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Bolex H8 REX Lenses


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#1 Trevor Jahner

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Posted 20 September 2012 - 09:10 PM

So after researching this on the web for a few hours I'm still lost as to what lenses can be used on an H8 REX. Bolex Collector seems to say that only the H8 RX Kern lenses will work, along with the Vario Switar zooms 36 and 36EE. I've also seen some on eBay with Som Berthiot zooms and Pan Cinor 40 zooms, but I'm not sure whether these are suitable. Bolex collector also says that other c mount lenses won't seem to work because of the 15.305 flange focal distance (shorter than 16mm c mount ffds) and issues with the beam splitter prism in the reflex viewing system, with the exception of longer focal length lenses.

Taking all that into account I'm wondering if there are any other options than the H8 RX primes and the two zooms mentioned on Bolex Collector. The obvious choices seem to be Wollensak c mounts or Som Berthiot c mounts (are these of comparable quality to the Kerns?). Are there any simple conversions that would work, for instance a d mount to c mount conversion ring? I'm guessing most conversion rings wouldn't be ideal due to beam splitter issue.

In any case if anyone has experience using non-Kern H8 RX lenses and can offer advice or generally provide a more comprehensive vision than the one I've pieced together on the web it would be much appreciated. It would be nice to know what options there are besides the H8 RX as these seem to be slightly rare and pricey compared to some of the other c mounts.

Thanks!
Trevor
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#2 Dom Jaeger

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Posted 21 September 2012 - 02:15 AM

Unfortunately there's not a lot of choice in lenses for the H8 RX. Apart from the 5.5, 12.5 and 36mm Kern Primes, the 8-36 Vario-Switar zooms and the Berthiot Pan Cinor 40 RX, there were H8 versions of the 100mm and 150mm Kern Yvars. They can also be converted from H16 to H8. A new mount can be fitted, which is about 2.2mm longer and is engraved 'H8 RX'. More tricky to identify are those that have simply had the back-focus adjusted, but may not be marked as H8.

With shorter focal lengths the problem is not just the different back-focus from H16 RX, but the difference in prism thickness. The H16 RX lenses are designed to correct for a thicker prism, and need more correction for the introduced spherical aberration than the H8 RX ones. I'm guessing that's why Kern didn't offer any other 16mm RX lenses for conversion. But it's possible to get them to infinity focus on a reflex H8 by shimming out the mount by 2.2mm (though the lens is then only screwed in by one or two threads), and by stopping down a little the aberration will diminish (if you even notice it).

Some older (non-RX) C-mount lenses have longer mount threads, so you could shim them out and still have enough thread for a more secure mounting. The effect will be a reverse spherical aberration to using H16 RX lenses (ie positive vs negative), but again a little stopping down will reduce it. It's a shame C-mounts are becoming so pricey though, I think it's a bit mad spending hundreds of bucks on a single lens for Standard 8mm.

In particular I don't know why the H8 RX primes are so expensive. There can't be that many H8 RX users out there, so it must be digital photographers, but why do they want to pay hundreds of dollars for a tiny image circle with spherical aberration? :blink:
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#3 Joel Pierre

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Posted 22 September 2012 - 11:25 AM

It is also possible — but not easy at all — to use a macro-zoom Schneider-Kreuznach designed for Beaulieu Super 8 cameras. The distance setting is done with the macro lever and must be modified for each zooming. It is very restrictive.

Edited by Joel Pierre, 22 September 2012 - 11:26 AM.

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#4 steve waschka

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Posted 29 September 2012 - 06:06 PM

i sold my primes. they produced pretty images but the folks online wanted them more than i did. i assume your noticing that about now while pricing them. i rebuild cameras. not for a living just to keep my stuff running and help others from time to time. occasionally that means going inside the lenses. fungus and bent iris leaves... you know... the simple stuff. the reason i sold my primes is because i have a pancinor 17-85 on there. i know... it shouldnt work and its been a while since i did this. but, in the older black models with the prism style dogleg viewfinder (removed i use the h8s finder)... the rear element is set in place with beveled packing washers. manipulating their direction and adjusting the retaining ring is similar to adjusting back focus on a betacam news rig. except a real pia to access. you gotta open and close the lens every adjustment. then i hand polished a stainless shim washer to the right depth and bingo. zoom lens on the h8. i used the thing for over a year and my primes just sat so i sold them. the process took hours and days of fiddling using a prismatic focusing aid on the gate to check the progress. and extreme care in handling the internals of a lens. i really wish kodak made vision stock in reg 8. you take 8mm and get it scanned nicely and its cool stuff. my h8 is newer (block base and the big viewfinder) but ive never had any weave or frame jumps. its depressing to think, but one day... and prob soon, that machine is gonna be a paper weight.
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#5 Glenn Brady

