Monday, 18 February 2019

Arri 16St Service

Arri 16St Service

In 1937 Arnold and Richter revolutionised cinematography when they introduced the Arriflex 35, the first spinning mirror reflex camera. It was an idea that would eventually become the model upon which all professional movie cameras were based, continuing right up to the present day with Arri's top-of-the-line digital camera Alexa Studio still utilising a spinning reflex mirror for its optical viewfinder.

Building on the success of their 35mm design, Arri released the 16 St in 1952, the world's first 16mm reflex spinning mirror camera. There had been some pretty high end 16mm cameras before, notably the Cine-Kodak Special and the Movikon 16, but the Arri 16 St really elevated 16mm to a professional level, and opened the format up for news gathering and documentary work.

With its ergonomic design, pin-registered steady image and durable construction, the 16 St has become something of an icon, and many people's favourite camera of all time. Countless documentaries, music videos, shorts and feature films have been shot with this camera, including Cassavetes' Shadows, Robert Rodriguez's El Mariachi and much of Sam Raimi's Evil Dead.


It's imperative to use the right tools for working on precision machinery like the 16 St. This means using screwdrivers that are the right fit for the screw head and not burred or damaged in any way. Most of the screws are slotted, but a few require specialised drivers with two pins. You can make your own by filing down the centre of an appropriately sized slotted screwdriver, or fitting steel pins in the end of a custom machined driver. Occasionally circlip pliers are needed, or spanners. A pair of good tweezers will also be invaluable. A compartment tray to store the various tiny components is a very good idea and can help to separate the elements of each sub-assembly. I would also recommend using clear plastic containers upside down to cover fragile parts like the mirror or optics and protect them while you work. A multimeter is necessary to check microswitches or do continuity checks, and a regulated power supply handy to check current draw.

For cleaning I use compressed air, cotton buds, an old toothbrush, soft bristle brushes, cotton cloth, tissues and occasionally a brillo pad or brass brush. An ultrasonic bath is used for bearings and movement components. Kim-wipes and swabs are invaluable for cleaning delicate surfaces like viewfinder optics and the mirror. The cleaning agents I use are isopropyl alcohol, shellite (or napthal), CRC 2-26 for corrosion and a good quality lens cleaning solution like Pancro. Acetone is occasionally handy, but be careful of painted surfaces or plastics.

To properly check the camera flange focal depth a precision depth gauge and perfectly flat 16mm wide backing plate to fit in the gate are more or less indispensable. If the movement is removed, there's no other way to check this critical distance when things are reassembled (other than a collimator and a perfectly collimated test lens, but this won't allow you to check gate flatness).


The main recommended lubricants for the 16 St are Arri's "special grease" Isoflex LDS 18/05, a molybdenum disulphide grease called Molykote G-n paste and a relatively viscous, golden oil called Chronosynth 1/8. I generally apply grease with a good quality flat brush (one for each grease), and oil with a syringe. Arri used to supply an oil pen for applying oil to the various oil holes that are sealed with a sprung ball, but you can easily depress the ball with the wooden shaft of a cotton bud and apply the oil with a syringe instead.

Service Procedure

The first step is to check the camera over and look for obvious issues - corrosion, contamination, structural damage or signs of a drop. I check that the lid fits well, nothing seems bent, the camera manually turns over without tight spots, the pressure plate looks OK, the power cable is in good condition. Then I run the camera from a power supply set to 8.5V and check the current draw, listen to the sound of the camera, apply some drag by holding the take-up spindle. I fit the magazines and run the camera again. I check that the buckle switch is working, and that it's also activated when the sprocket roller carriage is opened. I check the turret and the mirror, and the state of the viewfinder image. The flange depth and gate flatness is measured with a depth gauge.

To begin the camera disassembly, we start with the run plunger/buckle switch module at the bottom of the film chamber. First loosen the upper screw visible inside the motor cavity that fastens the wire heading off into the body, and pull the wire free with some tweezers.

Then undo the 2 screws in the camera base and remove the assembly that houses the run plunger.

At the back of the assembly is the microswitch that cuts motor power when the buckle is tripped. If the buckle switch wasn't properly activating, check that the positioning of the various components is correct (including the set screw that holds the flat spring at the right depth), and that the microswitch itself is not faulty.

Undo the 4 screws securing the drive belt cover plate, and remove it.

Undo the 2 screws holding the right angle plate beneath the gate and remove it.

Unloop the drive belts from their spindles and undo the screws around the edge of the platine.

Leave the 4 screws between the sprocket drive and feed spindle, they hold the tacho and do not need to be removed.

Using a 2 prong driver undo the screws holding the spindles.

Note that the feed and take-up spindles have differently angled ratchets underneath, allowing them to spin freely in different directions. Mark which one is which, or make a drawing.

Now the secret trick. Peel back the lining at the top of the gate using a bit of acetone to soften the glue. Beneath it are 2 screws that need to be undone.

Now the platine can be removed, with a bit of finesse..

 reveal the drive gearing, counters and tacho on the underside, and the movement beside the gate.

These gears drive the sprockets and counters and since they don't require precise rotation like the movement gears, they use plain bearings (shafts in bushings) lubricated with oil. The ball spring lubrication points on the chamber side feed into the tubes you can see and supply oil to every bearing.

If the sprocket rollers are not turning freely the carriage needs to be removed to be able to remove the rollers for cleaning and lubrication. There is a spring at the carriage arm pivot that hooks into holes which will need to be re-fitted for re-assembly. The spring and pivot are lightly greased.

Note the small spring at the roller carriage end which pivots in a hole and forces either one roller or the other away from the sprockets. The rollers can now be removed, the shafts and holes cleaned, and one or two drops of oil applied before reassembling.

Once the rear roller and guide plate is removed the belts can be unhooked from the sprockets and cleaned in an ultrasonic bath.

The grooves where the belts run are cleaned.

The guide can be cleaned and a drop of oil applied to the shaft for the rear roller.

When reassembling, use two film thicknesses to check all the spacings, and make sure the guide is not scraping against either sprocket. Remember to fit the belts back first!

With better access to the motor cavity, now is a good time to clean any dirt or corrosion present, especially the un-anodised strip near the locking lever. The ground voltage (negative on the battery) is conducted through the camera chassis to the motor via this strip.

Also clean any dirt or corrosion from the corresponding part of the motor body..

and the positive (+8V) contact at the front. Excessively corroded or pitted contacts can be polished with a leather strip and some jeweller's rouge.

The motor shaft end engages in this rubber socket to drive the camera. A common problem is hardening or splitting of the rubber, which can cause the motor shaft to slip inside rather than form a secure connection. I use a rubber rejuvenator to help prolong the life of the socket, but it may need replacing.

The turret should rotate smoothly and click snugly into its 3 positions. There should be no axial play, or the flange focal depth will be compromised. The releases either side of each mount should operate without sticking, and lenses should seat firmly. Some old Arri St mount lenses rotate within the mount when focussing, which can cause wear to the mounts over time. To remove the turret requires a special 2-prong driver.

Carefully remove the turret, noting the 3 spring rollers. They should compress smoothly into their holes.

The view under the turret, revealing the mirror.

I clean the old grease from the edge of the turret cavity. On reassembly I grease it lightly with a honey textured grease recommended by Arri called Catenera KSB, but any relatively viscous dampening grease would probably do.

To remove the movement/mirror assembly we need to remove this support bracket, held by 2 screws.

Then remove the 2 screws inside the turret cavity..

and carefully remove the assembly from the back.

Note any shimming material where the assembly seats (see the silver shims in this example). This is determining the crucial focal flange depth.

A view of the assembly showing the movement.

After opening the hinged pressure plate I check the side rail spring to make sure it moves freely. Note the prism at top that diverts an image to the viewfinder.

