Anyone looking to buy c-mount lenses is, sooner or later, going to come across the so-called RX lenses made for the 16mm Bolex Reflex cameras. There has always been a lot of confusion as to the exact difference between these and ‘standard” c-mount lenses, even sometimes among filmmakers and photographers of considerable experience. This article explains the difference.


RX lenses have exactly the same 25.4mm diameter mount as other c-mount lenses and, contrary to a common misconception, back focus is adjusted to have exactly the same flange focal distance (in air) of 17.52cm. The difference between the two lens types lies in the optical design to eliminate or reduce the various aberrations or faults and, in particular, spherical aberration.


First, lets take a look at spherical aberration. On a simple, uncorrected lens (Fig. 1), the light passing through the side of the lens forms an image closer to the lens than the light passing through the centre. In other words, the back focus distance is slightly shorter for light rays passing through the edge of the lens than those passing through the centre. This can be corrected (Fig. 2) so that all the light rays passing through the lens focus at the same distance.


Most reflex cameras, both for film and still photography, use a displaceable mirror set at 45° to the lens axis that reflects the light up towards the viewfinder. When an image is exposed, the mirror flips up for an instant so that the light travels to the film plane instead of the viewfinder. However, a movie camera exposes many frames every second so this system results in a flickering image in the viewfinder. In 1956, Bolex presented their H-16 Reflex camera which used a permanent glass prism instead of the moveable mirror. This prism was made of two 45° glass prisms joined together with a semi-reflective coating at the join (Fig. 3). The coating reflected 25% of the light up towards a ground glass viewfinder (the upper surface of the prism) while letting the remaining 75% of the light pass through the prism to the film plane beyond. Bolex thus avoided the flickering viewfinder but, by adding a new glass element, altered the optical path between the lens and the film plane.

This glass prism, 9.5mm thick, refracts the light traveling from the lens to the film plane so that the image no longer forms at 17.52mm, but 3.24mm further back at 20.76mm. In fact, the Bolex Reflex cameras have a film plane at 20.76mm from the lens flange, and this is probably the reason why some people think the back focus distance of RX lenses is different. It isn’t: put either an RX or a standard c-mount lens on a Bolex camera with a prism and both will focus at 20.76mm. Take them off and, just through air, they will both focus at 17.52mm.

So far, so good. The problem is that this prism bends the rays from different parts of the lens by different amounts, reintroducing the spherical aberration that our hypothetical lens had been corrected for in Fig. 2 above. This time, however, it is negative spherical aberration – the light passing through the edges of the lens focuses further back than the light passing through the centre, resulting in a loss of contrast and sharpness (Fig. 4). Kern Paillard (followed by some other manufacturers, see below) therefore introduced the RX lenses where the optical design is optimized to compensate for the aberration effects of the Bolex prism (Fig. 5). Use one of these lenses without the prism, though, and the aberration reappears.


That’s the theory, but what about in practice? Can we use an RX lens on micro 4/3, Scarlet Cinema or another camera without the prism. Or conversely, what about putting an AR (standard c-mount) lens on a Bolex Reflex? Here the question gets more complicated (and views more subjective). Let’s start by saying that the correctly optimized version of the same model lens is always going to be better – it’s just a question of whether the difference is noticeable. This is going to depend on factors such as the resolution of the camera, the focal length of the lens, f stop used and the particular design of the lens itself, but we can make some general observations.

First of all, the longer the focal length the less the difference. Bolex themselves decided that above 50mm focal length the difference was negligible so Kern Paillard only made RX lenses up to 50mm and, as far as I know, no manufacturer made primes at longer focal lengths than this (there are zooms, of course, such as the RX version of the Angenieux 2.2/12-120mm). Secondly, the aperture used will make a huge difference. Since spherical aberration is caused by the light rays being bent differently at the edge of the lens from the centre, the more we stop down so that only the central rays are used, the more the aberration is reduced and the sharper the image. Therefore, one would expect the AR version of the Switar 1.4/25mm used at f1.4 on a digital camera to give better results than the RX version. Ultimately only testing and use of each individual lens can decide, although personally I would hesitate to buy an RX lens for digital use.


