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Some advice on lenses?

Francis Elvans

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Sorry to bring up what has probably been mentioned in some form many times before.


I have questins about the use of 1. Non RX C mount lenses and 2. A c mount-nikon lens adapter with nikon lenses.


I am aware that there can be two nasty symptoms of using non RX lenses on the Bolex reflex cameras, one is soft images on lenses under 50mm and the other is an inability to focus to infinity.


I have read a general rule that stopping down to f4 and beyond will avoid the soft images, but is this true of all focal lengths under 50mm? I have some regular c mount lenses, Zeiss Jena 10mm, 16mm and 25mm, do you think these shorter lenses will be usable even when stopped down to at least f4?


With regard to the focusing to infinity issue, does this occur on all non rx lenses? Can this be avoided (I read something about unscrewing the lens slightly, (3 point something mm), which suggests there is a possibility of using a washer of somekind on the lens screw)?


Will the focus-to-infinity issue be present whilst using a nikon adapter and nikon lenses (esp 24mm and 50mm)?


Once again, apologies for bringing up this old lens issue, but finding a precise answer has so far been difficult. I know that crucially the answer will be in testing, but I hope you can point me in the right direction so that I can save on resources and do more useful tests.


All the best,



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Stopping down will sharpen soft images. F4 prob wont be enough to cut it and you lose your dof control... not acceptable much of the time.


When light travels through anything behind the lens it throws off the backfocus. It also can refract. The rx cameras have that prism which the light has to travel thru. The rx lenses accommodate. My experience with the bolex rx issue is that the biggest problem is the backfocus. As i have adjusted this before and the refraction softening wasnt really evident. Now I havent done a detail chart comparison and it would probably show up to some degree. I would think it would have to. Or bolex would have just sold the equivalent of a macro ring that would have solved it.


I use 35mm lens to cmount adapters. The prob of the non rx is still the issue. Once you address that, adapters with accurate FFD work ok.

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Thank you for your help.


Forgive me for asking what might be a simple question (I am quite new to this) but how would one adjust the back-focus? Does this mean that the focus will be wrong even though it appears correctly in the view-finder? Or is it a case of the focus not correctly matching up to the focus scale on the lens (this is what I previously thought)?


Also, I would be grateful if you could recommend a decent Nikon to c mount adapter.


Thanks again.

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I think if it looks sharp in the viewfinder and your viewfinder is properly adjusted then yes its sharp at the film plane. BUT bolexs are all old. Things get out of alignment with time. So if you want to be certain you need to check at the film gate. I would acquire one of the bolex prism critical focus checkers that attach to the film gate. It looks like a mini periscope / microscope. You take the pressure plate out and this thing snaps in via a magnet and allows a view of what is coming through the film gate. Get as close to what you figure is operating sharp thru the critical focus checker. Adust your viwfinder to match and you should be good to make some test exposures. I would then do some test exposures and scrutinize them magnified to see if what you are getting really is truly sharp. Do this at infinity all the way close and a few steps in between. Measure your distances with a tape and notate so you can compare to the distance scale on the lens barrel. If all looks good, and your confident in your settings, you're ready to start filming.


Backfocus is focusing behind or in front of what the lens was zero calibrated to throw as far as a film plane image. Nearly all lenses are NOT adjustable for backfocus. So you have to shim them. Glass such as viewfinder prisms require an adjustment to that backfocus to accommodate for the light going thru the glass before hitting the film plane. In this case you would need to increase the flange focal distance to accommodate for the viewfinder prism. So thats why you read a 3mm spacer works for a lens that was not designed to use the rx viewfinders. I cant verify that 3mm is perfect. But that seems like it might be right. Switar RX lenses have this all built into them. They optically correct for the backfocus change of the prism and recapture any scattered beams to produce a nice image with a proper distance scale on the barrel.


Do not forget that you are talking about a viewfinder PRISM. Not and optically clear pane of glass. I have no idea what kind of refraction is going on thru that thing. But i do know from experience if you use the methods i stated above to set your rig up, you will at least be in control of the situation. If you dont have any issues with the test images you may not have any issues with your final film. BUT some things like highlight edges may show some issues youre not happy with. Of course thats true of any lens. No matter who made it or how its set. Its ALL about testing before you shoot. Youre just adding more variables into the mix to worry about than just using a matched lens to begin with.

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Ah the Bolex prism..


So much confusion about this little chunk of glass - light loss, back-focus, spherical aberration, RX vs non-RX lenses - maybe Bolex should have gone with a pellicle or a guillotine mirror like Beaulieu.


The 3mm back-focus adjustment thing is a common misconception, based on the fact that the physical flange depth (ie the actual distance from lens mount seat to film plane) on reflex Bolexes is 20.76mm while a C-mount lens is designed to image 17.52mm behind the mount. But this is confusing the optical distance with the physical distance. The truth is that RX lenses will also form an image at 17.52mm if the prism isn't in the optical path. Any C-mount lens made for 16mm (including RX lenses) fitted to a reflex Bolex will have the light rays coming out the back refracted a bit by the prism and the lens focal plane extended out to around that 20.76mm point. The problem with a non-RX lens is that while the central rays will focus at that point, the rays passing through the outer regions of the lens aperture will focus a bit past it, causing a blurring of detail. As you stop down, you use more of the central rays and the spherical aberration diminishes. (I should clarify here that the outer or marginal rays are not referring to the outer edges of the image, just the edges of the light ray path.)


Now, because the central rays are focussing at 20.76mm but as the rays become more marginal they focus further and further beyond this point, the total error at full aperture when all the rays are combined can throw the focus scale off. You can adjust the lens back-focus slightly (certainly not 3mm, closer to about 0.1mm or the equivalent of unscrewing a C-mount by 1/8 of a turn) to re-align the focus scale at full aperture, but as you stop down the point of best focus will shift back and your adjusted scale will now be a bit out. The softness caused by the spherical aberration won't change either, you're just playing with where the focus scale lines up. It won't make any difference if you're focussing by eye.


You should be able to reach infinity with non-RX lenses, if anything the introduced aberrations will cause the lens to pass through it. If you're not reaching infinity the lens or camera or adapter has an issue.


All this complication is why it's better to just use RX lenses or use non-RX lenses stopped down a bit. But once you shoot some tests you might find the issue isn't even that noticeable.

Edited by Dom Jaeger
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I just checked an rx8 i left setup with a non-rx berthiot zoom. Its setup with the same spacer ive used for other non rx lenses and its about 3mm. To make sure I havent changed my tolerance for whats in-focus over the years, I just checked it on a siemens star at all focal lengths at full open. Its really pretty dang sharp. 2 weeks ago i ran across some footage i shot with that rig and was enjoying it thinking it looked nice. So i wont debate the science or anybody elses experience. Im just saying heres an example in working status.

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Lenses for Bolex 16mm Cameras

March 21, 2007 -- Bolexcollector.com

The following technical information is taken from a 1974 bulletin, published by Paillard Inc., NY, explaining the use of RX mount lenses with the Bolex H16.




Among the lenses for 16mm Bolex cameras are some designated for use on non-reflex 16mm cameras only, some which are usable on reflex and non-reflex models, and some designated RX types, meaning they can be used on Bolex 16mm cameras with reflex viewing only.


Since all these lenses have the same "C" type thread (1" diameter, 32 threads per inch), the question naturally comes up regarding the difference between the various types and whether or not other makes of lenses can be used on Bolex 16mm cameras.


Since the mechanical distance from the filmplane to the lens seat appears to be longer on reflex cameras, the opinion expressed most often is that RX and "C" mount lenses are adjusted for a different lens seat-to-filmplane distance. This, however, is not so.

Figure 1. RX and standard C mount lenses.

All Bolex lenses (Figure 1), whether RX or not, are adjusted for the same back focus and form an image at exactly the same distance of 17.52mm (.690") from the lens seat when set at infinity, providing there is nothing but air between the rear element of the lens and film. All these conditions are met in non-reflex cameras.

In reflex cameras, on the other hand, part of the space between lens and film is filled with the prism, which reflects part of the light into the reflex finder. The light which comes out of the lens no longer travels in air only.

Figure 2. Light is refracted by the reflex prism.

The light (Figure 2) which would normally form an image at A, enters the front surface of the prism where it is refracted (bent), and then comes out of the glass at the rear surface where it is refracted again, but in the opposite direction. As a result, the image is now formed at B, at a distance of 20.76mm, which explains why the mechanical distance in a reflex camera is longer.

