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.
BOLEX REFLEX CAMERAS
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.
WHAT ABOUT IN PRACTICE?
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.
TELLING THEM APART
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.
LINKS AND REFERENCES
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