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Bolex H16 exposure compensation


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#1 Elliot Rudmann

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Posted 25 December 2005 - 04:02 PM

Wasn't sure if I should have posted this in the lighting forum, but here's my question. I was recently gaffing on a student short film where they were using a Bolex H16, however, the DP told me to read the exposures on the light meter from 1/80th shutter speed, not the normal 1/48 (for 24fps). We were shooting at 24 fps, and he told me that the camera itself doesn't shoot at 1/80 shutter, but that the mechanics of the camera cause some light loss, so it's best to expose based off a theoretical 1/80 shutter.

My problem now is that I am shooting a short film with this camera, and I have a Sekonic L-558 light meter at my disposal, however, I cannot obtain a shutter speed of 1/80 in the ambient mode (can't get anything between 1/60 and 1/125). What is the most accurate way of calibrating/changing my light meter to get this 1/32 sec difference? Any suggestions would be greatly appreciated. Thank you!
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#2 Mike Rizos

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Posted 25 December 2005 - 06:01 PM

One way would be to set the ASA on your meter 1/3 stop faster than what your film is rated, and use 1/60sec. for your readings.
I would like to know what's the shutter opening of the Bolex Reflex, and how the 1/80 sec. speed is attained. I've heard of this before, but it doesn't make sense to me unless the Bolex Reflex has a different shutter than the EBM.
The EBM has a 170 degree shutter. The EBM manual that I have states that the "photometric" speed at 24fps is 1/67 sec. as opposed to the real exposure time of 1/50 sec. This is close enough to just use 1/60 without any compansation for the EBM.
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#3 Robert Hughes

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Posted 25 December 2005 - 08:49 PM

The 1/80 second effective shutter speed is a workaround for the light loss thru the reflex optic system, not due to a narrow angle shutter.
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#4 Sam Wells

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Posted 25 December 2005 - 10:59 PM

The 1/80 second effective shutter speed is a workaround for the light loss thru the reflex optic system, not due to a narrow angle shutter.


It's a workaround for both.

~ 135 degree shutter - 1/65 sec @ 24 on the Rex cameras.

-Sam
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#5 Boris Belay

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Posted 27 December 2005 - 11:31 AM

It's a workaround for both.

~ 135 degree shutter - 1/65 sec @ 24 on the Rex cameras.

-Sam

You're totally right. Bolex Reflex 'lose' about 25% light to their fixed prism system (justified by other advantages).
And the EBM, EL, and M models have a different fixed shutter angle of 170 deg.
Elliot, you should get a Bolex Reflex manual so that you have the table for actual exposure times and compensated exposure times for every shutter angle (variable shutter) and filming speed combinations. These are in the bak of the manuals.
They are also reproduced here (great site, by the way, check the rest out) :
http://www.city-net....ex/shutter.html

-B
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#6 Sam Wells

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Posted 28 December 2005 - 11:41 AM

At least 25 % for the prism.

What I usually do for exposure is treat Bolex Rex as if it was a 1/50 th shutter speed / "normal" camera and then rate the stock 2/3 stop lower (rate 50D as if EI 32 for instance).

The loss might not *quite* be 2/3 stop but it's close, and working in thirds is easy.

(The one place this doesn't work is on single frame time exposures, where only the prim light loss is a factor; the shutter angle is not relevant, only the shutter open time - if you're doing this kind of stuff on a Bolex)

-Sam
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