If I'm wrong, I'd love to see the photographs. Really!

50 rolls does not seem to very many over what I'd consider to be the expected life of a motor. At a wedding for instance I'd expect a professional photographer to take may be 10 rolls in one session. I'd have thought that 5 weddings over a summer season might not be unusual so that 50 rolls on a matter of a few months is perfectly possible.

I am simply trying to obtain evidence be that usage or engineering evidence that helps those converting 220 to 120 because of the price and shortage of 120 inserts, to decide if they risk their motors in so doing

I do not have a pre-set opinion on this. It would seem that as we stand, opinions are divided but there has to be a way surely to establish whether (a) there is a "strain problem" and (b) if so, how long might it take in terms of roll usage to manifest itself


asahijock

[grhazelton]

Perhaps this diagram will shed some light...

Looking at the drawing might a shim of the same thickness as backing paper attached to the pressure plate rail do the trick? It would "trick" the insert into believing that 220 film were inserted.

[asahijock]

I am not an engineer and admit to having a problem with understanding engineering drawings but from "itsdoable's" diagrams it appears that with a 220 back the pressure plate is thicker than the 120, such that the pressure plate on the 220 plus the film is equal to the 120 pressure plate plus film and backing paper.

If this is correct then why does using a 220 pressure plate in a conversion to 120 plus both the film and backing paper not increase the total thickness. From the pictures given to us by kaseki the only lower lip part of the 220 plate appears to be able to compensate for the 120 backing paper is the edges and not the central area where the film plus paper runs

Given this, I cannot see how using a 220 plate in any way compensates for the addition of the backing paper so my conclusion is that there will be more strain on the motor. Whether this in enough to lead to a problem remains open. Some believe it has and others report no problem

I may have completely misunderstood what is being said about shims, adhesive silver tape being the "cure" to any problem of extra strain

I do not have a 220 back to examine so I start at a disadvantage. I would be grateful if anyone can explain what the flaws are in what I have said. Has anyone measured both plates to check if they are of different thickness and if so what was the result?

asahijock

So 6 x $26 for 220 backs =$156. Six 120 backs at $100 each is $720. Difference is $564 which will buy a replacement 645N body IF the film advance motor fails. I picked numbers that I bought backs with.

[itsdoable]

Looking at the drawing might a shim of the same thickness as backing paper attached to the pressure plate rail do the trick? It would "trick" the insert into believing that 220 film were inserted.

That is what I did (and said in post #40). By default, the rails in the camera are set for 120 (film + backing paper), thus a flat pressure plate pressed against the outside rails provides the correct gap to hold the film flat against the film rails. The 220 back has the pressure plate machined away where it touched the outer rails so that it makes up for the paper backing thickness. This is what I did (plus the flipping the plastic thing).



Re: 50 rolls of film. That is about the # of 120 rolls I shoot recreationally in a year. It about 2 wedding worth if I'm primary - although the last time I was primary, only 1/2 was 120, the other half was 135 (and that was a long time ago!). When I'm not the primary (ie: shooting guest) it's 5~10 rolls.

[kaseki]

I think the question I was asked has been answered by itsdoable above. I would only suggest that itsdoable add the film backing paper to the drawing above to make the 120 solution fully obvious. As I noted before, the actual thickness of the pressure plate in the zone where the film is does not matter because it is the film side that rides the rails.

Bottom line: Add the tape as shown above on the 220 plate. The tape thickness will need to be determined from the cut depth on the pressure plate. I haven't got any 220 backs so I haven't had an urge to look for metal tape, but if I were to do so, I might start with McMaster-Carr, or even eBay (after determining what thickness is needed). The tape might be categorized as shim stock. Cut off the excess.

[asahijock]

I have been going over this whole thread tonight and mikebob in post #11 got an answer from Eric at Pentax( I assume he is the guru engineer on all things Pentax?). I have to say that in relation to the nub of this thread about a 220 to 120 conversion Eric's answer did not seem in any way to cover the question asked of him. mikebob then replied and his reply left little or no doubt about what information was needed from Eric. mikebob as of May 2010 was awaiting an answer but has not been back to say if he ever received one. I suspect not otherwise he would surely have given us the reply

So in the first instance Eric at Pentax seems not to have answered the question and in the second instance when mikebob made it very clear what the issue was, there appears to have been no answer in nearly 9 years

I always get suspicious when clear questions do not get clear answers. It suggests that while the person with the knowledge(in this case Eric at Pentax) has the answer he'd prefer not to give it. We need to ask ourselves why

I had a look at my 120 insert in an attempt to try and locate if there is the same part in it as in the 220 that allows the 220 insert to act like a 120 and stop at 16 frames but could not see anything. One member did provide photos but without an arrow to point out the part in question so I may be looking in the wrong place.