The mirror/shutter and ground glass assembly removed. During a movement disassembly any shims or spacers need to be carefully noted. I actually wouldn't recommend going this far without some experience and proper tools.

The movement/mirror driveshaft removed.

The gate removed.

Disassembling the movement.

The various parts after some have had ultrasonic cleaning.

Lubricating the claw cam follower..

its pivot shaft..

and the cam.

After reassembly.

The mirror/shutter refitted.

The pulldown claw and registration pin alignments checked with a steel film gauge.

Cleaning the optic in front of the viewfinder prism with a swab and isopropyl alcohol.

Once fully cleaned, lubricated and assembled, the movement/mirror assembly is carefully refitted, making sure the shims are in place. After the turret has been refitted a calibrated 52.00mm depth gauge is used with a steel backing plate in the gate to exactly measure the focal flange depth and flatness, from lens mount to film plane. If it needs adjusting new shims are cut and fitted.

The ground glass is under the eyepiece, and its depth is set by screwing the viewfinder tube in and out. To do so a set screw inside the door needs to be undone. A calibrated spacer at the back allows the tube to be screwed home to the correct distance. The ground glass is then aligned to the horizontal and locked by its lock ring. The ground glass should show a sharp image when a calibrated test lens is focussed on a test chart.

After reassembly, fresh film is run through the camera to check for scratching and smooth transport of the film. The basic camera functions are tested, the current draw is checked, and any magazines are checked for take-up tension and smooth function.

Monday, 1 December 2014

RX vs non-RX lenses

Ever since Bolex decided to utilize a beam-splitting prism in their cameras to make them reflex, there has been confusion and misinformation about the effects of the prism and the nature of the lenses that were designed to work with it. The first H16 Reflex Bolexes hit the market in 1956, and were supplied with a new range of Kern lenses that were labelled "RX" for focal lengths of 50mm and under. These lenses were also available in versions for non-reflex cameras, usually simply labelled "AR" (which actually stood for "anti-reflective" coating, but distinguishes them from the RX labelled ones). To confuse matters a little, the very first lenses Kern made for the reflex Bolex were labelled "DV" (which stood for "direct vision" I believe) as well as having AR engraved on them, but these are basically RX lenses.

Other companies also made RX lenses for reflex Bolexes. Both Schneider and Som Berthiot released primes that were labelled "RX" or "H16 RX", and some early Angenieux zooms were also designated "RX", or sometimes "Special P" (relating to Paillard).


Back Focus

The first thing to clarify is that the back-focus distance setting of RX and non-RX lenses is the same. In other words, both are designed to form an image 17.52mm behind the mount flange, which is the C-mount standard. You cannot simply "re-collimate" or "adjust the back-focus" of an RX lens to make it non-RX or vice versa.

Some confusion exists because the prism in a reflex Bolex acts to extend the light rays further back, so the "physical" distance between the mount and the film plane in a reflex H16 is actually 20.76mm, but the simple fact is that any C-mount lens fitted to a reflex Bolex will have its back focus distance extended back to this point.


The problem is that the prism causes the outer light rays from a lens to be bent at different angles to the central ones, so they don't converge at the same point, creating blurry images at the film plane. These aberrations are what the RX lenses have been designed to counteract. In the absence of a prism in the light path, an RX lens will exhibit the same sort of aberrations it was designed to negate, causing blurry details and flare around bright highlights and softness in the corners.

The aberrations are more pronounced at wider apertures, because this is when the light rays pass through the edges of the lens elements as well as the centre, exhibiting the most variation in how they are bent. When a lens is stopped down, the aberrations are reduced. 