Now we know the difference, but how can we tell them apart? This is actually quite a problem, since most of the RX lenses were also made as non-RX lenses and are identical apart from some small marking. Many, many c-mount lenses were never made as RX versions at all, so in absence of any indication we should assume they are standard c-mount versions. As far as I know, the only makers of Bolex Reflex lenses were Kern Paillard, Som Berthiot, Schneider Kreuznach and Angénieux (zooms only). Most reflex lenses are marked RX or H16 RX. The Angénieux zooms are marked RX on the mount but also have an additional system: B for non-reflex or C for reflex at the end of the name. Thus the “Type 10 x 12 B” is non reflex while the “Type 10 x 12 C” is reflex (the “Type 10 x 12 A” is a non-reflex lens with integrated viewfinder). I have read that some of the earlier silver 2.2/17-68mm zooms were marked “Type L2” for non reflex and “Type L3” for the reflex version which was also marked “Special P” (for Paillard). Some people think the AR marking found on non-RX Kern Paillard lenses refers to the fact that they are standard c-mount. In fact it stands for “anti reflection” coating – the RX lenses have the same coating but the AR initials were left off the marking.


Dennis Couzin, 1976, THE TRUTH ABOUT THE BOLEX PRISM http://www.city-net.com/~fodder/bolex/truth.html
Lenses for Bolex 16mm Cameras http://www.bolexcollector.com/articles/07_03_21.html
Post by Boris Belay http://www.cinematography.com/index.php?showtopic=10117&st=0&p=75641&#entry75641


  1. Jason, a concise article. I think you will find that it is the blue light end of the spectrum that is affected more by the b/s prism than other colours, so it can also depend on what subject is being filmed, also the rx difference is more noticeable when lens open greater than about f3.3. I have converted to S16 hundreds, serviced more, serviced & modified many hundreds of Kern lenses, having found that many outdoor users never film with lenses fully open &, therefore, tell me they don’t have any concern using non-RX matched lenses on an H16RX camera. Some workshops have permanently converted an H16 to Arri PL lens mount, but no PL-mount lens is RX-matched; no one complains. I find an easy compromise is to use an H16SB/SBM/EBM/EL with an adaptor to PL lenses, the adaptor made about 0.0005 to 0.001″ thinner. Again, no one seems to complain. Extremely short focal-lengths/wide angle lenses are the problem.

  2. Hi Jason,
    Thanks for a great resource – truly very well done.
    A question for you on the great RX lens debate. I have a Super 16 H16 film bolex, and use 35mm SLR lenses with it. These are mainly Nikon fit, with a C mount adapter. I must say I’m paranoid to get the sharpest and best results from this set up. I haven’t made the plunge to buy dedicated rx lenses, but have used everything from 35mm to 85mm Zeiss ZF SLR lenses and Nikon 50mm AIS SLR lens, made for the 35mm format. Like you have advised here, the rx correction is not so valid on focal lengths longer than 50mm. So – my question is this . As I’m using the centre of the glass on these 35mm lenses (due to the crop factor), and on top if I prioritise the longer focal lengths (so say my 85mm Zeiss ZF) – would I reach the nirvana of sharpness for the bolex? i.e. the centre of the glass (said to be the best area, and avoiding the edges where the diffraction happens), and the >50mm focal length rule, shooting at f8 onwards? What do you reckon?
    Best Regards……Julian

    1. Hi Julian,

      As you say, on those 35mm SLR lenses you are only using the central part of the image circle so the aberration should be negligible.
      Also, you aren’t using anything wider than 35mm, which also helps.
      The Nikon and Zeiss lenses should both be excellent, so I reckon your images should be about as good as you can get.
      At the end of the day, it’s a question of testing and seeing the results, but I would expect them to be fine even shooting at much wider apertures than f8.
      Regards, Jason

  3. Good stuff.
    I’d like to add two things, which are more of the curiuos nature than of any practical use.
    Small batch of the very first Kern lenses for H16 reflex bodies were marked DV instead of RX engraving. I’ve seen two 10mm f:1.6 Switars marked DV, their serials being just a few hundreds apart. Don’t know if other focal lengths with DV-markings exist.

    What comes to RX-lenses of higher focal length than 50mm, such beasts exist. At least some 75mm f:2.8 Yvars were adjusted internally to some extent. A small engraved RX can be found at back of the barrel, somewhere close to the threading. These were made only upon request and one can easily go through hundreds of Kern primes and never see one of these.

  4. I found your article interesting but challenging since I am newly learning about lenses/photography. I mainly am buying for my film student son for whom I purchased a Bolex Rex 5 camera that came with an unwieldy 18-86 Vario-Switar OE lens.
    I find the RX lenses very expensive compared to the normal Kern C-mount lenses. Could you tell me in simplest , non-technical terms what C-mount -nonRX-lenses I could safely get for this camera and what f stop adjustments would be necessary to use each? If possible , something like a chart or simple list of specific lenses would be most helpful. I suspect that this information is already in your writeup here , but please forgive my confusion. Thanks much. Gid

    1. Hi Gid,

      Thanks for reading and for the question.
      The 18-86 Vario Switar OE should be a very nice lens so I would certainly keep it. However, it is a big lens and your son will probably want to use some smaller prime lenses.
      As you probably read in the article, in theory at least only the RX lenses are optimized for the Bolex Rex 5. In practice, you can probably get very respectable results with non RX lenses, especially in the longer focal lengths (the more wide angle the lens, the more the RX optimization is likely to be noticeable).
      Since he is a film student, my advice would be to buy your son some relatively cheap 1″ (non RX) TV lenses. He can then experiment with these and, with time, come to understand which lenses he finds most useful and whether it is worth replacing any of them with more expensive RX lenses of a similar focal length.
      There are lots of them, so it is difficult to give an exhaustive list, but here are a few ideas:

      1. Fujinon TV lenses: 1.4/12.5mm, 1.4/25mm, 1.4/50mm, 1.8/75mm
      2. Canon TV-16 lenses: 1.5/13mm, 1.4/25mm, 1.4/50mm, 1.8/50mm, 3.2/75mm
      3. Computar TV lenses: 1.3/12.5mm, 1.3/25mm, 1.3/50mm, 1.8/50mm, 1.3/75mm, 1.4/75mm
      4. Cosmicar/Pentax TV lenses: 1.4/12.5mm, 1.9/12.5mm, 1.4/16mm, 1.4/25mm, 1.9/25mm, 1.4/50mm, 1.8/50mm, 2.5/50mm, 1.4/75mm, 1.9/75mm, 2.8/100mm, 2.8/135mm, 3.2/150mm

      As far as f-stop adjustments are concerned, if your son is a film student he should be learning all about that. The wider the aperture (f-stop), the more light the lens lets in and the shorter the depth of field (more areas out of focus apart from the plane which is in focus). Lenses that give a wider f-stop (e.g. the 1.3s and 1.4s in the list above) can therefore be used in lower light levels and can better pull a subject out of the surrounding (out of focus) background. They also tend to be more expensive. None of the above lenses should be very expensive however – certainly not in the Switar price range (the Cosmicar/Pentax lenses will usually be the cheapest of the four).

      Hope this helps a little,

    1. Yes, they are right. The 10mm Switar will be the equivalent of about a 29mm lens on a full frame camera. It will give a field of view of about 63°.
      This of course assumes that the Switar will cover the entire S16 sensor which I rather doubt.

  5. Hi Jason, I have just stumbled upon your website and I too would like to congratulate you on the excellent information contained therein. Firstly I am a collector of cine cameras, not a user. Contained in my collection are about 70 cameras that utilise the C mount fitting, mainly 16mm but some for the 9.5 and 8mm guages. These cameras have between them space for about 180 lenses and I have about 100 to go around them. I regularly buy cameras without lenses and have in the past bought lenses on their own (before they shot up in price) Some of the cameras came with lenses that were either date or manufacturer inappropriate and as I like to display my cameras with the correct lenses some swapping about is neccasery. I have some excellent source material from which I can deduce the correct lens required but do have difficulty at times in working out the date of manufacture of some of them. Do you know of any means by which the serial number of any particular lens can be used to determine its age.

  6. hi jason, i need some help from anybody that can tell me what my 1956 16mm bolex movie camera is worth. i inherited when my father passed away 35 yrs ago and i have never done anything with it. now i need money and want to sell it but the prices i see on ebay seem to vary a lot from one camera to the next. so my father bought it brand new when he was in world war 2. its in excellent condition hardly used except the zipper on the leather caring case is torn loose. it has three schneider krueznach lenses on it. one tele xenar 1:3,8/75 and 0ne xenon1:1,9/16 and one xenon 1:1,5/25. the camera has a few small scratches on it and there is 2 rolls of 16mm 100 ft long 30m. can you or anyone get me close to a asking price cause i have no idea where to start and i cant find anyone on the internet to give me an appraisal. whatever works for anyone either post it here or call 231-881-2579 if you or anyone wants to talk on the phone i sure would appreciate it or email me at deanparker21@hotmail.com thank you and peace out!

      1. Jason:

        I find myself in a situation not unlike Dean Parker’s in January
        where he asked if you could help him re pricing a 16mm Bolex
        with 3 lenses (due to variable prices on Ebay and other sites).

        I actually have (2) 16mm Bolex Reflex Cameras (same model)
        but with different lenses and accessories.

        Camera 1: Cosmetic condition is excellent, no scratches or rust
        Lenses: 3 Kern-Paillard Switar lenses 16, 25 and 75mm AR (all
        in excellent condition and clean.
        Original Manual, Handcrank, filters, lens shades, caps, pistol grip, etc.

        Camera 2: Cosmetic condition is excellent, no scratches or rust.
        Lenses: (1) Kern-Paillard Switar 1:1,6 10 mm lens, (1) Lytar SOM Berthiot 1:1,8 25mm lens, (1) Wollensak CineRaptor 2″ f 1.5,
        (1) Bell & Howell long thin 4″ lens (102mm), f 4.5. Manual, Handcrank
        orig. filter set in box, lens shades, caps, level.

        I see that some sellers are separating the lenses from the camera
        body and pushing up the price on each considerably. What do you

        Any advice you can provide will be greatly appreciated.

        Your website is terrific

        1. Hi Sara,

          Thanks for your message and appreciation of the site. That is quite a collection you have there.

          If you want to maximize the price I would separate them and sell each lens separately. Many people are using these c-mount lenses on some of the new mirrorless digital cameras such as the micro four thirds format and these people are interested in the lenses not the cameras. However, when you list the cameras you might want to refer to the lens listings to make sure the camera buyers know the matching lenses are also for sale: the four Switars in particular make a very nice matched set of lenses.

          The 75mm Switar is worth quite a bit. You don’t mention the maximum aperture, but if I remember correctly there are only two of them, both 1:1.9, one macro and the other not. The non-macro is worth upwards of $500 in good condition, the macro version more – maybe $800. The Wollensak 1.5 2″ Cine Raptar and the Switar 1.4 25mm should raise about $250 each. The other lenses rather less – maybe around $100 each.

          Hope this helps – good luck!!

          1. Jason:

            Thanks so much for your very informative reply – it will be so
            helpful as I figure out how to sell these items

            I don’t know how to figure out whether the Switar 75mm AR is
            a macro or non-macro lens. You asked about the maximum
            aperture: i can’t tell by looking at the lens, but the aperture range is from 1.9 – 22 and FOV is 1.5 – 200. The model no.
            on the lens is No.67238. I have tried researching myself but
            have not had any definitive results.


          2. Sara,
            It’s maximum aperture is 1.9 then.
            I think if it says AR it is the normal non-macro one but I may be wrong.
            To help in identification have a look here: http://www.bolexcollector.com/lenses/60kern.html
            The Switar 1.9/75mm macro is fairly well down the page.
            Then go to the page of 1950s Switar lenses (link on the left) and you will find the non macro version. They are not the best photos but it might help.

  7. Jason,

    This is the most indepth discussion on Bolex lens I have seen.
    We all know when we use Bolex lens on M43 cameras the focus length doubles. How about aperture and DOF? Do they change as well? For example, when I use the Kern Paillard 25mm 1.4 lens, can I use its original DOF table?


    1. Thanks for the message.

      Aperture and depth of field remain the same so yes, you can use the original DOF table for your Kern Paillard 1.4/25mm.

      You say that when you use a Bolex lens on a micro 4/3 camera the focal length doubles – it doesn’t actually, a 25mm focal length lens remains a 25mm whatever camera you put it on. What changes is the Field of View and this is directly proportional to the frame size. A normal lens is usually considered to be a lens that gives a angle of view of about 50° and where the focal length is approximately equal to the diagonal of the frame. So, if you put a 25mm lens on a 35mm camera it is considered a wide angle lens (if it covers the frame), while if you put it on a m4/3 camera it is considered a normal lens. Focal length, aperture and DOF all remain the same though.

      As far as 16mm cine lenses are concerned, 25mm was actually considered the focal length of a normal lens (just as it is considered normal on m4/3). It is actually longer then the diagonal of the 16mm frame size (which is smaller than the m4/3 frame), but since that standard was established wide angle lens design has improved and we have become more used to wider fields of view.

      Hope that helps.

      1. Jason,

        Thank you so much for such a clear answer!
        So, the focal length of the a lens is always defined based on its 35mm Field of View.
        On an m43 camera a 25mm lens will provide Field of View equivalent of a 50mm lens on 35mm film. Therefore original DOF will not cange.
        I got it. That is why there are many 16mm cine lenses with very short focal length.

        1. Well, you’re pretty much right! :)
          But the focal length of a lens is not defined based on its 35mm field of view.
          Focal length is an optical formula which measures the distance over which a lens brings the rays of light to focus. That is why longer focal length lenses are physically longer.
          For the rest you are right and yes – that is why 16mm lenses are generally of short focal length.

          1. Jason,

            Thanks a lot for the response. I got it. A 25mm lens on M43 camera the focal lenth is still 25mm. But the ourcome of the field of view is equivalent to a 50mm lens on 35mm film.

            I have one more question about 16mm cine lenses on M43 cameras:
            I see obvious vignette with focal length equal or less than 25mm lenses, but not with 26mm and above, why?


          2. It’s just that the longer the focal length the larger the projected image circle tends to be. Don’t take it as a hard and fast rule – I have seen longer focal lengths that vignette on M43, especially in the longer focal lengths of some zooms.

  8. What about the light loss?

    There is unavoidable light loss as a result of having a prism in the light path, which is resolved how exactly? I assume the RX lenses are faster then they are marked to compensate? Or what?

    1. Thanks for your message and your very good question.

      Yes, there is a light loss of 25-33%. This has to be adjusted for by overexposing around 1/3 of a stop.
      The lenses are not faster than they are marked – the light loss is the camera’s business not the lenses!
      Also, many of the RX lenses made by Kern Paillard are also available as non-RX versions with exactly the same apertures.

      See here:

      1. Jason,

        The slight loss in light was allowed for in the little exposure meter Gossen made for the 16 Reflex. This was calibrated for the RX16 and will be out by roughly 1/3rd of a stop if used for normal camera metering.

        Also, may I be permitted to make a slight correction to your narrative about the maximum prime focal length lens available. If I have understood you correctly, you believe that 50mm was the maximum focal length of a prime, but I have a 75mm Switar to supplement my 25mm and 10mm Switars.

        Thank you for your explanation of the difference between the standard and RX versions of the same lens. I’ve always wondered.

        1. Terry,

          Many thanks for the comment.
          Interesting about the Gossen light meter for the 16 Reflex – I didn’t know that but it makes sense.

          I think you misunderstood my comment about the maximum prime focal length lenses available: I wrote that the longest RX (reflex) lens was a 50mm. Above that length the lenses were the same for Reflex and non-reflex cameras. Thus, I think you will find that your 75mm Switar is not marked RX (I stand corrected if it is!).

  9. Hi Jason, Nice site, and nice to see that you read my postings on this topic on cinematography.com If you need more info or some help on C-mounts for movie cameras, don’t hesitate to write me. I could help with your list of lenses too. Best, Boris

    1. Hi Boris,
      Thanks for visiting and for your encouraging comment. I have a pretty good long list of lenses – I just need to get it up here on the site!! – but sooner or later I’m sure some help would be welcome. I’ll e-mail you.

  10. Thanks Jason for fantastic work about Bolex lenses and other c-mount glass
    p.s. i’m looking for any info about history of Tevidons from CZ Jena
    anyway, great job.

    1. Thanks for your comment – it is much appreciated.
      I’m working on posting about individual lenses right now. Trouble is I’m on the As and Zeiss is right at the end!! Maybe I should skip a few!

      The Tevidons are great lenses. They are post-war Carl Zeiss Jena (i.e. East German) lenses. They were made mainly for high end security cameras (used especially by the STASI – the East German secret police). They were also installed aboard the Russian MIG fighters I believe. They are found both with c-mount and with a bayonet mount that you find only on Tevidons.
      The following lenses were made:

      Prime lenses:
      Tevidon 2/10
      Tevidon 1.8/16
      Tevidon 1.4/25
      Tevidon 1.9/35
      Tevidon 1.8/50
      Tevidon 2.8/70
      Tevidon 2.8/100

      Zoom lenses:
      Vario-Tevidon 2/18-90
      Vario-Tevidon 2/15-150

      They were all made to cover a 1″ sensor. The zoom lenses cover Super 16mm. The 25mm lens will cover a micro 4/3 sensor with some vignetting, longer focal lengths do not vignette. I believe the 50mm lens will cover the NEX sensor and the 100mm lens is the only one to cover 35mm. They are very solidly made, earlier versions in brass which are pretty heavy, and later versions in aluminium. The 35, 50, 75 and 100mm versions also changed design from a wide, bulbous barrel to a narrower one when alumium construction was introduced.

      Carl Zeiss stopped making Tevidons on German reunification when the West German Zeiss took over CZJ. A new company, Docter Optics (http://www.docteroptics.com), was started up near Jena (by ex-employees of CZJ I believe) and bought the rights to continue making Tevidons. They still make them today – although the range has changed somewhat.

      Hope that brief summary helps.
      All the best,

      1. Say, Jason…I was wondering if you knew of a reliable way to tell the difference between reflex and non-reflex Schneider-Kreuznach lenses, as you didn’t mention any difference in the article.

        1. The reflex versions should all be marked “RX” after the name (cine-cinegon RX, cine-xenon RX, Variogon RX). They often also have “für Bolex” on the barrel of the lens near the mount. I believe the only RX Schneider lenses are the black and silver coloured primes (black with zebra focus ring) and the f2/16-80mm Variogon RX. There may be an RX version of the Variogon 2/18-90mm but I don’t think so. In the case of the black and silver coloured primes, the non-reflex versions are marked “AR” after the name. The non-reflex versions of the Variogon and the all chrome primes have nothing after the name.

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