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Genlock, projection and filming 24fps, 144-degree shutter


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#1 Jay Young

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Posted 21 October 2015 - 11:19 AM

So I have some effects that I was making on 16mm, just really animations and visual loops - And to preface, I understand I could likely do all this in After Effects and project it digitally but what's the fun in creating organic effects if one is not going to shoot on film.

 

Anyhow,  I have a few 16mm projectors available, but I'm just not understanding how this Genlock thing works.

I'm also pretty sure the 1200w lamp in my larger 16mm projector is brighter than anything I can get digital at a reasonable price.  

 

I might have an Arri SR3 available, but the other cameras I have do not have adjustable shutters, hence the title.  What is the possibility of shooting a 16mm projected image at 24 frames per second with a 144 degree shutter?  One of my older machines has the ability to project at 16.7 (unsure actually) frames per second if that will make a difference.

 

I think the thing I am worried about is the two shutters being in sync and filming "nothing".  I can't recall if one of the projectors has a three blade shutter or if they all have 2-blade type.   Thoughts?


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#2 J. Winfield Heckert

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Posted 21 January 2016 - 10:00 PM

Well you would want to get the camera in sync with the projector, to control the speed. Your camera will be crystal locked at 24 but the projector speed will vary up and down.

I only know of syncing projectors to video, and then you use a 5 blade shutter. But even then the image brightness varies.
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#3 Gregg MacPherson

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Posted 21 January 2016 - 11:57 PM

If you like experimenting directly with these issues you could get a Lomo tank and just run a test. From memory (1986?) I have shot some 16mm, 175deg shutter with 16mm back projection, half silvered mirror, where we just kept starting the camera untill we saw a darkening in the viewfinder. So assumption was that the shutters were out of phase. Result looked perfect. The 3M back projection screen was spot metered...but it all was a long time ago.

Find an easy, fast, inexpensive way to test. Lomo tank...?
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#4 Mark Dunn

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Posted 22 January 2016 - 05:39 AM

You could just stuff a short clip test, 5' or so, in a Paterson or similar stills film tank. Or a bucket in the dark. The hand-processed aesthetic might suit the project.

The 144 deg. shutter is really for shooting video.


Edited by Mark Dunn, 22 January 2016 - 05:42 AM.

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#5 Gregg MacPherson

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Posted 22 January 2016 - 06:36 AM

More thoughts.  You should get away without a lock, if the shot is not too long.  If you are unsure if the projector is accurate enough,  you could test that.  Think of something,  like project a junk roll with flash frames cut in at exactly 0 and 400 ft (use your syncroniser or flatbed frame counter) and use even a simple stop watch.  If your reaction time is 1/2 sec then 0.5/600=0.00083 sec,  meaning 0.02 of a frame error in that experiment.  The design of some projectors may be such that they don't have slippage..... driven by an induction motor with a tootthed belt,  gears etc......

 

When shooting the test(s), if you are looking to visually identfy the phase shift through the viewfinder,  take notes on what you are seeing,  then you know what you are trying to reproduce.

 

I don't think the camera shutter angle is that important.  I supose a smaller angle makes it easier to avoid being in phase.  But your tests should show that being a tiny bit out of phase is no biggie,  just affecting background exposure a tiny bit.

 

So is that chap in the flying jacket going to fly in a dog fight or something?


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#6 Jay Young

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Posted 22 January 2016 - 04:39 PM

I don't think the camera shutter angle is that important.  I supose a smaller angle makes it easier to avoid being in phase.  But your tests should show that being a tiny bit out of phase is no biggie,  just affecting background exposure a tiny bit.

 

So is that chap in the flying jacket going to fly in a dog fight or something?

 

I"m shooting a short in May and I'm going to project clouds out the cockpit in at least 100-degree field of view so the actors have something to work with.  Ive been researching three-projector setups and I think at this point I'm going to use digital for that.  There is also a map-table rear projection sequence where the image has to change and I was going to use 16mm projector for that - but that might as well be digital also at this point. 

 

Had I another $10,000 in the budget or so, I might have the resources to source more 16mm projectors and do some frame animation - it's just not in the budget at this point unfortunately.


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#7 Gregg MacPherson

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Posted 22 January 2016 - 09:05 PM

Hey Jay,

I did make one small (but huge) faux pas in what I have said sofar.  Those familiar with old school film effects may have spotted it.  Why did he need a half silvered mirror for his back projection?

 

It was, actually,  front projection,  which is what you may well use.  The background was stock Air New Zealand footage of flying through clouds,  sunny skies.  The forground was the back view of a somewhat arcane looking woman riding a bicycle.  it intended to be a bit sureal,  a dream,  but looked quite realistic.  The screen was about 12 or 18' wide,  three pieces of 3M reflective material glued on,  couldnt see the joins.  I think it was made for front projection.  I seem to remember it had a fairly narrow viewing angle.

 

The camera and projected image were on the same effective lens axis,  by using a half silvered mirror.

 

Anyway,  it was all done on the cheap.

 

Can you find a way not to need a 100 deg projected image? 


Edited by Gregg MacPherson, 22 January 2016 - 09:07 PM.

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#8 Jay Young

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Posted 23 January 2016 - 07:58 AM

Anyway,  it was all done on the cheap.

