not a repair question....camera film advance...

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

Hey there, I was just curious about mechanical and electronic film advance mechanisms in film cameras. Can anyone enlighten me on how they work? Specifically what decides the advance length? Thank you!

peterqd · Expire: 2014-01-04

A 135 film has notches (or holes) which engage on the teeth of a sprocket roller. One wind of the lever turns the roller and the film is
advanced by exactly 8 notches, which is a standard number for one 36mm wide frame on all films. Cameras designed for taking half
frame pictures advance the film only 4 notches per frame.

120 Medium format film has no notches. On earlier cameras the film was advanced by hand roller, lining up printed numbers on the film
backing paper with a small window in the camera back. More modern cameras advance the film automatically by one turn of the lever.

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

It's just pure mechanics - typically there's a circular piece with a notch that is connected to winder shaft. At the end of the wind notch hits a stopper and blocks further movement. Releasing shutter will also unlock the stopper and allow next wind cycle.

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

I totally get the perforation count and advancement. I'm really curious if it would be a big deal to make a modification so that it only advances half the frame as in a half frame camera.

Lloydy · Expire: 2022-01-01

The shutter opens for the full frame, you would have all sorts of problems with blanking half the frame, and depending on the type of shutter - the direction it moves etc - would the exposure be compromised?

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

It would be easier to acquire a quality half-frame camera, like Pen.

kansalliskalaCafe · Posted: Fri Jan 29, 2016 9:17 am Post subject:

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

There shouldn't be an issue with the shutter system and exposure. A camera would be doing the same thing with a full or half frame. I'm looking at one thing at a time. Film advance first, and wondering how the electronic advance works. Is there a mechanical sensor or is it a servo? I have no idea. I have thoughts on how to blank off half the frame but if the 4 perf pull is impossible I won't even look at the rest. I'm looking at the Pen FT's but I like the idea of a modern half frame and after dealing with cine cameras and seeing them modded I was just wondering about a 35mm...

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

A. You do realize that film is not pulled by perforations, right? They are more like guides/stays. The mechanics that determine length of the advance are under the bottom cover.

B. You need to advance a full frame to cock the shutter. You can bypass that manually if the camera has double exposure button, but modding the camera to make that automatic? You'd have to have some serious mechanical engineering skills to design the parts, and a place to manufacture them.

C. You'd need a camera with vertically traveling shutter, otherwise your exposure times will be messed up.

D. You'd need to design an efficient system to block the light from the other half of the frame.

Actually, here is what you can do. Get a good film SLR with proper shutter - something like FM or FA. Block half of the frame. Carefully measure how far you need to move the arm to advance half the frame. Put a stopper in the top cover that would not let the arm move past the desired point. Then your your shooting would be like this - advance, press double exposure button, cock shutter, fire. You can probably do a second mod, to make double exposure button always on.

visualopsins · Expire: 2021-06-22

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

Lloydy · Expire: 2022-01-01

Sat here messing with two cameras, an old Zenit with cloth shutter and a Chinon CE-5 with metal leaf shutter.

The Zenit has two curtains moving sideways in line with the film, designed to give the right aperture / speed by both curtains acting as one to give an image circle that is centred.
The Chinon's metal leaf shutter works in a vertical plane - right angles to the film - but again has two leaves that work in unison and gives a central image circle.
As shutter speeds increase the closer the two shutters are together, effectively a thin strip moving across the film plane, but in different directions for each camera.

Any blanking of the image circle would surely have to be done equally on each side of the center? If so, then the time the curtain / shutter travels past the blanked off section would be lost time, which would require adjustment to achieve a correct exposure time?

or am I overthinking this? Question

peterqd · Expire: 2014-01-04

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

Maybe the best reference on shutters is the book "Photography - Theory and Practice" by J.R. Clerc.:

https://ia601403.us.archive.org/6/items/PhotographyTheoryAndPractice/aa001.pdf

Shutters are in chapter XII

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

Lloydy · Expire: 2022-01-01

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

bratboy_y2k · Joined: 29 Apr 2011 Posts: 16

peterqd · Expire: 2014-01-04

If you think about how a focal plane shutter works, I hope you'll agree that it doesn't matter what frame size you have or where it is positioned.

The shutter has two curtains - the first which allows light through to the film/sensor and the second which blocks it again. The amount of
exposure time for any "pixel" on the film or sensor, wherever it is within the frame, is determined by the time between the first and
second curtains.

Obviously both curtains need to travel at a constant speed across the frame to obtain an even exposure, and this speed remains constant
whatever exposure time is selected. The exposure time is adjusted, not by the travel speed of the shutter curtains, but by altering the
width of the gap between the curtains, i.e. the time between when the first curtain opens and the second curtain closes. For slow
exposures the gap is actually wider than the frame itself, so it appears that the shutter opens fully for a short time. For faster speeds the
gap is smaller than the frame width, so it appears that a slit moves across the frame. However the important point to understand is that,
whatever exposure time is selected, every "pixel" within the frame receives the same amount of exposure time, but not all at the same
instant.

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

In fact, the curtain speed doesn't need to be constant. A real curtain comes out of immobility and its speed increases continuously as it runs through the focal plane. For the exposure to be uniform, it is the necessary and sufficient that the graphs of distances traveled versus time for both curtains be parallel (a shift in time, or a delay) as shown below. In a well adjusted shutter, the size of red segments, which give the exposure time, is constant, but the size of green segments, which give the width of the slit between curtains, increases continuously.

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

peterqd · Expire: 2014-01-04

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil