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10mm Switar Lens on Eclair ACL


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#1 Super16Eclair

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Posted 22 May 2006 - 07:05 PM

Hi, I've just used a Switar 10mm H16 RX lens on my Eclair ACL (Super 16). The lens looked super sharp through the viewfinder, but on researching this lens I have found that it was designed particularly for the aberation caused by the prisim in the Bolex camera. Does this mean my shots are actually going to be out of focus on the film??
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#2 Brian Rose

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Posted 23 May 2006 - 10:43 AM

The lens you have is meant to correct for light loss due to the Bolex's prism viewing system. A certain amount of light (I believe it is a half stop--others correct me if I'm wrong) is directed to the eyepiece for viewing, instead of passing directlyl to the film. As a result, with most c-mount lenses, you must compensate for this loss by opening the aperture an extra half stop to a stop on top of what your exposure meter tells you. Some lenses, like your switar however, were designed specifically for the Reflex, and already compensate for the light loss due to the reflex viewing system, thus negating the need to overexpose. What that does mean, on the other hand, is that if you use a lens intended for a Bolex Reflex on a camera like an Eclair that uses a mirror viewing system, you will have to underexpose by a half stop or so, to compensate for the additional light that would have been directed to the film, were it a Bolex. So, although your image ought not to have any sharpness issues, the film may be slightly underexposed, depending on the type of film stock you used. Understand?
Best,
Brian Rose

Edited by Brian Rose, 23 May 2006 - 10:46 AM.

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#3 Ian Marks

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Posted 23 May 2006 - 11:25 AM

I think Brian missed an important point - the Rx lenses are designed to take into account the optical effect of the Bolex's behind-the-lens prism. The ACL has no prism - the light from the lens passes directly to the film during exposure.

Since you've apparently already shot some film, your results will determine conclusively if the Rx Switar worked properly, although if the image on the ACL's ground glass was nice and crisp, I'd say you're probably going to be okay. Also, shooting stopped down below, say, f4 ought to minimize any problems - although the best advice is usually to test, test, test. If there's a problem, at least you know that non-Rx C-mount lenses are plentiful. You might look for a Schneider Cinegon 10mm instead of the Switar.
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#4 Super16Eclair

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Posted 23 May 2006 - 04:37 PM

Thanks Brian and Ian for your responses. Firstly, the extra half a stop will not be a problem, as I was shooting negative film, which can easily handle that amount of overexposure. I can simply do some correction in telecine. The image was certainly very sharp in the viewfinder, and in most cases I was shooting at least F4 (although a few shots were almost wide open, so it will be interesting to see the results). After I posted the question I performed a crude test on my camera. I took the magazine off the camera, fitted the 10mm lens and pointed the camera at a bright subject. I then held a piece of thin tracing paper firm against the film gate and found a sharp image was projected onto the tracing paper. Not the most accurate of tests, but it restores a little confidence.

Regarding the Schneider Cinegon 10mm. Does this cover Super 16? How does it compare to the quality of the Switar?

I'll let you know how the telecine goes next week. I'm holding my breath until then...
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#5 Ian Marks

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Posted 23 May 2006 - 04:52 PM

Regarding the Schneider Cinegon 10mm. Does this cover Super 16? How does it compare to the quality of the Switar?


The "tracing paper in the gate" test is probably no better than the "looking through the viewfinder at the groundglass" test, unless you suspect that your ground glass is out of adjustment (in which you have a bigger problem). Tracing paper can flex and flop around. That being said, I've done the same thing with a little "invisible" scotch tape.

There are two c-mount 10mm Cinegons. The one that covers Super 16 - the one that I'm suggesting you look into if the Switar doesn't work out - has a larger front element and accepts a 55m screw-in filter. I'm no expert, but I think both the Schneider and the Switar are fine lenses.
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#6 Super16Eclair

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Posted 29 May 2006 - 06:32 AM

Just had my footage telecined and it looks amazing. Not a problem with the Switar lens, it was pin sharp. In fact, I could hardly pick the difference between the Switar and my beautiful Zeiss 12-120 zoom. The Zeiss seems a fraction more contrasty. Exposure didn't seem to be off either (although the telecine process can hide slight exposure variations). So a 10mm Switar can be used on an Eclair ACL. It even covered the Super 16 frame without a problem.

The only problem I had was a slight pulsating from some domestic fluro tubes while shooting at 50fps, which is not lens related of course.
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#7 Sam Wells

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Posted 29 May 2006 - 09:05 AM

I wouldn't give any exposure compensation with the Switar RX lenses used on a mirror reflex camera.

Strictly speaking they will be overcorrected for spherical aberration and astigmatism when used on your Eclair but may well look good anyway as you've discovered; and as Ian said by f4 you are probably pretty safe.

-Sam
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#8 Clive Tobin

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Posted 29 May 2006 - 06:55 PM

The lens you have is meant to correct for light loss due to the Bolex's prism...


This is absolutely false and I don't know why this misinformation gets repeated and repeated.

Bolex has made it very clear in all their literature that the RX lenses are not calibrated differently. On the contrary, they clearly state that "adapted" shutter speeds are used in the Bolex light meter instead, to allow for prism loss. NOT in the lens calibrations.

When using other than the Bolex light meter, it is very well known in the industry that you need to set 1/80 second in the speed dial, or else lower the film ASA setting by 1/3 of a stop, to correct for the fact that the light efficiency is reduced by 20% from the physical 1/60 exposure time.

The only difference in the RX lenses is that they correct for dispersion error caused by the prism. Dispersion causes red, green and blue information to focus at slightly different distances. Even this slight error disappears when you stop down to f/2.8 or smaller, or are using a lens of 75mm or more focal length. Some lenses seem to work equally well on RX and C mount cameras but the only way to know for sure is to test.
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