 

Can you find a way not to need a 100 deg projected image? 

 

I"m trying my best!  Front projection for sure.  I had thought of building a screen out of white spandex material (seems popular in the DIY home theatre crowd), curved around the front of the set.  The problem with limiting the projection is that if I move the camera setup (which will happen), the I have to reset the projector/screen.   If I just build a three screen projection wall around the front of the set, I can tie three projectors together (nifty hardware from Matrox) and have the projection where ever I need.    My problem is finding three projectors to rent!


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#9 Gregg MacPherson

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Posted 24 January 2016 - 07:39 PM

If you can abstract the images a bit then perhaps the viewing angles can be small,  or macro,  so the required screens size(s) is small.  Actors will respond ok to something less literal out of frame.  Oh,  these cues seem interesting,  a carboard aeroplane wobbling along an arc out of frame.  Put it in the movie?


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#10 Roberto Pirodda

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Posted 29 January 2016 - 01:42 PM

hi Jai, it may be only a weird idea,
you could do this experiment: remove the shutter from your projector , then put a switch that on ( or off) every frame like those on home made telecine, after all. Now you have to synch this pulse from the projector to your cine camera ( and this could be the main issue if your cinecamera can't be sinced externally). In short your projector will drive your camera, so no flicker or out of synch at all IN YOUR 16mm FOOTAGE, but ofcourse your eyes will see blurred projected images.


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#11 Carl Looper

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Posted 17 February 2016 - 04:41 PM

I've regulated projector speed to get one projector in sync with another projector, and this would work for a camera as well. The technique I've used is to switch the projector's power supply off and on at a rate determined by a sync signal (from the other projector - which could be a camera as well). It's basically a feedback loop where software increases/decreases the switching frequency to shift the projector into sync (and to maintain sync) with the sync signal. It's called pulse width modulation (PWM).

 

One could also do this the other way - having the camera slaved to the projector. I work with a Bolex in which I've attached my own motor and shaft to that camera, and can regulate it's turn-over rate (very precisely).

 

One has to ensure both rate and phase are adjusted. For example, one might have both going at exactly the same rate, but if they are 180 degrees out of phase with each other, one will have the undesirable situation where the camera shutter is open when the projector shutter is closed, and vice versa!

 

C


Edited by Carl Looper, 17 February 2016 - 04:50 PM.

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#12 Gregg MacPherson

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Posted 17 February 2016 - 05:52 PM

I have done this, once, where we did not use a lock. The shots were 10 or 20 seconds long, and there was no observed phase shift in the viewfinder or shift in exposed image. We did do a few takes in case there was a problem with phase that we weren't seeing in the VF. I can't remember if some were no good.

From memory, we looked to see the shutter obscuring the middle of the GG image, which seemed a good guess at the projector and camera being in phase. Our obsession at the time, being with the projected image and the GG image being out of phase. As Val Kilmer (?), as stoned hippy weed dealer in Entourage put it, the "nega-negative"....is a positive. So yes, the projector shutter and camera shutter should be appropriately in phase if you see the projector shutter obscure the image on the ground glass.
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#13 Doug Palmer

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Posted 18 February 2016 - 05:37 AM

It's fairly straightforward making a physical connection from projector to camera.  So there's no density fluctuation on the image.  Assuming there's an accessable 1:1 shaft on the camera.

http://filmcamblog.b...lex-camera.html


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#14 Carl Looper

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Posted 18 February 2016 - 06:56 AM

It's fairly straightforward making a physical connection from projector to camera.  So there's no density fluctuation on the image.  Assuming there's an accessable 1:1 shaft on the camera.

http://filmcamblog.b...lex-camera.html

 

Nice one Doug. Love it.


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#15 Doug Palmer

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Posted 19 February 2016 - 05:20 AM

Using a flexible shaft to make the camera sync to projector doesn't have to be as involved as my set-up with the Bolex.  Any simple DIY connection should work, as long as you trust the camera not to jam (so maybe a cheapo one is a good idea)   The projector can be inched to find the best position for locking the cable, to get the most light from the shutters of both camera and projector.  Maybe try this :lol: set-up ?  The camera can be hand-held perhaps for extra realism.

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#16 Gregg MacPherson

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Posted 19 February 2016 - 04:15 PM

Hey Doug,
That is pretty cool, rootsy stuff. Did you have any phase lag from the speedo cable? Did you just guess on that, twisting the cable with your fingers to estimate?
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#17 Doug Palmer

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Posted 20 February 2016 - 04:26 AM

Hey Doug,
That is pretty cool, rootsy stuff. Did you have any phase lag from the speedo cable? Did you just guess on that, twisting the cable with your fingers to estimate?

Lately I've been using quite thick cables, but when adjusting the locking position (so that the shutters are phased properly) any lag seems constant.  The main thing is to ensure there is plenty of time gap when the camera shutter is open, for the projector shutter to open and close. So any discrepancy is avoided.  It also simplifies things if the auxillary shutter blade of the projector (that is just used for flicker-removal), is cut off.  

 

Caution: If the film in camera jams for any reason, the strong projector motor will likely cause damage in the camera as it continues running. So it's best to either use a cheap camera or provide a way for the cable to slip in emergency.


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