Official Bolex literature claimed that the focal length of a lens would also affect how pronounced the aberrations were. 50mm was the longest focal length prime that any manufacturer designed as RX, because it was decided that beyond 50mm the effects of the prism were not noticeable enough to worry about. It's actually a little more complicated than that. A crucial causal factor is the position of the exit pupil (the apparent position of the iris as seen from the rear), which along with the rear element size determines the angle of the cone of light departing the rear of the lens. A deeper exit pupil creates a more parallel cone which will produce less aberrations when a refractive material like glass is placed in its path. While a longer focal length will often have a deeper exit pupil than a wide angle lens, this is not something that is always directly related to focal length. As we shall see, the 10mm RX Switar for example exhibits less aberration without a prism in the light path than the 25mm RX Switar does, because the 10mm has a much deeper exit pupil than the 25mm. The longer Kern primes all happen to have deep exit pupils and slower apertures, which is the reason they didn't need RX versions.


Another area of confusion is the f stop markings on RX lenses - some people believe that the aperture rings have been marked to compensate for the light loss caused by the reflex prism diverting about a quarter of the light to the viewfinder. This is incorrect. The f stop marks are the same on an RX lens and a non-RX lens, and relate to the lens only. The light lost to the viewfinder needs to be factored in to the stop calculation for any lens fitted to a reflex Bolex. (This should be obvious when we remember that longer focal lengths were not marked as RX, so having exposure compensated marks on some lenses but not others would be a very ridiculous and confusing state of affairs.) 

The question of whether one lens might seem brighter than another at the same f stop relates to the coatings and the internal light loss caused by reflections and scattering. Many cinematography lenses are marked in T (for "transmission") stops to account for this. Typically a prime lens might lose a third of a stop, while a zoom might lose a half (or more for older models). An older version of the same lens may have earlier, inferior coatings - leading to more light being reflected or scattered at each glass to air surface, and a dimmer, less contrasty image at the same f stop. Damaged coatings will do the same.

Measuring the difference

Many people are aware of the difference between RX and non-RX lenses, but few can say exactly how much the image quality may suffer when the wrong type is used on a particular camera. This blog post is an attempt to show in simple terms the different optical characteristics of some lenses that are available as RX and non-RX variants, when used without a 9.5mm thick glass prism in the light path. 


The following images are photographs of a lens test projection. In simple terms, a test slide is placed at the film plane and light is shone through it and then through the lens and onto a screen. We are effectively reversing the normal light path from subject, through the lens and onto the film plane. It allows us to see exactly how a lens recreates the fine pattern details of the test slide, and thus how well it would resolve or distort a subject placed at the screen distance.

The photos were taken with a Nikon D200 camera, and much of the actual resolution of the test image is finer than the Nikon camera/lens can capture, but even so it is easy to see the difference between an RX and a non-RX 25mm Switar projection.

Only a quarter of the whole test pattern projection is shown, since the image deterioration is quite symmetrical (unless a lens is faulty). For the 25mm projections that follow, the crop is the bottom left quarter. The centre of the lens is always the sharpest, seen in these photos at top right.

Kern Switar 25mm AR @ f/1.4

Ken Switar 25mm RX @ f/1.4

At f/1.4 (wide open) the RX lens exhibits considerably more halation and flare than the AR lens.

Kern Switar 25mm AR @ f/2.8

Kern Switar 25mm RX @ f/2.8

Notice that while stopping down improves the corner sharpness of the non-RX lens considerably, the RX lens is still quite soft in the corners. Even at f/5.6 it is still not as sharp in the corners as the AR lens. While the RX lens halation at f/1.4 is mainly caused by spherical aberration which disappears fairly quickly as you stop down, the corner softness seems to be mainly the result of astigmatism, which is not affected as rapidly by aperture changes.  

The following photos are of 10mm Switar projections, cropped to the top left quarter. The test pattern centre is therefore at bottom right. As mentioned previously, there is less difference between these RX and non-RX images because the exit pupil is quite deep inside the lens, so very little astigmatism is introduced by the prism that needs correcting. The maximum aperture is also slightly (a third of a stop) slower than the 25mm.

Switar 10mm AR f/1.6

Switar 10mm RX f/1.6

Notice that the centre of the AR lens is sharper, but the corners are slightly worse. Wide angle lenses of this era often have issues with corner sharpness, due to various aberrations including field curvature, coma and chromatic aberration, I suspect the prism corrections in the RX lens have reduced some of the aberrations causing corner softness in the non-RX lens. The centre of the RX lens shows considerably more halation however. 

