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Bolex question


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#1 Damien Bhatti

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Posted 20 June 2007 - 02:39 PM

Hi, I was just wondering whether someone could enlighten me as to the shutter angle of the bolex ebm, how it relates to light reading ? ie - what calculations need to be done when taking a light reading? Also I have heard that a percentage of light is lost due to the layout of the prism in other rx's - is it true with the ebm? if so how much should you compensate by? Thanks!
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#2 Kip Kubin

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Posted 20 June 2007 - 03:15 PM

Hi, I was just wondering whether someone could enlighten me as to the shutter angle of the bolex ebm, how it relates to light reading ? ie - what calculations need to be done when taking a light reading? Also I have heard that a percentage of light is lost due to the layout of the prism in other rx's - is it true with the ebm? if so how much should you compensate by? Thanks!


Download a manual for the Cam... they give a rundown of shooting speed to exposure ratings.

%25 of the loght goes to the viewfinder

If it's the same as an EL

Film Speed Exposure Real Photometric

10fps 1/21 1/28
12fps 1/25 1/33
18fps 1/37 1/50
20fps 1/42 1/56
24fps 1/50 1/67
25fps 1/53 1/70
32fps 1/67 1/93
40fps 1/83 1/110
50fps 1/106 1/1/40
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#3 AdamBray

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Posted 20 June 2007 - 03:46 PM

This is for REAL, then ADAPTED.

12fps 1/33 1/40
16fps 1/45 1/55
18fps 1/50 1/60
24fps 1/65 1/80
32fps 1/90 1/110
48fps 1/130 1/160
64fps 1/180 1/220


It's my understanding that you open up by 2/3 of a stop to compensate for the prisim light loss.
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#4 Bryan Darling

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Posted 20 June 2007 - 06:35 PM

This is for REAL, then ADAPTED.

12fps 1/33 1/40
16fps 1/45 1/55
18fps 1/50 1/60
24fps 1/65 1/80
32fps 1/90 1/110
48fps 1/130 1/160
64fps 1/180 1/220
It's my understanding that you open up by 2/3 of a stop to compensate for the prisim light loss.



2/3 seems a little high to me. 25% is what I've always done, which would be 1/4 stop which I often round to a 1/3 to make it easy. The simplest way to calculate exposures for something like a Bolex is to take your exposure compensation for the light loss of the vf and the difference in the shutter angle from the standard and translate that into an adjusted ASA. That way you can set your meter for a specific ASA and never have to worry about doing any math in your head when your out and about.

As a side note, I use the L-508 cine and it has a wonderful function allowing you to dial in your shutter angle. It makes shooting a Bolex and Super 8 cameras a breeze.
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#5 Nick Mulder

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Posted 20 June 2007 - 08:15 PM

The EBM shutter angle I think is the same as the EL at 170deg - ie. open %47 of the time - close enough to call 180deg ...

The prism takes %17 of the light - this is from actual measurement ...

For a run down of the arguments/confusion re. filter factors/stops and Log10 vs. Log2 that will no doubt arise in this thread - check out this one I prepared earlier:

http://www.cinematog...mp;#entry174916
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