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Urgent Bolex Question


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#1 Sage E

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Posted 11 October 2008 - 10:33 AM

The Bolex I am using is a H-16 Refex. I'm unsure if it is one of the REX-1 through REX-5s, but it's serial number begins with 203.

I was wondering how much I should compensate for light lost in the reflex, or if the wider (narrower?) shutter angle already compensated for this. I will be shooting at 24fps but have been told variously to meter for 1/60th shutter speed or 1/80th. Now I am just really confused. Any one shot on this or can shed some light on the matter?
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#2 lluis

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Posted 12 October 2008 - 02:42 AM

The Bolex I am using is a H-16 Refex. I'm unsure if it is one of the REX-1 through REX-5s, but it's serial number begins with 203.

I was wondering how much I should compensate for light lost in the reflex, or if the wider (narrower?) shutter angle already compensated for this. I will be shooting at 24fps but have been told variously to meter for 1/60th shutter speed or 1/80th. Now I am just really confused. Any one shot on this or can shed some light on the matter?


1/60th if you use Switar RX lens.
1/80th if not.
Best,
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#3 Jean-Louis Seguin

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Posted 12 October 2008 - 07:25 AM

For 24fps, just set your meter to 1/80th sec.

It has nothing at all to do with the lens.

Cheers,
Jean-Louis
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#4 Tim Terner

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Posted 12 October 2008 - 07:52 AM

I'm also confuded by this Jean-Louis. Why are the lens marked RX ? and if I use the RX lens on my eclair are the T stops then accurate ? Hope you can shed some light
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#5 Simon Wyss

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Posted 12 October 2008 - 09:26 AM

I'm also confuded by this Jean-Louis. Why are the lens marked RX ? and if I use the RX lens on my eclair are the T stops then accurate ? Hope you can shed some light

The RX mark means that the particular objective has an additional lens as compensation for the prismatic focus error introduced by the reflex prism block. To compensate for this error you stop a non-RX objective down to f = 3.3 or more. Such information was published years ago. By chance I know of. It's about 9 millimeters of glass between lens and film and even more between lens and ground glass (actually the etched top surface of the prism block). The inner combined surface of the two prisms deflects 25 %. That is the additional quarter from 1/80 to 1/60 second.
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#6 Jean-Louis Seguin

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Posted 12 October 2008 - 11:04 AM

I'm also confuded by this Jean-Louis. Why are the lens marked RX ? and if I use the RX lens on my eclair are the T stops then accurate ? Hope you can shed some light



The RX designation on lenses has nothing to do with exposure.
It refers to the fact that the lens is designed to give optimum sharpness when used on a Bolex H16Reflex camera using a beamsplitter in the image forming path. The beamsplitter in the camera is considered as if it is part of the lens design.

As an added consequence of the beamsplitter, about 25-30% of the light that would normally reach the film is diverted to the viewfinder system. To compensate for this exposure loss, 1/3 stop is added to the shutter speed value for an effective exposure time at 24fps of 1/80th second.

As far f/stops and T/stops are concerned, to the best of my knowledge, all Bolex Switar/Yvar/Pizar lenses are marked in f/stops not T/stops.

Cheers,
Jean-Louis
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#7 Tim Terner

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Posted 12 October 2008 - 02:47 PM

Thank you Simon And Jean-Louis.
I bought an Eclair NPR that came with 3 Switar preset RX lenses (10, 26 & 75) but have never used these on the Eclair but only on the Bolex I have that came without lenses. Am i correct in thinking that if using these lenses on the Eclair fully open, it means focusing by tape and not by eye ?
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#8 Richard Tuohy

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Posted 12 October 2008 - 06:14 PM

Thank you Simon And Jean-Louis.
I bought an Eclair NPR that came with 3 Switar preset RX lenses (10, 26 & 75) but have never used these on the Eclair but only on the Bolex I have that came without lenses. Am i correct in thinking that if using these lenses on the Eclair fully open, it means focusing by tape and not by eye ?


