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10-07-2017, 09:59 AM   #1
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P645Nii - What is the mechanism determining recognition of the start of the film

I possess a P645N not the Nii but I think the above mechanism is the same in both cameras. A thread on another forum involving a P645Nii has prompted me to ask this question. Is it simply a change in thickness where the film starts so if backing paper only were to be rewound onto an empty spool the camera would simply wind the roll on to the end looking for the change in thickness?

I take it for instance that if the start of the paper leader were to be damaged and had to be cut off and a new leader cut to shape the roll would still be perfectly OK? If there were to be damage to the backing paper horizontally along the backing paper such that a section was torn horizontally rather than vertically, this would make no difference other than the obvious one that if it extended a long way the start of the film might be exposed to light but provided the paper was OK to be loaded onto the take-up spool such that it ran through this would not affect the camera recognising where frame 1 was?

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10-07-2017, 10:21 AM   #2
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Based on Occam's razor, I would think that the way it "knows where the film starts" is that there is a standard length of paper ahead of the film; it always advances the same amount each time, and doesn't have any sort of sensor to detect where the film is.

---------- Post added 10-07-17 at 10:25 AM ----------

The fact that you line up the Start arrow when you load the film seems to support my assumption.
10-07-2017, 11:21 AM   #3
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Thanks. So if the backing paper is damaged and even a small amount has to be cut off this will lose you the corresponding amount of film i.e. the first frame starts later and last frame finishes later. Not necessarily a problem if the start( in terms of frames available) is only a few cms later but if the backing paper had to be cut much further it could lose you a frame or two.

Without wishing to appear to be challenging your conclusion, is this supposition on your part or definite knowledge of how it works in a P645N?

asahijock
10-07-2017, 12:01 PM   #4
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QuoteOriginally posted by leekil Quote
Based on Occam's razor, I would think that the way it "knows where the film starts" is that there is a standard length of paper ahead of the film; it always advances the same amount each time, and doesn't have any sort of sensor to detect where the film is.

---------- Post added 10-07-17 at 10:25 AM ----------

The fact that you line up the Start arrow when you load the film seems to support my assumption.
I have both the 645N and 645N II and have used the Pentax 645 in the past, and this is exactly how they work. The manual and mechanical Rolleiflex has the function the OP mentioned where it senses the change in thickness when the film proper starts, but most other medium format film cameras "knows here the film starts" based on the lenght of the leader.

10-08-2017, 01:38 PM   #5
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QuoteOriginally posted by asahijock Quote
Without wishing to appear to be challenging your conclusion, is this supposition on your part or definite knowledge of how it works in a P645N?
It's a supposition on my part, but a well reasoned one (if I may be so bold).
10-09-2017, 09:32 AM   #6
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QuoteOriginally posted by leekil Quote
It's a supposition on my part, but a well reasoned one (if I may be so bold).
Thanks. Yes it was as demonstrated by other replies


asahijock
10-31-2017, 03:54 PM   #7
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It seems the 645N uses a photosensor, see this quote from a technical article on the 645N (google translated from japanese):

QuoteQuote:
Film feeding mechanism _

Like the conventional 645, the film feeding mechanism is configured to transmit the power of the main motor to the first stage and the second stage using belts and the subsequent using gears. 645n For development,

① Improve hoisting speed

② Improvement of film feed accuracy

③ Increase number of shot

We have been developing with the goal of.

First of all, ①, changing the gear ratio, shuffling, review the mirror charge system, stabilize the motor load
As a result of realizing a hoisting mechanism with low noise level, improvement of 2 frames / second was achieved.

With regard to ②, accuracy was dramatically improved by detecting the film feed, which had been mechanically controlled conventionally, with the photosensor detecting the rotation of the pulser gear linked to the film roller.

As for ③, as a result of improved feed accuracy, the interframe interframe dimensions become stable, and as a result of searching for the number of frames satisfying the flatness within the range defined in the ISO standards, 16 films at 120 films, 220 films Thirty-three shots can be taken.
11-02-2017, 01:06 AM   #8
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Though that sounds like it might only be related to detecting the accuracy of how much the film is advanced per shot, in order to keep the spacing between the frames to a minimum, to allow extra shot(s) per roll.

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