Tag Archives: 16mm film

BOLEX REFLEX (RX) LENSES: THE DIFFERENCE.

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.

BACK FOCUS

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.

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

C-MOUNT DEFINED

C-Mount is an internationally recognized standard that is followed by all manufacturers of C-Mount lenses. Here’s the full definition for those who are new to them or may have doubts.

A C-Mount lens has a screw thread with a diameter of 1 inch (25.4mm) and 32 TPI (threads per inch). The “flange focal distance” is 17.52mm (0.69 inches). This distance is measured from the lens mounting plane (i.e. the rear surface of the lens which butts up against the camera) to the focal plane (i.e. the plane of the film or sensor). The C-Mount standard also defines the thread length as being 3.8mm.

The C-Mount standard does not define the coverage or projected image circle of the lens. This can differ widely: C-Mount lenses may be designed for 1/2″, 2/3″, 16mm, 1″ or other formats. It should also not be confused with the CS-Mount standard, which has the same thread mount but a smaller flange focal distance of 12.52mm.

Concerning adapters

As we see, the C-mount standard does not define the width of the flange itself. As some Micro Four Thirds and Sony NEX users have found out, wider lenses may not be able to obtain infinity focus when mounting them with an adapter. Although both the Micro 4/3 and NEX formats have very short flange focal distances compared to other DSLRs, they are still slightly longer than the C-Mount one. NEX has a flange focal distance of 18mm, Micro 4/3 of 20mm. When using C-Mount lenses on these cameras this means that the lens must actually be set nearer to the sensor than the flange plane of the camera. The adapter must allow them to be recessed slightly inside the camera’s mount opening, but this obviously means that the flange width cannot be wider than this opening. Exact measurements vary with the make of the adapter and the geometry of the back of the lens can also make a difference, but as a general rule the flange diameter of lens cannot be wider than 37.2mm for use at infinity focus on Micro 4/3, or wider than 44mm on NEX. Some lenses can be modified to fit, others will not be able to obtain infinity focus but can still be used for closer focusing or macro work.

C-Mount to Micro Four Thirds Adapter

Actual configurations may vary depending on manufacturer and model. Some have two or more “steps” instead of one. There are also “ultra thin” models that may help obtain infinity focus on certain lenses.

C-Mount to Sony NEX Adapter

LINKS AND REFERENCES

Schneider Optics FAQ, http://www.schneideroptics.com/info/faq/industrial.htm#qu4

C-MOUNT LENSES: A brief history.

C-mount lenses have been enjoying a period of popularity with still photographers over the last couple of years due to their use with an adapter on the new micro 4/3rds format. Some photographers have also been trying them out on the Sony NEX cameras and it is likely that with the launch on the market of the upcoming Scarlet 2/3 Cinema (interchangeable lens) digital film camera, their popularity is only due to increase. But the C-mount standard has been around for a long time and an interesting history: let’s take a look!

16mm film is invented

C-mount lenses were first used for early cine cameras: the “c” in the name probably stands for “cine”. The earliest beginnings are a little vague. The c-mount standard is for a 1″ diameter screw mount with a 32 thread per inch pitch and a 17.526 millimetres (0.6900 in) flange focal distance. It does not refer to the coverage of the lens, but it was almost certainly established for use with 16mm film. This was introduced to the market by Eastman Kodak in 1923. Prior to this, the main format used was 35mm. Up until the early 1920s, 35mm film was a negative stock that had to be printed to be projected, was made of an inflammable nitrate base, and was altogether too large, dangerous and costly for amateur home use. The new 16mm film was a reversal film that needed no printing, was cellulose acetate based “Safety Film” and cost much less, opening the path for a new amateur market. The same year, Bell & Howell introduced the first model of its famous Filmo camera, the Filmo 70A. Placing ads in popular nationwide US magazines, Bell & Howell marketed the Filmo to home movie makers for the first time. Costing $180, the Filmo was certainly not within the reach of everyone, but amongst the wealthy a new hobby was born (the cheaper 8mm format, which further encouraged film making as a hobby, arrived in 1932).

The Bell & Howell Filmo 70 Camera

The Filmo 70A was a single lens camera which probably had a C-mount. What is certain is that it’s 1927 successor, the Filmo 70C, had a three lens revolving turret which mounted three c-mount lenses of varying focal length. Bell & Howell also made a kit for retrofitting the turret to the the 70A and the 70B (a high-speed scientific version similar to the 70A). All the early Filmo 70 cameras were fitted with lenses made by Cooke of England. A 1″ f3.5 universal focus anastigmat was sold as the standard focal length, but the Cooke range stretched from a 2/3″ f2.5 “wide” to a 6″ f5.5 “tele”. It seems likely that this was the first range of c-mount lenses.