Hi Tim,
no. If anything, the converse. The markings on the lens are very accurate (if the lens is correctly collimited) but ONLY when used on an RX camera. The thing is the RX lenses were designed for the RX cameras reflex system (the prism mentioned above). You could use them on other c-mount cameras, but it would be advisable not to use them fully open at all with such cameras. Stopped down, you will get away with it without too much trouble. Save them for the rx as they are ideal.
rt
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#9 Tim Terner

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Posted 12 October 2008 - 09:32 PM

Understand fully now, thank you Richard
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#10 John N Campbell

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Posted 21 December 2008 - 11:47 PM

If the serial number begins with 200 its probably a Rex 2 or Rex 3. The Rex means it has a reflex viewfinder which sucks about 25% of the incoming light away from the film and channels it into the viewfinder. All you need to do is add an extra 1/4 stop to compensate. So if your light meter tells you to expose at f8 (or t8), open the lens up to f5.6 and 3/4. When in doubt it's always better to overexpose than to underexpose. If you overexpose, the print can always be printed darker, but an underexposed negative is harder to print lighter and still have detail in in the image.
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#11 Richard Tuohy

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Posted 22 December 2008 - 06:32 PM

If the serial number begins with 200 its probably a Rex 2 or Rex 3. The Rex means it has a reflex viewfinder which sucks about 25% of the incoming light away from the film and channels it into the viewfinder. All you need to do is add an extra 1/4 stop to compensate. So if your light meter tells you to expose at f8 (or t8), open the lens up to f5.6 and 3/4. When in doubt it's always better to overexpose than to underexpose. If you overexpose, the print can always be printed darker, but an underexposed negative is harder to print lighter and still have detail in in the image.


this isn't correct I am afraid. See the above posts. In a nut shell, the reflex system accounts for a 1/3rd stop loss of light. The 135 degree shutter accounts for another 1/3rd stop difference from the 180 degree shutter a normal hand held light meter will assume. Thus the correct compensation is 2/3rds of a stop. For example, a normal cine meter set to 24 fps will give readings based on a 48th of a second shutter speed. The 135 degree shutter means that such a reading will be 1/3rd wrong. A 60th of a second is closer to the actual shutter speed. But then the reflex viewfinder loss of an additional 1/3rd stop means that an effective shutter speed of 1/80th is required. This accounts for the 2/3rd difference between what a normal meter will read and what you should set the camera to. I find it better to compensate with the asa rather than the shutter speed. Thus with a 100 asa stock I set the light meter asa to 64.
You are right in saying that with negative film it is better to err on the over exposed side.
richard
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#12 Chris Millar

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Posted 23 December 2008 - 04:25 AM

:rolleyes:
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#13 John N Campbell

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Posted 28 January 2009 - 12:22 PM

In a nut shell, the reflex system accounts for a 1/3rd stop loss of light. The 135 degree shutter accounts for another 1/3rd stop difference from the 180 degree shutter a normal hand held light meter will assume. Thus the correct compensation is 2/3rds of a stop. For example, a normal cine meter set to 24 fps will give readings based on a 48th of a second shutter speed. The 135 degree shutter means that such a reading will be 1/3rd wrong. A 60th of a second is closer to the actual shutter speed. But then the reflex viewfinder loss of an additional 1/3rd stop means that an effective shutter speed of 1/80th is required. This accounts for the 2/3rd difference between what a normal meter will read and what you should set the camera to.



Sorry Richard but your reply has several errors which I feel I must correct. I double-checked with Deiter Schaeffer at Procam and he confirmed that the Reflex system causes a 25% loss of light, so 1/4 stop more is the correct figure. Deiter is probably the most knowledgeable person in this country on Bolex cameras and I've been using the 1/4 stop compensation figure for 18 years now with perfect results. 1/4 stop more is also the figure taught to me in film school by Chris Malkiewicz who wrote the seminal book "Cinematography". So the figure I'm quoting is from at least 2 highly respected sources.

Most Bolex cameras have a shutter opening of 130 degrees (not 135). With the variable shutter open fully, running at 24 fps the exposure per frame is 1/65 of a second. Compensating for the 25% loss of light through the reflex system equates to an exposure of 1/80. (In single frame mode the exposure is 1/30, adapted to 1/40). I'm getting these figures from a table provided to me by Dieter at Procam. All these fractions and shutter angles are enough to make your head explode if you try to do the math in the heat of the moment, so just keep it simple and add 1/4 stop more. It's simple and it's accurate!

On a practical note, compensating either 1/4 or 2/3 stop (Richard's figure) on today's wide latitude negative stock is probably not going to matter much. It's just nice to know the correct figure in case you're in a situation where multiple small errors could add up to cause a real problem.
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#14 Chris Millar

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Posted 28 January 2009 - 05:21 PM

One thing that I learned at the beginning of my bolex experiences is that uncompensated footage comes out fine anyway ;)

Yeh sure with everything else equal its a smidge underexposed, but that is never the case and with all the other factors at play on an average set its nothing.
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