Switar 10mm AR f/2.8

Switar 10mm RX f/2.8

At f/2.8 the AR lens has improved markedly in the centre, while the RX lens still has spherical aberration halation. Stopping down has not improved the corner resolution of the AR lens all that much, in that part of the image the RX lens is still superior. It's interesting to note that the spherical aberration at f/2.8 is better in the 25mm RX than in the 10mm RX at the same stop, but the astigmatic corner sharpness of the 25mm RX is much worse.

I don't have a 50mm AR Switar on hand at the moment to show comparisons with the RX version, but I have projected and analysed the 50mm RX. It has an exit pupil depth halfway between the shallow 25mm and the deep 10mm.  Considering the focal length and the fact that it's an f/1.8 lens, one might expect the 50mm RX to exhibit minimal aberration, but in fact it shows about as much halation and actually more corner degradation than the 10mm RX, which reinforces the idea that the exit pupil position is a more critical factor than the focal length.


It can be difficult quantifying the effects of aberrations that vary with both aperture and lens design, but from these projections we can at least get a sense of the degrees of difference between RX and non-RX lenses. They show that RX lenses exhibit halation caused mainly by spherical aberration, and astigmatic corner softness in lenses with shallow exit pupils. The aberrations are most pronounced at full aperture, still visible at f/2.8 and the halation reduces more quickly than the astigmatism.

For reflex Bolex users wanting a guideline on non-RX lens choice, I would say any lens with an exit pupil distance of more than about 50mm from the film plane should be fine stopped down to f/2.8 and beyond, with the effect diminishing as the pupil distance increases. A lens that has the iris apparently positioned directly beneath the rear element (so an exit pupil distance of maybe 20mm from the film plane) will need more stopping down, and may still exhibit soft corners at f/5.6. Naturally this is all dependent on the final viewing parameters, these guidelines assume a pretty critical Circle of Confusion. And of course many old C-mounts have inherent aberrations anyway, so it can all be rather moot.

For digital camera shooters wanting to use C-mount lenses the issue gets even more complicated because of the fact that almost all digital cameras do in fact have an extra block of glass in the light path, situated between the rear of the lens and the sensor. It's the sensor stack, consisting of cover glass, optical low pass filter and IR filter, which can vary from 0.5mm to over 4mm in thickness. Micro 4/3 cameras typically have the thickest sensor stacks at around 4mm. While this is less than half the thickness of the 9.5mm Bolex prism, it does mean that RX lenses are probably not much worse than non-RX lenses on Micro 4/3 cameras, since one type is over-correcting for the sensor stack and the other is not correcting at all. For a good discussion of the effects of sensor stack thickness on image quality (and the importance of exit pupil distances), see Roger Cicala's LensRentals articles at:

One last note about RX lenses. For their 8mm reflex models, Bolex commisioned Kern to make 3 primes -5.5mm, 12.5mm and 36mm - and 2 zooms, all designated "H8RX". For reasons best known to Bolex, these lenses have the standard 1" C-mount thread and so are often advertised as C-mounts, but the back-focus is actually 2mm shorter than the C-mount standard. This means that if they are used on C-mount cameras (other than a H8RX) or with standard C-mount adapters they won't focus any further than a few feet away, in fact the 5.5mm won't focus past an inch.  Plus of course they only really cover a tiny 8mm frame, perhaps the 36mm covers more but with severe fall-off. Considering that I have seen H8RX Macro-Switars go for extraordinary sums on ebay, with their short back-focus never mentioned, I think this should be more commonly known. Though I guess it's not a problem if the buyer only wants these lenses to photograph flowers and insects anyway.

Thanks to Dennis Couzin for first questioning the official Bolex literature nearly 40 years ago, and later suggesting the importance of rear exit pupil depth. See: