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Bolex Light Loss Revisited


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#1 Giuseppe Valentino

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Posted 12 March 2008 - 02:01 PM

Hello everyone.
I'm currently looking for a definitive answer to the, now mithical, Bolex Light loss.
I've been around, I've read almost everything that has something to do with the prism light loss on this cameras. I own a Bolex Rex 5 and a manual light meter which doesn't give me intermediate shutter speeds with precision.
I'm asking here if is possible to compensate for the light loss changing the speed which seems to make the things easier. Plus, how can I measure with precision when the shutter is full open (130°) or 1/2 (65°) or at 1 (32°-35°) if the adapted shutter speed are not pointed out on my meter? If somebody says 1/87 will be, on the light meter of mine a value somewhere between two written values.
I'm starting over and it is not a math problem but lack of experience. If would be possible to give back the light loss with the sensibility of the film would be better and easier when I use the shutter speed at 65°, for instance, there will be no problem using the real values of 1/90 at 24 fps which are reaceable on my light meter far more easier than a given 1/112.
Can please anyone help me out? Speaking about ISO instead of stop or shutter speed?
Concluding, can I play around with the film speed and have precise metering measuring the light with 1/50 shutter speed?
Thanks a lot, you don't know how important for me is going to turn out the answer.
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#2 Nick Mulder

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Posted 12 March 2008 - 03:35 PM

prism light loss - ugh - searching for info is a nightmare of contradictions and 'rules' ...

In the end though considering the range of negative stocks its not actually that much and most Bolex users (run/gun/fun) could just forget about it and not notice.

That being said you want to a professional and actually know whats going on ? Well, you have to get out your Log2 calculator to convert percentages into Log2 compensations - its ass math that aint going to translate into anything practical with a meter like yours (maybe on something with filter factors and shutter angles like the digital sekonics etc...)

I did some threads on it a while back, prob just adding to the myth ... So anyway, after having learned all this 5th dimension quantum entanglement people just revert to sneaking the iris a little more open anyway ;)

If you dont mind telling >> what is mission-critical about your Bolex shoot ?
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#3 Giuseppe Valentino

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Posted 12 March 2008 - 04:35 PM

Hello Nick, thanks for the quick reply. I'm currently shooting a documentary with both 16mm and video, in the next days I'll be shooting with both cameras and I'll use the 16mm, I'm justing look after some good material in the end, because I'll cut the footage together for a 5 minute presentation, I'm looking for some money and I'll like to shoot with both formats, even some Super8, but that's not me, just a kid who loves the support, so if I'm going to present and convince someone to put money in this project I'll like to show some good 16mm footage aka, I'll like to know something clear about the light loss just as simple as possible.
I'm not sure if I understood your theory about the speed. Can we figure out how much should we tweek the speed to compensate? Is that maybe nonsense?
Allright, I get the beautiful notion, which is pretty K.I.S.S. rule, to just open up a stop, is then a stop?
If my meter says 5.6 will be 4? Right, with the shutter speed on 1/50 or adapted for the Bolex?
Thanks a lot again, please, try eventually to cope with my ignorance.

Thanks, Giuseppe
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#4 Nick Mulder

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Posted 12 March 2008 - 04:47 PM

No, a stop is too much (but still within the capabilities of negative stock anyway).

Light loss is around %18 - as in %18 of what comes in is lost - not %50

So I dunno (actually I did once, but now don't worry so much) go for a 1/3 of a stop or so - just a sneak ...

You have many other things to think about that are more important than this ;)
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#5 Giuseppe Valentino

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Posted 12 March 2008 - 05:30 PM

Oh, yes Nick, your are right of course but for the plenty of other things I can just found answer here and there, stuffthat I can, with limits understand.
So you mean 1/3 of a stop, correct me if I'm wrong than you mean if the meter points 5.6f you go for 5f?
Here I'm lost again and again I ask forgiveness for my lack of experience, How can I get a 1/3 of a stop less? I'm sorry Nick, what about the shutter, if I'm 1/3 more the shutter values are going to be calculated the usual 1/50?
Thanks

Giuseppe
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#6 Adrian Sierkowski

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Posted 12 March 2008 - 09:47 PM

IIRC it was 2/3rds of a stop from my bolex days which gave me a nice negative, but it's pretty liberal in a lot of ways and I have a feeling it might depend on which bolex (i think this was a REXIV?)
in any case 1/3rd of a stop would be, at an F5.6 reading, 2/3rds of the way between F4 on the lens and F5/6. Basically splitting the space between those two iris positions into 3rds.
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#7 Satsuki Murashige

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Posted 13 March 2008 - 01:18 AM

I haven't used a Bolex in a few years, but I was taught in film school that the light loss from the prism was approximately 1/2 stop. If your light meter could only assume a 180 degree shutter, then the Bolex's 130 degree shutter would be another 1/2 stop loss, for a total light loss of 1 stop. This made exposures fairly easy to calculate - meter at 24 fps assuming a 180 degree shutter (1/48 sec) and then open up one stop (or just meter at 1/96 sec and expose at that stop). My exposures with this method were always solid, generally printing around the high twenties-low thirties. So it probably is overcompensating a bit, but with negative stock it's better to err on the side of overexposure anyway. Just my 2 centavos.
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#8 Nick Mulder

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Posted 13 March 2008 - 03:34 AM

As you can see there are many solutions - all working for each individual for whatever reason ...

What I suggest is you study up on stops, ASA and some math (reciprocals and Log base2 in particular) and/or just shoot some footage and see what happens, you'll soon work out how it comes together and be able to distill your own rules from your knowledge rather than reading all the gumph on the net (as good, bad, confusing and contradictory as it may be) ... then forgetting it the next day.
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#9 Sam Wells

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Posted 13 March 2008 - 10:10 AM

In mot cases, downrating the film stock's Exposure Index 2/3 stop works quite well, then your metering can be set for an 'assumed' 172 - 180 degree shutter or 1/50 sec.

So you set your metering for EI 64 with an EI 100 stock etc.

(Where this breaks down is for long exposures or time exposures where the shutter angle is not really an issue...)

-Sam
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#10 Dirk DeJonghe

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Posted 13 March 2008 - 03:22 PM

You loose 1/3 of a stop because of the 130 degree shutter instead of 180 degree normal, plus 1/3 of a stop for the semi-transparent mirror, so 2/3 in totql as several posters have pointed out.
With negative exposing a full stop more would not hurt anything. Instead of 100 EI make it 50 EI and so on (divide EI by 2, couldn't be easier).

There is much more latitude on the overexposure side of the negative than on the underexposure side.
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#11 Leo Anthony Vale

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Posted 13 March 2008 - 04:49 PM

In mot cases, downrating the film stock's Exposure Index 2/3 stop works quite well, then your metering can be set for an 'assumed' 172 - 180 degree shutter or 1/50 sec.

So you set your metering for EI 64 with an EI 100 stock etc.


The actual Paillard Bolex manual states that the effective shutter speed at 24fps is 1/80th of a sec.
This is a combination of the 130* shutter and the prism's light loss.

This is a 2/3 stop difference from 1/50 sec. If one's meter doesn't have a 1/80 setting, Mr. Well's suggestion will give the same result.
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