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Bolex H16 variable shutter angle question


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#1 Elliot Rudmann

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Posted 08 February 2006 - 02:37 AM

Here's a beginner's question for you all:
I am currently working with a Bolex H16 reflex for a short film in my production class. My professor has told us that it has a 135 degree angle variable shutter and that we can use this to create in camera fade-ins and fade-outs, however, I was wondering if this particular technique (when the variable shutter knob is closed 1/4 or half way) would affect the motion blur of the image - making it more strobe-like. Or does it just cut out light? Let me know. Thanks!
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#2 Brian Pritchard

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Posted 08 February 2006 - 02:58 AM

Here's a beginner's question for you all:
I am currently working with a Bolex H16 reflex for a short film in my production class. My professor has told us that it has a 135 degree angle variable shutter and that we can use this to create in camera fade-ins and fade-outs, however, I was wondering if this particular technique (when the variable shutter knob is closed 1/4 or half way) would affect the motion blur of the image - making it more strobe-like. Or does it just cut out light? Let me know. Thanks!

Changing the shutter angle changes the exposure time of each frame and therefore enables the production of a fade out or fade in. If the aperture is adjusted to compensate for the reduced shutter angle it will produce a sharper image by reducing the motion blur and will enhance the strobe effect. The effect will depend on the speed of motion of the objects being photographed.
Brian
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#3 Robert Hughes

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Posted 08 February 2006 - 04:15 PM

If you're looking for a non-strobing fadeout, here's the filmschool standby: assuming you're shooting at f/5.6 or so, slowly close your iris to f/22, then cap the lens. Look ma, No strobe.
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#4 Elliot Rudmann

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Posted 23 February 2007 - 11:05 AM

Instead of starting an entire new thread, i decided to reactivate this one, because it deals with virtually the same issue: Bolex shutter angle.

My question (this time) involves the exposure compensation needed when adjusting the shutter angle knob. For my upcomming film, I want to acheive a strobe like motion effect by manipulating the shutter angle. I just checked the Bolex cam we will be using and on the shutter angle knob there are the numbers "1/2" and "1" - as far as I know, the 1/2 means that the normal 133 degree bolex shutter will be reduced by half if I set the knob there, which means I will have to open up 1 stop for compensation. This is what I have read from manuals online, but they don't say anything about how to make exposure adjustments for the "1" setting...I assume it would be an extra stop? So open up 2 stops if I set it at "1" ? Let me know what you think - thanks!
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#5 Nick Mulder

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Posted 23 February 2007 - 05:05 PM

Hi,

After opening up a Bolex and having a look to be sure I think its the other way around - a setting of '1' actually halves the shutter angle and it is referring to a 1-stop reduction in exposure...

When the shutter is set to 1/2 it is physically not close enough to the 67.5deg (135/2) it would have to to cut exposure by 1/2 ... So I'm figuring it refers to a 1/2-stop reduction instead

confusing huh!

Bolex shutter and prism info is notoriously incoherent on the net - some websites that are quite well designed with nifty graphics, tables n wotnot (ie. someone has taken care) are giving incorrect info .... and theres no way for you to tell if I'm just another one of them so... Opening one up and having a look for yourself is the best way to go in my opinion :lol:

... think of all the 1/2 stop under and over Bolex footage out there :ph34r:
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#6 flavio filho

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Posted 27 January 2012 - 10:17 AM

Hi,

After opening up a Bolex and having a look to be sure I think its the other way around - a setting of '1' actually halves the shutter angle and it is referring to a 1-stop reduction in exposure...

When the shutter is set to 1/2 it is physically not close enough to the 67.5deg (135/2) it would have to to cut exposure by 1/2 ... So I'm figuring it refers to a 1/2-stop reduction instead

confusing huh!

Bolex shutter and prism info is notoriously incoherent on the net - some websites that are quite well designed with nifty graphics, tables n wotnot (ie. someone has taken care) are giving incorrect info .... and theres no way for you to tell if I'm just another one of them so... Opening one up and having a look for yourself is the best way to go in my opinion :lol:

... think of all the 1/2 stop under and over Bolex footage out there :ph34r:



Hi all.

Indeed quite confusing. But anyway, anybody has a definitive answer to that?
I'm looking to test shooting with the same effect, so would be nice to know.