So does the 120 insert have the same piece and if so a photo of it would be greatly appreciated

Thanks

asahijock

[itsdoable]

....I had a look at my 120 insert in an attempt to try and locate if there is the same part in it as in the 220 that allows the 220 insert to act like a 120 and stop at 16 frames but could not see anything. One member did provide photos but without an arrow to point out the part in question so I may be looking in the wrong place.

So does the 120 insert have the same piece and if so a photo of it would be greatly appreciated

The 3 images in posted #25 show the black plastic widget (between the 2 spool holders, fasten by 2 philips screws) that sets the insert to 120/220. The 1st image shows the piece with a finger up. The second shows the piece re attached with the finger down (rotated 180 deg). The 3rd shows the piece with the finger up again.


There is nothing in the 120 insert to stop the frame advance at 15 frames (or 16). The finger trips a micro-switch lever in the P645 body that tells the firmware that the insert it 120, and the firmware stops advancing the film at 15 frames (or 16 if you have that modification).

So: adjusting the gap on the pressure plate makes room for the extra thickness of the backing paper, and flipping the plastic block between the spools on the insert tells the camera to stop advancing frames at 15 (or 16) exposures. If you don't adjust the pressure plate, you can still use 120 film - Lomo and older stock films use a thicker film base, so the pressure plate of the 120 insert creates drag anyways. Not flipping the plastic piece means the camera keeps advancing after 15 (or 16) frames, which does not affect the exposures, but if you don't notice the frame count, you may continue to take blank frames.

[asahijock]

Thanks, itsdoable. I have now located what was being referred to. What threw me was that everyone was using the plural word "teeth" so I was looking for some part with teeth whereas a pointing finger might be a more accurate description. It's the old story: when you know what you are looking at, it is difficult to understand why others cannot easily see what you see.

There has to be a reason why the 220 pressure plate is lower at the edges and your explanation would cover it in terms of increasing space as only the emulsion is involved in a 220 film.

Metal tape might do the job but I suppose that a shim of the exact thickness and permanently attached to the edges might be the better permanent solution.

If the supply/demand problem were the other way around i.e. 120 film had been discontinued then converting 120 inserts to 220 would also work but might give film alignment issues? However that is me entering into the realms of a pure academic discussion

What remains disappointing is that the one Pentax source that has been mentioned either chose to not answer mikebob or the latter chose not to relay what Eric from Pentax said to him.

asahijock

[kaseki]

...
What remains disappointing is that the one Pentax source that has been mentioned either chose to not answer mikebob or the latter chose not to relay what Eric from Pentax said to him.

asahijock

Or had moved on, or was directed by Pentax not to further support the conversion. By now, there may be no Pentax employees who were involved in film back engineering.

Pentax not helping with the conversion may not be due to any "nefarious" reasons. They may wish to not take any responsibility for modifications to their products which haven't been vetted by their engineering and legal departments. For example, the customer having removed a screw that the factory had used a "Loctite" type product on, can Pentax be confident that the customer will reinstall with a similar product so the screw will remain retained and not get loose and fall somewhere into the camera guts? What if too much Loctite is used? Will the screw torque be consistent?

What about the choice of pressure plate shim -- will it stay attached and not cause a problem? Will its thickness be optimal? For manufacturers, sometimes these modifications become a nightmare can of worms, just as modifying a recently-made automobile's engine has a lot more legal implications than modifying a flat-head Ford engine 60 years ago. And anything that does go wrong can affect the manufacturer's reputation.

We have cameras that are out of production and out of support. It is up to us to determine what works and what doesn't work. Retroactive engineering can shed insight, and I think itsdoable's contribution here, along with contributions from others on much earlier threads, has produced the needed insight.