Enlarging an 8mm gate

#1

Hello,
For my 8mm telecine, I am planning on enlarging the gate in order to ensure that I capture the full exposed image. I plan on modifying the width either by filing or milling with a small milling machine. I could just start hacking away until I’m satisfied, but I don’t think that is the best approach. Furthermore, I suspect that different films will have different widths, so I might undershoot depending on which film I use as my guide.

So I was wondering, are there exact dimensions available that I should follow? Obviously, the sprocket area needs to be widened the most. I am not sure if the opposite (non-sprocket) side should be widened or not. Either way, what would the dimensions of the “extensions” be? Has anyone done some analysis or found a reference to do so, or is it really just trial and error?

Thanks!

#2

Hi Greg! Here is a reference I looked to when doing some of the testing around advancing film through my gate. (Jim Inger has a curious setup to scan film that’s pretty
S/R 8mm spec

I haven’t compared it with my films extensively as I noticed some variation in the exact frame size and position of the frame on the film when different cameras were used, but the example worked for getting a good gate size with plenty of “look-around” room to change framing.

#3

Hi John, thanks for the info. I believe I’ve been to this site before and the image is really just a dimensional specification for 8mm film (which is also available on Wikipedia - see link below).

What I’m really looking for are the dimensions of the maximum exposed area of 8mm film. That information doesn’t seem to be available so I would only be guessing at “how much to file”. If someone has figured this out, that would be much better than me simply guessing via trial and error.

#4

That you may have to experiment with still - the specs show a certain dimension, but the implementation within the camera that shot the film will ultimately determine the shape of the frame. I know from the films I’ve examined so far, the shape of the frame is partially determined by the gate in the camera (can vary from manufacturer to manufacturer!) as well as the lens that was used. For example, I have some Regular 8 films that have basically useful image area extending all the way over to the sprocket holes (and a little beyond if you don’t mind seeing the sprocket hole).
The spec frame of 8mm is certainly smaller than that, but in this case the camera had no gate material masking the film area between the “legal” 8mm frame and basically the edge of the image circle. Other films from different cameras in my collection have totally different shaped frames, all a product of how closely the makers wanted to adhere to the standard, classic YMMV.

Generally manufacturing techniques improved over time, so depending on the vintage of the originating camera(s), you may have a much more uniform experience than me!

#5

Hi there,

I spent actually quite some time to figure out the dimensions of Super-8 film. The research is summarized in the pdf-file below. I think that these are the real dimensions of the Super-8 film format.

Note there are two different frame dimensions - one for the camera (blue in my drawing), one for the projector (red). The camera frame is specified slightly larger than the projector frame, which kind of makes sense. Note also that the centers of both frames do not match!

Generally, if you actually look at projectors and cameras, they do not use these given dimensions exactly. For example, cameras tend to have larger frame sizes, often extending right into the sprocket, as well as above and below the sprocket. However, this is rarely a usable screen estate.

Enlarging a given projector frame to capture the full camera frame or even more is certainly possible, I did it by myself. However, since the film gates are usually made from strong material and are small in size, it is not that easy. After milling, you need to polish the rough edges in order to avoid film scratches. Also take a note at the gate geometry - if you are not careful, you might mill away tiny grooves designed to gate the film. Otherwise, it is certainly doable.

Best, cpixip.

Here’s the promised pdf-drawing: _Super 8 Format with Sound 2.pdf (208.8 KB)

  • have fun!
#6

Thanks for that. I hope to find something similar to that for 8mm as that is what I am working with :slight_smile:

#7

Greg,

the width of Normal 8 is the same as the width of Super-8 stock. So you can run Normal 8 stock through a Super-8 gate.

Now, if you do not use the transport pin of the film gate (for example by utilizing the Kinograph concept of transport rolers and independent sprocket registration/detection), you can simply use a Super-8 film gate for Normal 8, without the hassle of milling away anything, to capture the surrounding area of a Normal 8 frame.

Of course, that approach will not work if you are planning to use the transport mechanics of an existing Normal 8 projector, as the sprocket position is different from Super-8 stock.

However, the standard projection window of a Super-8 gate is wide and high enough to show you a single Normal 8 frame with surrounding screen estate. Your camera will also see part of the Normal 8 sprocket (compare the Wikipedia image you posted above).

There used to exist projectors which handled both Super-8 and Normal 8 with that “trick”, simply masking off the wider screen estate of Super-8 material in the case of Normal 8 film, and adjusting the sprocket pin position and movement accordingly if you switched format.

#8

@cpixip Is that PDF still available? I keep getting error 404 when I try to access it.

#9

@jankaiser - it seems that the Kinograph forums do no longer allow to store .pdf-files directly in a post; also I could not edit the link above any longer.

So I created a new topic with the file to download - please look here:

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#10

I’ll check on this, @cpixip.

1 Like
#11

I rebaked the post (click :wrench:, rebuild HTML–I can’t remember if you have to be a moderator for that to work)) and the PDF link now works.

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#12

confirmed working @cpixip. cc @pfaffman

#13

@cpixip Thank you very much! It looks like some great and very helpful information.

#14

Thank you, I’ll have to see if this will work for my existing 8mm gate without disturbing the film advance mechanism.