Another thing... on the LIGHT METER L-398a...
I was measuring light of a scene with the incident light... It happened that the light was too much so the meter put it to the maximum. I know I'm supposed to use the HIGH "lid" and read the HIGH stops... But that would work?
Isn't it possible that I could still put too much light on the film?

I don;t know the results cause I didn't process my film yet.

Thanks,
Flavio
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#7 Charles MacDonald

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Posted 27 January 2012 - 10:38 PM

LIGHT METER L-398a...
I was measuring light of a scene with the incident light... It happened that the light was too much so the meter put it to the maximum. I know I'm supposed to use the HIGH "lid" and read the HIGH stops... But that would work?
Isn't it possible that I could still put too much light on the film?


If you use the HIGH insert on the Sekonic 398, then you must use the High index on the meter. to get your settings.

how much light you get on teh film depends on the lens setiings, and perhaps the need for a ND filter.
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#8 Chris Millar

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Posted 27 January 2012 - 10:55 PM

Indeed quite confusing. But anyway, anybody has a definitive answer to that?


After opening up a Bolex and having a look to be sure I think its the other way around - a setting of '1' actually halves the shutter angle and it is referring to a 1-stop reduction in exposure...



makes sense anyway.

If it were angles you'd go from 1/2 angle which is the same as %50 (1-stop) to 1 which would be %100 (infinity-stops). But if you crank the shutter around with it set on 1 you can see it's still somewhat open (open 135/2 degrees) thereby negating that theory. Which is not to say proving the other, but I think it's right myself.
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#9 Simon Wyss

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Posted 28 January 2012 - 01:52 AM

Truth is that the shutter opening angle is 144 degrees but only about 135 degrees are effective due to the distance shutter-film.
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#10 Chris Millar

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Posted 28 January 2012 - 05:29 AM

Truth is that the shutter opening angle is 144 degrees but only about 135 degrees are effective due to the distance shutter-film.

Sorry, I don't quite follow ...

'distance shutter-film' ?

I'm interested to learn though.

I would have thought 144deg is ... 144deg ...

If 144 degrees becomes 135deg what happens at 9degrees physical ?


When it comes to Bolex people seem to love taking one factor and representing the effects of it in terms of another factor - hence all the confusion (non-linearities and so on)

*edit*
1000th post ! or should I say 991th ? Posted Image

Edited by Chris Millar, 28 January 2012 - 05:33 AM.

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#11 Simon Wyss

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Posted 28 January 2012 - 08:41 AM

I am sorry to have thrown that at you like nothing.

See, if a film movie cameraʼs shutter really were in the focal plane which it apparently canʼt weʼd have the exact opening angle. Due to physical necessity we have the shutter somewhat off, about three millimeters with the Paillard-Bolex H cameras, by which fact, in every camera, a smaller angle becomes effective. Every point in the focal plane is given light and obscured rather in a fading manner behind kind of unsharp shutter edges.

Do continue to calculate with 135 degrees for Bolex H Reflex.

Itʼs not Bolex specific. We could easily talk about Beaulieu R 16 details or the 1923 Filmo.
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#12 Dom Jaeger

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Posted 28 January 2012 - 11:39 AM

After opening up a Bolex and having a look to be sure I think its the other way around - a setting of '1' actually halves the shutter angle and it is referring to a 1-stop reduction in exposure...


I would have thought it would be easier just to look at the manual.. B)

But yeah, the markings on the variable shutter (1/2 and 1) refer to the diaphragm adjustment required to maintain exposure. The first mark down from open is 1/2 - the shutter is a quarter closed so to maintain exposure the lens should be opened up 1/2 a stop. The next mark is 1 - the shutter is half closed, so the lens should be opened a full stop.

Confusing? But wait, there's more! The table of exposure times in the H16 Reflex manual (actual times, not adjusted to compensate for the prism) has 1/65 sec for 24fps at full shutter - giving a shutter angle of 133 degrees. But it's 1/90 sec when the shutter is a quarter closed - a shutter angle of 96 degrees (instead of 100 degrees which is 3/4 of 133). And it gives 1/150 sec for a half closed shutter - giving a shutter angle of 58 degrees, which is nowhere near half of 133. The times given for other frame rates yield the same shutter angles.

