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Strobe lights and 35mm? Sync Issues?


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#1 Evan Andrew John Prosofsky

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Posted 10 October 2011 - 10:11 AM

Hi All,

This coming weekend I'll be shooting a music video on 5219, this will be my first experience with strobe lighting. The scene takes place indoors in a rather large loft space, with strobes providing the main source of light. Having never used a strobe before I'm curious if there is anything special I need to look for in terms of the bright flashes syncing up with the film. (we'll be shooting primarily at 24fps but perhaps some slow motion at 48fps and even 120fps). Our original idea was just to rent "strobe party lights" from a party lighting store, as the film co-op here does not have any special lighting units designed specifically for motion pictures.

Secondly, is there a particular technique you all like to use to determine exposure when using the strobe? I'd like the room to be entirely black and only illuminated by the strobe flashes intermittently. Thanks very much!

Evan
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#2 Phil Rhodes

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Posted 10 October 2011 - 11:22 AM

Can't you just shoot it under continuous lighting and occasionally cut in a frame in the edit?

P
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#3 dan kessler

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Posted 10 October 2011 - 12:44 PM

Well, there's the Unilux strobe system, which is routinely used
at high frame rates for nice, crispy slo-mo splashes, crashes, etc.
Very common in TV commercials. Of course, the flash rate is fully
synched with the camera shutter, so the lighting appears continuous.

For a party strobe effect, you don't want that perfect sync, obviously,
but then again, a random flash rate is going to be hit-and-miss with
the shutter timing.

Surely it will work, just don't know if you can be too picky about
controlling it. Sounds like something you really should test beforehand.

Exposure? What about using a light meter designed for flash?
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#4 John Sprung

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Posted 10 October 2011 - 12:44 PM

The main thing to remember is that with a mirror shutter reflex camera, what you see in the finder is exactly what you *don't* get on the film. Run the camera without film and watch several of the strobes with one eye on the finder and the other open. The ones you *don't* see in the finder are the ones you have on film.

As for exposure, shoot bracketing tests. Strobes because of their short duration get into the area where reciprocity failure becomes an issue.




-- J.S.
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#5 Chris Millar

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Posted 10 October 2011 - 01:45 PM

yup,

A stills flash meter will give you an idea ...


As for the syncing a party strobe - are you happy with partial shutters or do you want full on or full off only ?

For optimum control you need to build a sequencer or simple pulse gen that can fire off the pulses when you want them. It'd 'fire' and hold a voltage that feeds a bitwise AND until the other AND input would receive a go from a shutter sync (shutter open) sensor. This way you'd have resolution within 1 frame period. You yourself could be the sequencer just pushing a button.

Sounds complex, but for an electronics nerd it's not too much
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#6 Evan Andrew John Prosofsky

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Posted 11 October 2011 - 01:02 PM

The main thing to remember is that with a mirror shutter reflex camera, what you see in the finder is exactly what you *don't* get on the film. Run the camera without film and watch several of the strobes with one eye on the finder and the other open. The ones you *don't* see in the finder are the ones you have on film.

As for exposure, shoot bracketing tests. Strobes because of their short duration get into the area where reciprocity failure becomes an issue.




-- J.S.


Hi John thanks for the response. Very Helpful. Won't be able to shoot any bracketing tests to determine the reciprocity (though I'd love to), any idea where a good place to start will be?

Furthermore, isn't it dangerous to run the camera without film in it? Or am I going crazy? I seem to remember someone telling me the mirror shutter can be damaged? We're shooting with the BL4 EVO if that makes any difference.

Alternatively, would it be a good idea to shoot with an open shutter if I want all the strobes to register? Would this work? Obviously I"m aware of the way it will change the way motion is rendered..

Thanks!

Edited by Evan Andrew John Prosofsky, 11 October 2011 - 01:04 PM.

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#7 John Sprung

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Posted 11 October 2011 - 05:40 PM

Furthermore, isn't it dangerous to run the camera without film in it?... I seem to remember someone telling me the mirror shutter can be damaged?


No, that's just misinformation. Go with the largest shutter angle you can, mainly to be sure you get all the strobe flashes that don't hit the mirror. Since you can't test, go with what the stills flash meter tells you.