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Posted 11 October 2012 - 01:46 PM

. . . there were H8 versions of the 100mm and 150mm Kern Yvars. They can also be converted from H16 to H8. A new mount can be fitted, which is about 2.2mm longer and is engraved 'H8 RX'. . . .


I was aware that there were H8 and H16 versions of the 100mm and 150mm Yvar lenses and that a factory conversion from one to the other was possible at one time. The Bolex factory representative whom I contacted here in the USA about such a conversion denied it was possible. If such conversions are still done, who does them? Wouldn't the H16 mount be longer and not the H8 (the H8 RX ffd being 15.305mm and the H16 RX ffd being 20.76mm)?
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#6 Dom Jaeger

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Posted 11 October 2012 - 08:44 PM

I was aware that there were H8 and H16 versions of the 100mm and 150mm Yvar lenses and that a factory conversion from one to the other was possible at one time. The Bolex factory representative whom I contacted here in the USA about such a conversion denied it was possible. If such conversions are still done, who does them? Wouldn't the H16 mount be longer and not the H8 (the H8 RX ffd being 15.305mm and the H16 RX ffd being 20.76mm)?


It's a very simple conversion, just the lens mount gets swapped over. On long focal lengths with a large focus travel it's possible to adjust the back-focus instead, bringing the collimated focal plane 2.2 mm forward. I haven't done that with those Yvars personally, so I'm not sure if you lose some range at minimum focus. Swapping over the mounts means you don't have to adjust the back-focus. Any lens tech should be able to do it easily, they just need the part.

Here's what the 2 mounts look like:

Posted Image

The H8 RX mount is longer - it effectively shims the lens 2.2 mm out from the mounting surface, bringing the focal plane forward so that the lens images 15.3 mm behind the mount instead of 17.52 mm as all standard C mount lenses do. The 20.76 mm figure for H16 RX cameras is the physical flange depth of the camera, because the prism in the path diffracts the light rays and extends the focal plane back, but in air H16 RX lenses are still set to image 17.52 mm behind the lens mount, just like non-RX C mounts. The only difference is the RX lenses are corrected in their optical design for the aberrations introduced by the prism.
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#7 Jean-Louis Seguin

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Posted 12 October 2012 - 07:54 AM

I believe these interchangeable mounts illustrated by Dom are only for the later Macro-Yvar series of lenses. I have seen one regular 75mm Yvar marked H8RX and in this case, the whole rear helicoid is a different, longer part.

Cheers,
Jean-Louis
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#8 Jean-Louis Seguin

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Posted 28 December 2012 - 08:53 AM

I have been sent some pictures taken by a non-member of this forum to illustrate the Yvar 75mm f/2.8 lens in H8RX version.

I have posted them full-size here:
http://www.flickr.com/photos/bolextech

Here is his message:


Attached are few unedited shots. Feel free to edit and crop them as You wish.
Here's the measurements:

Diameter of the opening in the push-in stop - 15.95 mm
Diameter of the slot where the stop is pushed - 21.35 mm
Outside depth of the threaded c-mount section - 3.82 mm
The short flange between c-mount section and the knurled ring - about 0.80 mm
The knurled ring - 6.79 mm

The width of the knurled ring is 33 mm.

There's a pic with the focusing set to infinity, the visible length of the black barrel between the knurled ring and the moving focusing ring flange is 3.25 mm.
And another where the focus is as close as it gets, just below 5 feet. The visible length of the black barrel is 7.20 mm.

You probably have access to some 'standard' 75 mm Yvars. They seem to have a threaded stop instead. Also the minimum opening is set deeper into the barrel.

That's about it. If You can suggest other measurements or something else, I'll do my best.


Cheers,
Jean-Louis
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