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Daniel Meier

Bolex Rex 5 - Infinty Focus and Parfocal Issues

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Dear community.
I recently got me a Bolex Rex5 camera. The set included two lenses.
The Kern Vario-Switar 18-86mm/f2.5 zoom and a Schneider Cinegon 10mm/f1.8 prime lens.

It seems like the focus of the 10mm won't reach to infinity.
Here is a video demonstrating that:

https://vimeo.com/426998946/3241902a4f



The zoom lens also gives me trouble keeping focus throughout the zoom range.
A Bolex user (Ben Ericson on this forum) told me that his 18-86mm is parfocal.
On my camera it doesn't work as a parfocal lens. Here is also a video demonstrating that:

https://vimeo.com/426999919/166835a0f8

 

Things I already tried troubleshooting:
- checking multiple diopter settings of my viewfinder (making sure the groundglass is in focus - I guess you can also tell from the video samples that it is)
- making sure the lenses are tightly attached on their mounts


Do you have any idea what might be the problem here?
Is it the camera itself that needs adjustments? 
I bought the set for about 1200€. The seller said that all components are in perfect conditions.

 

 

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do you have a MFT mount stills camera or similar camera which can be used with C-mount adapters? 

it should be pretty easy to test the lens with such a camera to see what it exactly the problem. I personally use GH4 with C-mount to MFT adapter for this type of tests. easy to see the lens characteristics by using the 1x or 2x magnify on the screen

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That's a good idea. 
Although I think the lenses are ok.

But they might make a good fit on my BMPCC4K.

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First off, make sure you are focussing the eyepiece diopter on the ground glass. Point the camera at a diffuse light source like the sky and focus the eyepiece until you see the grain of the ground glass. Don’t focus on specks that may be on the optics just behind the ground glass, look for a uniform fine grain surface. 

If the lenses still seem off, the camera may need adjustment. Less likely is the possibility that both lenses need adjustment.

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Ok, well then both lenses are focussing short which is a symptom of lenses sitting too far away from the film plane. But if you’ve thoroughly checked that nothing is stopping the lenses from being screwed in all the way, then you need to have the kit looked at by a Bolex technician. You could check the lenses on another camera like Aapo suggested, or check the camera with another lens, but in the end it seems either the camera or the lenses need adjustment which is something a technician with a collimator and the right tools needs to do. 

Or ask the seller for a refund.

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Bolex RX (Reflex) cameras need RX lenses to focus properly at wide angles. This is due to the beamsplitter prism altering the flange distance, compared to the original non-reflex cameras. Non-RX lenses intended for use on Bolex cameras without a reflex viewfinder are sometimes labeled AR. I’m sure someone more familiar with Bolex cameras can explain better. The Switar 10mm RX was a common wide angle lens used on the RX cameras.  

Non-RX lenses are not a good idea on Bolex RX cameras until you get to around 25mm focal length, if I recall correctly. After that, the depth of focus behind the lens is deep enough to cover the issue. I’m not surprised the zoom won’t maintain parfocal performance, though you might be able to focus to infinity on the long end.

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Hi Satsuki, nice to see you back on the forum!

The Vario-Switar is definitely a parfocal zoom, and was designed specifically for reflex Bolexes. It should hold focus through the zoom range, unless it’s out of tolerance.

The 10mm Schneider came in both normal and RX versions (it will be written on the barrel), and it’s true that an RX one would be better to use on a Reflex Bolex. But if this one is not RX it would not throw the focus out to the point where the lens won’t reach infinity, rather it would just exhibit aberrations in the image at wide apertures, a bit of spherical aberration and astigmatism. The difference between RX and non-RX lenses has nothing to do with back-focus, it’s whether the lens has been optically corrected to compensate for the prism or not. You don’t need to worry for lenses 50mm and up.

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Cheers Dom. Had a family health crisis to deal with, but everything is more or less stable now. What did I miss?

Thanks for the correct Bolex info, haven’t used one in over a decade and didn’t know it that well to begin with!

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7 hours ago, Satsuki Murashige said:

 What did I miss?

Oh a few heated discussions.. after one on the pandemic response I think Richard B stormed off the forum and deleted half his posts, which is a shame.. been pretty quiet since then..

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Tried both lenses on my BMPCC4K today using the Metabones C-Mount to MFT adaptor.
Both worked great.

The 18-86mm was nearly 100% parfocal on the BMPCC4k.
And I could focus even past infinity on the 10mm.

Must be a misalignment in the Camera then, right?

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Can you handle a micrometer?

It would be a measurement of the FFD from lens seat, the rim ahead of the mount thread, to aperture plate (lateral rails). Be careful to not scratch the rails. You should read .8175" or 20,7645 mm. 20,75 mm is good.

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A technician needs to check the flange depth with an autocollimator, a test lens and a mirror block in the gate.

This measures the “optical flange focal distance“ which is what needs to be measured on late model reflex Bolexes, because there is a tolerance range of the thickness of the prism (9.30 to 9.52mm), which will affect where the film plane lies. Measuring the mechanical depth is therefore not a reliable indicator, and is difficult to achieve accurately anyway with a micrometer. 

6D4E7D14-1029-4B58-ABAF-2773CB484A2C.jpeg.6d4dabfd312fd915034baf3e8f5db171.jpeg

 

Judging from the focus footage, the OP is also judging focus only through the viewfinder, rather than a film test. This is a seperate (though linked) setting to the flange focal distance, and also needs checking.
 

620076C6-42EF-4B67-80A1-76A87191126C.jpeg.6f8a47934728a485c6978bf997ef09ce.jpeg


Note that there are many tools and jigs needed to properly work on a Bolex, though a skilled and knowledgeable tech can get by without some of them. An autocollimator is pretty essential though.

It’s unusual in my experience for the Bolex flange depth to be out by much, unless the turret itself is loose or deformed, so the whole camera really needs an experienced tech to go over it. It may have been fiddled with, or something is causing the lenses not to seat properly.

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