Two early c-mount lenses
Two early c-mount lenses: a Carl Zeiss Jena Biotar 1.4/25mm made in 1930 (left) and a Hugo Meyer Makro Plasmat 2.7/50mm from 1932 (right)

The 16mm format was a success: by the late 1920s many manufacturers had introduced their own model, while famous lens makers like Cooke, Dallmeyer, Kern, Hugo Meyer, C.P. Goerz, Wollensak, Schneider Optics and others were making c-mount lenses to equip them. Rather than an amateur format, 16mm ended up being used for news gathering, reportage and small budget productions. The Filmo 70 itself became the most famous news gathering camera of all time, landing with Allied troops on the Normandy beaches, covering the action during the Vietnam conflict and being adopted by the early TV studios when they started sending out reporters to get live footage.

The Vidicon Tube

Not all early c-mount lenses were made for 16mm film but most of them were. Some were also made for 8mm film, although this format generally used the smaller diameter D-mount. From the 1950s, however, the situation became more complicated because C-mount lenses began to be used not only for film, but also for early TV cameras and then for industrial and CCTV cameras. The earliest TV cameras used very large video camera tubes or pickup tubes. The coverage required was very large and so were the cameras and lenses: c-mount lenses were not suitable. In the early 1950s RCA developed the smaller Vidicon tubes. Typically of 1″ or 2/3″ diameter*, these could be used in much smaller, portable cameras which could mount new and existing c-mount lenses. At the beginning, it was probably existing lenses and designs that were used. Many of the earlier TV lenses seem to be of much higher quality than later ones. The Canon TV-16 series of lenses, for example, is so named because they were designed for both 16mm film use and for TV – hence the name “TV-16”. The early c-mount Cosmicar lenses were marketed as a TV lens, but exactly the same lenses were also marketed for 16mm use under the name Kinotar Professional.

Early Zooms

The TV cameras didn’t require the same resolution in a lens as 16mm film however, and as time went by lenses started to be made specifically for use on TV cameras. Many of these were zooms, typically in the 12.5-75mm or 16-100mm range, and they often boasted wide maximum apertures to capture as much light as possible. In the meantime, c-mount zoom lenses had also been developed for the 16mm film cameras. The earliest of these, like the Som Berthiot Pan Cinor 20-80mm f2.8, came with a parallax corrected viewfinder, but by the late 1950s and early 1960s, Som Berthiot and Angenieux were producing zoom lenses with an integrated viewfinder offering through-the-lens viewing. The introduction of the Bolex H-16 Reflex camera with it’s own prism viewing system brought with it new lenses optically corrected for this camera, the RX c-mount lenses (a separate article on this will be published on this site shortly).

The CCD Sensor

The use of c-mount lenses on TV or video cameras lasted for about thirty years – from the 1950s to the 1980s. Then the Vidicon tube was supplanted by the CCD sensor. The market for video cameras split into a lower end with fixed zoom lenses and a higher professional end with larger interchangeable zoom lenses, using bayonet mounts, that were optically designed for use with 3 ccds behind a beam splitter prism. Bayonet mounts were introduced into the world of 16mm and Super 16 film as well: today, the majority of new cine lenses are fitted with the Arri PL mount.

Although a few cine lenses are still produced in c-mount today, by far the largest part of the market for new c-mount lenses is for CCTV and industrial lenses. The sensor sizes these are designed for differ enormously: many are for 1/4″ or 1/3″ sensors while others are designed to cover 1/2″, 2/3″, 1″ or even (rarely) 22mm. Quality of these lenses differs hugely too: at one end of the scale are cheap, plastic lenses, sometimes with a fixed focus or aperture, while at the other end there are quality, precision lenses offering high resolution and solid build designed for various applications from quality control to scientific research.

However, with Micro 4/3, NEX and Silicon Imaging’s SI-2K already on the market and with the Red Scarlet  and Ikonoskop A-Cam dII Interchangeable lens cameras due to join them, we are already beginning to see new C-Mount lenses designed for photographic and cine use. The number only seems likely to grow.

* A lot of confusion exists on the exact meanings of formats and sensor size – a separate article will come on this soon.

Links and References

The history of the B&H Filmo Camera http://www.tfgtransfer.com/filmo.htm
List of manufacturers of vintage cinematographic equipment http://www.xs4all.nl/~wichm/cinelist.html#B&H
The Bolex Collector http://www.bolexcollector.com
LabGuy’s World: The History of Video Tape Recorders before Betamax and VHS http://www.labguysworld.com/index.html