why doesn't frame spacing increase as film winds on?

[Poohblah]

i've been wondering this for a while. basically, the premise of my question is that, as film winds on to the take-up spool, the effective diameter of the take-up spool increases because there is more film built up around the take-up spool. so, by the laws of geometry, if the angle that the take-up spool rotates through as one frame advances is constant, then the frame spacing should increase as more film is wound on.

why, then, does this not actually occur?

 

[2F/2F]

It does occur on film without sprocket holes!

 

[DWThomas]

Many cameras figure out the amount of film motion from an idler roller next to the supply spool. The spool diameter has no effect there. My Perkeo II (120 film) works that way. Other cameras have some sort of magic cam mechanism to adjust the rotation as the film spool diameter changes: I believe that's what my Bronica backs do (but haven't ripped one open yet).

DaveT

 

[Ian Grant]

Older cameras had a magic window so you could just line up the frame number printed on the back of the film. Come to think of it so do some modern Panoramic cameras

Ian

 

[jimgalli]

Your winding mech is counting sprocket holes. That is the constant.

 

[Ian Grant]

Your winding mech is counting sprocket holes. That is the constant.

On 120 film ?????? It'll wear out before it finds one

Ian

 

[nsouto]

on 35mm, it's sprocket holes.
on the pentax 6x7 it's a rubber roller before the take up spool.
on the 6X6 Arax (Pentacon six offshot), same.
on the rb67, I don't know but I'll find out: you got me thinking now and it won't go away...

 

[jimgalli]

On 120 film ?????? It'll wear out before it finds one

Ian

The question was in the 35mm Cameras section. But you're right. With my Mamiya Universal 6X9 the spacing did change as the film progressed from one side to the other.

 

[2F/2F]

The question was in the 35mm Cameras section. But you're right. With my Mamiya Universal 6X9 the spacing did change as the film progressed from one side to the other.

...which is why I thought this was common to all MF cameras. The Super 23 was my first MF.

 

[AgX]

Pooblah,

The solution to this problem is very, very simple:

The spacing between the frames is controlled by the transport mechanism via the sprocket wheel.
Its gear drive is designed that way that one full stroke of the transport lever will turn the wheel as much as it needs for 1-frame+spacing.
The take up spindle is geared to this mechanism that way that one full stroke of the transport lever will rotate it that much to accept 1-frame+spacing.

As you realized, that would lead to problems as with advancing film the diameter of the roll of film on the transport wheel and thus its outer circumference will rize, which means that it would need to get more film per stroke than the sprocked wheel would deliever.

The solution is a friction drive inserted in the link between transport mechanism and the take up spindle.

As simple as that.

 

[David A. Goldfarb]

This is incidentally the difference between the old knob wind Linhof Rollex rollfilm backs and the later Super-Rollex backs with the advance lever. The older backs turned the takeup spool a constant amount for each frame, so frame spacing would increase. Unfortunately, they also assumed a certain film and paper backing thickness, so with thinner modern 120 films, they often produce overlapping frames. The later Super-Rollex back actually meters the film travel with the large rubber roller, so that it never has frame spacing problems.

The Voigtlander Superb TLR from the 1930s didn't have an auto-stop film advance, but it did have a ratcheting lever and a feeler wheel to measure the amount of film travel and transfer that information to a frame counting cylinder.

 

[kodachrome64]

Why is it that such a "modern" camera as my F100 can't space all frames evenly? I've never been able to understand why 35mm cameras can't advance a set number of sprocket holes (at least the ones I've had).

 

[keithwms]

Some cameras can advance very precisely. For example the konica hexar AF. I recall that one could remove film half roll and switch rolls and then later come back to where you were.... without missing any frames. So the technology exists...

 

[David A. Goldfarb]

Some cameras just have better registration than others. If the frame spacing drifts back and forth, then it's a registration issue. If the frame spacing gets progressively larger, then the camera isn't metering the actual film travel.

 

[Jim Jones]

Some cameras can advance very precisely. For example the konica hexar AF. I recall that one could remove film half roll and switch rolls and then later come back to where you were.... without missing any frames. So the technology exists...

This is true with perhaps most 35mm cameras if one always carefully starts the film in the same position and with the shutter ready to shoot so the film advance mechanism is uniformly positioned. Being a klutz, I usually sacrifice a blank frame to allow for operator error.

 

[Akki14]

On my finetta 88 camera it doesn't count sprocket holes and the spacing varies from nearly nonexistant at the beginning of the roll to about 2-3 times wider than average spacing towards the end but I still get the right number of exposures from the roll of film so it doesn't bother me.

However, it ticks off lab techs to no end. I had a 1hr photo place here whinge at me for not letting them cut apart my negatives and it was a roll unevenly spaced and for some reason their lab tech had to do everything very slowly which took approximately 6 hours. I didn't get any refund for the hassle either.

 

[John Hermanson]

Because the takeup spool turns on a slipping clutch. As the effective diameter of the spool increases, it has to turn less. The sprocket pulls film through at a fixed amount, but the takeup spool compensates for it's own change in diameter. Not doing so would pull too much film and rip sprockets. I saw this once in an OM-1 I was repairing. The customer had poured epoxy in through the spool slots so it wouldn't slip anymore......ha ha. Kohn, www.zuiko.com

 

[MikeSeb]

You people have WAAAY too much time on your hands.

 

[John Hermanson]

Happy to help. Also, if the OM (1 thru 2N) takeup clutch wears out, the spool won't takeup enough, film bunches up in the chamber, has nowhere to go, then frames start top overlap (and the wind action itself feels kind weird while this is going on). John, www.zuiko.com

 

[Poohblah]

gotcha. thanks guys.

 

[nsouto]

Some cameras can advance very precisely. For example the konica hexar AF. I recall that one could remove film half roll and switch rolls and then later come back to where you were.... without missing any frames. So the technology exists...

Nikon F. You can even rewind a few frames, then wind again, the framing will be fine. To count frames exactly, the dot in the rod of the shutter button is the key: one revolution of the dot, exactly one frame. Backwards or forwards.

I used this all the time many years ago, when I was into multiple exposures: I could take a series of shots, then rewind those frames and shoot them again!
No other slr can do that, as far as I know.

 

[Jerry Thirsty]

No other slr can do that, as far as I know.

Pentax LX

 

[Nicholas Lindan]

Mamiya Universal 6X9 the spacing did change

The lever wind Mamiya 23/Universal backs have a pinch roller that meters out the film and stops the winding mechanism. TTBOMK, the knob wind back uses a cam in the winding mechanism that compensates for frame spacing. The cam approach is approximate. Without compensation the amount of film/turn of the take-up spool almost doubles from the beginning to the end of the roll.