Simple maths would suggest the actual exposure times for a half closed shutter should be half that of an open shutter, but not according to the manual. 1/65 at full shutter becomes not 1/130 but 1/150 when half closed.

I've never bothered to measure the shutter angles at the variable lock positions, but I know the mechanism itself has a bit of play in the gearing, so it's not particularly precise. It may be that the locked shutter angles are in fact slightly smaller than quarter and half closed, and the exposure times are all rounded off. Perhaps also Simon's theory is at work and as the mechanical shutter angle diminishes the actual angle becomes even smaller due to the the edge blur (though I imagine that would be somewhat dependent on the lens type and focal length).

It's all fairly academic anyway, since the largest discrepancy, between the 1 stop (half closed) mark and the printed exposure times difference of about 1 and a third stops, is pretty minimal. But it's confusing nonetheless.
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#13 Chris Millar

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Posted 28 January 2012 - 05:31 PM

But yeah, the markings on the variable shutter (1/2 and 1) refer to the diaphragm adjustment required to maintain exposure. The first mark down from open is 1/2 - the shutter is a quarter closed so to maintain exposure the lens should be opened up 1/2 a stop. The next mark is 1 - the shutter is half closed, so the lens should be opened a full stop.

Confusing? But wait, there's more! The table of exposure times in the H16 Reflex manual (actual times, not adjusted to compensate for the prism) has 1/65 sec for 24fps at full shutter - giving a shutter angle of 133 degrees. But it's 1/90 sec when the shutter is a quarter closed - a shutter angle of 96 degrees (instead of 100 degrees which is 3/4 of 133). And it gives 1/150 sec for a half closed shutter - giving a shutter angle of 58 degrees, which is nowhere near half of 133. The times given for other frame rates yield the same shutter angles.

Simple maths would suggest the actual exposure times for a half closed shutter should be half that of an open shutter, but not according to the manual. 1/65 at full shutter becomes not 1/130 but 1/150 when half closed.

Gidday Dom Posted Image


A 1/2 stop adjustment in the iris is necessitated by a (1 - 2(-1/2)) = 0.29 closed shutter - which is to say %29 closed from the physical 135/144deg fully open shutter. Significant enough difference from %25.

The calc in general for % terms is:

100*(1 - 2(-x)) = %shutter angle reduction from full open

with 'x' being your stop reduction in absolute terms

The inverse is:

x = log2((1-(y/100)) where y is the %shutter angle reduction from full open

(1-0.29)*135 = 95.45 a bit closer to 96deg - but I used 135 - and still not quite there anyway huh ?!

I really do think at least one of those Bolex supplied tables is wrong, I mean, it has to be in that it's internally inconsistent within the confines of its own logic. I thought maybe that they had thrown in the prism offset which would stay constant and therefore throw off the linear straight line (via a log scaling) extrapolations we make in our heads, but yeh losing interest when it comes to practicality as you're correct in that what does it matter with all the other factors at hand (bad metering technique in the first place) + those sloppy shutters Posted Image

To anyone wonder what the flip we're on about, I know it's confusing, and why give us anymore credibility than the other x, y and z 'official' websites telling you so and so the opposite - I say learn it for yourself once - test, open one up - whatever makes you feel like you've done a proper job - then sleep well and ignore the internet from then on - it'll be fine Posted Image


See, if a film movie cameraʼs shutter really were in the focal plane which it apparently canʼt weʼd have the exact opening angle. Due to physical necessity we have the shutter somewhat off, about three millimeters with the Paillard-Bolex H cameras, by which fact, in every camera, a smaller angle becomes effective. Every point in the focal plane is given light and obscured rather in a fading manner behind kind of unsharp shutter edges.
Do continue to calculate with 135 degrees for Bolex H Reflex.



I still don't get it and I'm the type who like to know things...

~There is an offset - tick
~There are leading and trailing fading edges - tick
~Not sure if it's relevant but I know that different parts of a frame at any given time are experiencing slightly different things - (the centre of rotation of a shutter is a finite distance away and we aren't talking about focal plane blind shutters)

I'll try to see if you can explain how it works at the extremes of possibility and in % terms of duty cycle also - so I see a shutter as a device that allows us control over the duty cycle of a exposure over a given frame rate - you're telling me that a physical structure of 144/360 = %40 duty cycle equates to %37.5 duty cycle - a difference of %2.5...

oh wait - heh

I get it now

the offset between 144 and 135 scales with the shutter angle as the duration of the lead and tail shadows stay the same, whereas the fully exposed part gets shorter in shorter in relative terms

got it - interesting !