Figure 3. Distance of image formed by lens is increased by reflex prism.

This change in the image distance is caused completely by the prisms and happens to any lens, whether RX or C (Figure 3), wide angle, telephoto, zoom, which explains why some lenses, like the Macro Switar 75mm, Macro Yvar 150mm, Pan Cinor 85, can be used on reflex cameras as well as on non-reflex models without an adjustment of any kind.


We can, therefore, state the following facts:

  • Without the prism, all the lenses form the image at 17.52mm -- with the prism, at 20.76mm.
  • Any 16mm lens adjusted for the standard "C" mount distance of 17.52mm will properly focus on all Bolex H16 cameras, whether reflex or non-reflex models.
  • All Bolex H-16 cameras, whether reflex or not, have the same opticaldistance from the lens seat to the filmplane.

Since all lenses adjusted for 17.52mm form the image at 20.76 when used in a reflex camera, the above statements still do not explain why special RX lenses are necessary. To find the answer, we must discuss the optical correction of lenses.


In an optically well-designed lens, the various faults or aberrations, such as chromatic, spherical aberrations, coma, astigmatism, etc., are minimized to a degree where they are no longer visible and objectionable on the film.


As an example, let's take spherical aberration, which is the fault of a single lens where light rays passing through the edge of a lens form an image closer to the lens than light rays passing near the center (Figure 4).

Figure 4. Spherical aberration.

In a lens corrected for spherical aberration, all light rays meet at the same point, form the image at the same distance (Figure 5).

Figure 5. A lens corrected for spherical aberration.

"Regular" 16mm lenses are designed to have a minimum amount of spherical aberration when the light behind the lens travels in air only. When we add the prism to such a corrected lens, light rays entering the edge of the lens reach the prism at a different angle than light rays passing near the center. They are refracted differently and, as a result, form the image in a different plane (Figure 6). The prism has re-introduced spherical aberration, resulting in a loss of sharpness and contrast when used on a reflex camera.

Figure 6. Spherical aberration introduced by the reflex prism.

Overcoming this loss of quality was the purpose of designing special RX lenses, which are lenses with a 17.52mm back focus but designed to provide maximum quality when combined with the prism (Figure 7). If such an RX lens is mounted on a non-reflex camera with a 17.52mm back focus, the image will focus properly but will show a loss of sharpness and contrast, at least at the larger apertures.

Figure 7. Aberration of RX lens corrected by reflex prism.

While the description up to now explains the difference between "regular" and RX lenses, it still does not answer why it is possible to use some lenses on either camera.


While the prism affects the optical correction of every lens, the loss of sharpness and contrast on many lenses is so small that it is not visible on the film. This applies to most longer focal length lenses, such as the Macro Switar 75mm, Macro Yvar 150mm and some zoom lenses like the Pan Cinor 85.


On other lenses, a loss of definition may be visible at maximum aperture, but will be eliminated by simply closing the diaphragm one or two stops down.


There is no definite rule to indicate which lenses are usable since this depends on many factors, such as the size of rear element, position of rear element to the prism, focal length of lens, aperture or general design of lens.

A film test is necessary to determine the quality or loss of quality. If the quality is unsatisfactory, an adjustment in the back focus does not solve the problem.

1"Bolex Product News From Paillard," No. 5, (New York: Paillard Incorporated, June 25, 1974).

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Hi Steve,


actually for a H8 RX you're absolutely right, you need to mount a standard C-mount lens about 2.2mm out from the camera lens flange. The H8 RX is a strange beast, using the standard C-mount thread of 1" 32 TPI, but with a shorter optical flange depth than the 16mm versions. The prism is also smaller, so the specially designated H8 RX lenses have less correction for the introduced prism aberrations (and normal C-mounts - properly spaced out - will be less affected). If you try to use H8 RX lenses on a H16 RX they won't focus anywhere near infinity.


Another Bolex idiosyncracy.. ;)



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  • 2 weeks later...

Dom that figures. My 16s right now are m5s. I have messed with the rx 16s but its been so long now... I almost bought a couple last month. I was convinced I had the same experience with them as I did the 8. I tell you what.. I always used 35mm format lenses on adapters. If youre only really using the center of the lens, is it possible it changes the effect of the prism?

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