-- J.S.
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#8 Chris Millar

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Posted 11 October 2011 - 06:31 PM

Unless you've got a intermittent and rocket-speed mirror you'll still get partial hits.

You'll see part of the image exposed and the other not at an angle - the angle the edge of mirror was at when the flash hit

You might decide this is cool, if not, it's an issue
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#9 John Sprung

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Posted 11 October 2011 - 06:53 PM

True, and the shutter shadow is generally very soft edged, because of its distance from the film plane. It's another of those woulda-been-nice-to-test items....





-- J.S.
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#10 Chris Millar

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Posted 11 October 2011 - 08:01 PM

I didn't think about the distance softening it like that... My experience with film and strobes was with bolexes which have a plane-parallel shutter that is much closer to the gate. You still get a blur and might even fool yourself it was purely motion blur if you didn't know the small magnitude exposures that strobes give.
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#11 Evan Andrew John Prosofsky

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Posted 11 October 2011 - 10:37 PM

Unless you've got a intermittent and rocket-speed mirror you'll still get partial hits.

You'll see part of the image exposed and the other not at an angle - the angle the edge of mirror was at when the flash hit

You might decide this is cool, if not, it's an issue

Sorry I'm a little unclear what you're saying, could you elaborate a bit? What do you mean about a partial image exposure? Is there a way to counteract this?

I think I'll just be shooting with a typical 180 degree shutter on a BL4...though I'm considering a wider shutter angle to allow more light and to presumably allow for more strobe flashes to render on film. Am I correct in this thinking?

My apologies if I seem daft, thanks ya'll!
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#12 Evan Andrew John Prosofsky

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Posted 11 October 2011 - 10:41 PM

I didn't think about the distance softening it like that... My experience with film and strobes was with bolexes which have a plane-parallel shutter that is much closer to the gate. You still get a blur and might even fool yourself it was purely motion blur if you didn't know the small magnitude exposures that strobes give.

How small magnitude are these exposures really? I am hoping for the strobe to be bright enough to completely illuminate the frame (for a partial moment only, obviously) from complete darkness. Is this feasible? (I'm assuming it probably depends on the type of strobe, but...)
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#13 Chris Millar

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Posted 12 October 2011 - 01:12 AM

Hard to say what your strobes will be - a camera flash can be anywhere from 1/1000 to 1/50,000 sec...


It's the wild west out there ;)



Hard to say in any case as there is a major dependency on the strobe to subject distance - (which is the same for any light source)


Seriously, best bet is to get/borrow a flash meter
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#14 Evan Andrew John Prosofsky

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Posted 13 October 2011 - 10:39 AM

Hard to say what your strobes will be - a camera flash can be anywhere from 1/1000 to 1/50,000 sec...


It's the wild west out there ;)



Hard to say in any case as there is a major dependency on the strobe to subject distance - (which is the same for any light source)


Seriously, best bet is to get/borrow a flash meter


I have an old Minolta flash meter I could use, but I don't understand...is there a way to trigger the strobe flash via the meter? Just to confirm incase anyone is confused, this is a "party strobe light" and not a photographic strobe light.
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#15 Mark Dunn

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Posted 13 October 2011 - 11:32 AM

You can measure on 'open flash' by pressing the meter button then firing the strobe straight away, but you really need to measure a single flash.
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#16 Chris Millar

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Posted 13 October 2011 - 03:04 PM

I have an old Minolta flash meter I could use, but I don't understand...is there a way to trigger the strobe flash via the meter? Just to confirm incase anyone is confused, this is a "party strobe light" and not a photographic strobe light.


Flash meters usually have a setting where they are simply waiting for a spike in the light they are recieving (some kind of op-amp-ish set up in a differentiator mode). Once they get one hit they'll relatively immediately return a reading, so turn the strobe rate waaay down, then try to fire one off, if you get two then it's likely the meter wont register the second one anyway...

Most flash meters also have a sync output, your strobe may or may not have a fire input, but you need to know what kind of signals the strobe is expecting... If I recal sync outputs just close a circuit, either physically or via a transistor type set up (SCR's ring a bell??). Some old flashes send big voltages through the sync cable, enough to fry certain electronics sync outputs, so if yout don't know what I'm talking about best to find the manual mode in the meter outlined above...
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