Annoying in that I think I've actually encountered similar logic in other mechanisms


Posted Image



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#14 Chris Millar

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Posted 28 January 2012 - 06:09 PM

er, no

hang on

I've drawn some pictures here...

Wont the part shadow in the area of where the shutter is 'open' simply be accounted for by the partially exposed area where it should be shut ?

Or in other words - as the lens isn't a point source the penumbra from the leading and trailing edges of the shutter will affect not only the 'open' area but also the umbra (the 'closed' area)

In psuedo math:

umbra - penumbra + penumbra = umbra

umbra equates to shutter angle - i.e. 144 = 144
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#15 Simon Wyss

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Posted 29 January 2012 - 04:28 AM

Chris, oh!
The distance I mean is along the optical axis, the shutter away from focal plane and film towards lens.


Hereʼs a picture of a Revere 103 shutter. The trailing edges are cut not radially but angled. This is to compensate across the aperture but has nothing to do with the angle shortening as mentioned above.

Revere-Verschluss.JPG


There is another point about Paillardʼs tabellae, that is they rounded figures off with regard to —

films. Yeah, photochemical films react differently to increasingly shorter exposure times but in general more or less in the same sense. Perhaps you have noticed sometimes that collapse of the picture with fades. Times beyond 1/500th second or so result in strong underexposure. Depends on the type of stock (originating, duplicating or release positive), on the mechanism guiding the variable shutter in the (printer) camera, and on the overall conditions of the transition.


Edited by Simon Wyss, 29 January 2012 - 04:31 AM.

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#16 Chris Millar

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Posted 29 January 2012 - 03:11 PM

So they're compensating for reverse reciprocity failure in terms of shutter angle and exposure time now

It gets even worse.


heh heh

And yes, I know what you mean - I know about the non-radial angles (as a disclaimer I did mention that I wasn't sure if it was relevant)

hmmm, guess I'll scan those drawings I made as online diagrams of penumbra and umbra all relate to solar eclipses, it might confuse things

:D
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#17 Chris Millar

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Posted 29 January 2012 - 05:00 PM

ok...

excuse the drawings - all I have here is a burning twig - both to light my way in the cave and to draw with:

Posted Image


To keep things simple imagine the lens is pointing at something of uniform intensity - in that regard it projects as I have drawn in the diagram above. As you can see and as you know the shutter edges due to the offset form an area of less intensity leading to nothing. This effect happens on both the leading and trailing edges. The dotted lines show the extent of area on the film plane that would be exposed if the shutter were co-planar with the film plane. Moving on to the zoomed in drawing below:

Posted Image


As you can see the projection due to the offset causes a relatively shadowed area 'B' in side the normal image area (and I mean 'normal' both figuratively and literally). But it also causes an area of relative higher intensity 'A', this is an area that should be in the co-planar case be completely occluded by the shutter at this stage.

In terms of penumbra and umbra:
penumbra = A+B
umbra = the area defined outside of the dotted line (shutter 'closed' in the co-planar sense)

So in timing sequence film would see:


shutters backside
leading A
leading B
normal exposure
trailing B
trailing A
shutters backside

In relative terms compared to the normal co-planar exposure A adds, B subtracts, normal is normal ;)

+A-B+A-B = ?

Well, according to my hastily drawn diagram it actually looks like the deficit is on the side of A - as in the offset causes more exposure - so 144deg should be compensated as something higher. Making the lens projection much larger or smaller and re-drawing my lines doesn't change this, smaller actually makes 'B' project completely into the area that shouldn't be exposed, larger is more of the same, just larger 'A' and 'B' areas.

Imagine bolex made two shutters - one with a 90deg opening, one with a 180deg opening but then put 1-stop ND on it - you'd end up with the same integrated exposure, but the actual exposure time is different by a factor of 2 - are you saying that they are adjusting for the penumbral areas (as a sort of 'ND') and giving adjusted exposure for them ?


Posted Image



We haven't begun to talk about what happens when the two B areas meet each other...

Likely there is something wrong with my logic.

Edited by Chris Millar, 29 January 2012 - 05:03 PM.

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