H16 RX and Non RX Lenses why the difference?
Bolex produced lenses in two different versions up to 50mm in focal length. The first version (Non RX) was made for Non Reflex Bolex cameras but would work perfectly well on any other manufacturers cameras that didn't use a prism in its design (practically all). The second version was lenses made purely for use on Bolex Reflex cameras (H16 RX) which are designed to correct some very specific problems caused because there is a prism permanently in the light path always diverting 25% of the light inbound towards the lens to the viewfinder and the remaining 75% to the film. These lenses aim to remove the 2 problems the prism causes.
What Problems do H16 RX lenses Address?
Without going into extreme technical depth as can be found elsewhere on the web for practical shooting here's what's worth knowing:-
The f stops on an H16 RX lens internally let a 3rd of a stop more light through for each f stop to compensate for the 3rd of a stop light loss the prism causes the film to loose. If you shot say with a Non Reflex lens on your Bolex Reflex camera you would have to manually open by a 3rd of a stop difference the f stop to be correct. If the situation was reversed a H16 RX lens on a Non Reflex Camera you'd have to close by a 3rd of a stop. However when shooting modern negative film the difference is negligible. Its more important when shooting with Bolex Cameras (Reflex or Non Reflex) to make sure you are compensating for the correct shutter angle which is never 180° as most cine light meters assume rather than compensate for the small negligible light loss caused by the prism if your not using the right lens on the right camera.
All C mount lenses regardless of the camera there intended to be used on are designed to focus an image at a certain distance from the back of the lens, in film cameras the image is designed to be in focus at whats known as the film plane. The distance from the lens to the film plane is called the flange focal distance and is 17.52mm for all C mount cameras including the Non Reflex Bolex. The Bolex Reflex's Prism causes an image to focus at a new film plane and its flange focal distance is 20.76mm. However you can put a H16 RX lens on a Non Reflex camera and the lens will focus at 17.52mm and vice versa put a Non RX lens on a H16 Reflex camera and find it focuses at 20.76mm. So it appears H16 RX and Non RX Lenses are interchangeable between Reflex and Non Reflex Bolex cameras but there is a caveat..
The glass of a well engineered lens is designed to let all light rays incoming into the lens whether at the centre or the edge make perfect focus at the same point. So when you open up a lens to its lowest f stop to let maximum light through the extra rays coming through at the edges are designed to come as perfectly into focus as those through the centre of the lens to still make a sharp image at a low f stop. Anything that causes a focus shift based on f stop change can be classed as a spherical aberration. The work of a good lens manufacturer is in producing a lens free from any such errors, however there normal testing & design doesn't figure for a prism to be in the light path that may show up previously unseen spherical aberrations not present without it.
Generally the shorter the focal length of lens being used and the wider the aperture the greater the chance a particular lens MAY show a loss of focus and contrast as some lenses are better at imaging through the Bolex prism than others. So the rule of thumb is if you are shooting with a Non RX lens of 50mm focal length or less on a Bolex Reflex camera of which you've not had chance to do some proper testing with before being used on a shoot stop down the lens to at least f 3.3 or next highest. This causes light rays on the edge of a lens that are the cause of a focus shift caused by the prism therefore no entry and no chance of the lens exhibiting its spherical aberration if its prone.
At the time that Bolex introduced the Reflex Line of lenses they did state that a lens made by French company Som Berthiot a 25mm Lytar f1.8 introduced no distortion so was in effect up to being badged H16 RX.
If a lens exhibits spherical aberration above 50mm in focal length when combined with the Bolex Prism the focus / contrast shift will be so minor as to be negligible hence why you will never see H16 RX on a lens above 50mm as special lenses were not needed. There will be lots of lenses below 50mm in focal length out there that despite not being made by Bolex nor badged as H16 RX and that were never designed with any foreknowledge of being used in a Bolex Reflex camera with a prism in the light path will nevertheless produce no spherical aberrations with use. Only testing a lens fully open will find out.
You can normally take a steer from the choice of lenses a rental house chooses to supply with a Bolex for example Panavision UK rents Fujuinon TV 'C'-mount lenses with there Bolex H16 kits which must mean they think there lenses that show none of the problems.
Identifying an RX Lens from a Non RX Lens
This difference can be seen on all lenses by looking for the words H16 RX in red on the barrel of the lens. If present it indicates that the lens is meant to be used on a H16 Reflex camera only. Both pictures below show a Switar 1:1.4 f=25mm lens with non RX version on the left and an RX version on the right.
If a lens has the letters AR embossed on its face what does it mean?
If a lens has the letters AR it means that all the lens elements have been coated with an Anti Reflective coating to aid the reduction of incident reflections off each of the lens surfaces. This was standard Practice by Bolex on all there lenses after some date in the early 50's onwards after which they all had it applied. By marking up the barrel of the lens it was a way of differentiating to the consumer this extra feature of there lenses they wanted to promote at the time. A rule of thumb seems to have developed where if a lens has the AR marking it means the Lens is Non Reflex this probably came about because to fit H16 RX with all other essential markings meant they had to drop some text otherwise the barrel would look cluttered so they dropped AR. So its kind of true except for one exception (see next)....
If a lens has the letters DV embossed on its face what does it mean?
DV Stands for Direct Vision which is the name Bolex first came up with to identify there Reflex lenses before H16 RX when they first designed the Reflex Camera system.
There is no difference between this lens at all and the one top right bearing H16 RX. This naming convention of Reflex lenses if you see a lens marked as such tells you it was one made in the very first years of manufacture of Reflex Lenses and probably supplied for use on some of the early H16 Reflex cameras such as the H16 Rex 0 and Rex 1.
Shown left is the very same model of lens show above again and you can tell by looking at the serial numbers this one fits in between the two above as regards date of manufacture. You can also see this lens has AR on it proving AR and Reflex aren't mutually exclusive. The DV marking got dropped after only 1 or 2 yeas.