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: 

Wednesday, 2 April 2014

Arriflex 35 IIC

Very few movie cameras can equal the impact that Arnold and Richter's Arriflex 35 "Handkamera" has had on cinematography since it's introduction in 1937. Conceived as a compact, handheld news camera with easy loading and robust construction, its revolution was in the viewing system, a spinning mirror shutter that allowed parallax-free framing and focussing through the taking lens while the camera was filming. With little alteration, the reflex spinning mirror idea became standard and was eventually used in every professional motion picture film camera made and continues to be used in optical viewfinder digital cameras like Arri's flagship Alexa Studio.

The original Arri factory in Munich was largely destroyed during WW2 but after the war Arri began manufacturing their Handkamera again, as the Arriflex 35 II, with improvements to the movement, shutter, optics and other refinements finally resulting in the model IIC in 1964. The camera was quickly taken up by New Wave filmmakers and others, and was produced up until the late 70s. It remains Arri's best-selling film camera out of all their models.

An original Arriflex 35 Handkamera. This example is serial number 700 and probably dates from around 1942. Unlike later versions it has a single release for all 3 lenses, a fibre resin gate and only a 120 degree shutter angle, among other differences. The original matte box hood only allows for small lenses and vignettes wider than a 28mm.

The original movement was a simple eccentric, with a fairly flimsy claw arm. This design was less steady than the later cardiod cam movement, which created a dwell at the end of the pull-down which acted to stabilise the film before exposure. The early design also had a longer transport phase, which is why the shutter angle had to be smaller.

A IIC with a PL mount hard-front and a bellows matte box. With the motor fitted underneath, the camera needs a special head or a stand as pictured here to attach to a tripod head. Another type of stand extends out the front to support zooms.

A flat base allowed the motor to be attached next to the camera, and was often used for studio work. It made a more compact configuration and allowed the camera to fit in a sound blimp.

A variety of manufacturers made flat bases over the years, but they all followed the same pattern. The motor attached on the non-door side, the camera is normally secured by 2 screws. Some had inching knobs and counters, some were fitted with custom AC motors.

To attach the camera to a flatbase the motor is taken off (4 screws) and then this fibre gear needs to be removed. It was usually stored in a cavity of the flatbase, in the Cine 60 version above you can see the cylinder that holds it, and next to that some screw holes to store the locating screw at the front of the camera base, which also needs to be removed. Without the fibre gear the motor cannot drive the camera when put back in the handheld configuration.

35II cameras are mechanically quite simple, and generally don't need much more than basic workshop tools. However the tolerances are very fine - to correctly check the camera flange depth after disassembly it is necessary to have a 52.00mm depth gauge and a backing plate. Otherwise a focus test film needs to be made. Also, to accurately check side rail and pressure plate forces or mag take-up tension specific gauges are needed. Without these, one can only hope they are in tolerance.

The main lubricants I use are Isoflex LDS 18/05 grease and Chronosynth 1/8 oil obtained from Arri.

In the service manual, owners are advised to lubricate the movement after 25,000 feet of film, or every 6 months. Under the movement cover plate are 2 holes for oil that are visible when the claw is in a forward position, a drop in each will suffice. All moving parts of the movement should be lightly greased. On early models there is an oil hole above the movement to lubricate the intermediate gear bearing (marked with a white arrow), later this was replaced with a ball bearing that doesn't need lubricating.

After years of use or storage, or after contamination or simply neglect a camera will need a full overhaul. A professional will always do a much, much better job, but for many owners these days who wish to try it themselves this guide may help them avoid doing irreversible damage or missing a crucial check that could lead to wasted film and money. Some of the most common problems with cine gear that I come across as a repairman are actually introduced by poor servicing. Inexperienced or simply careless technicians (or tinkerers) can do far more damage than time and the elements.  I take no responsibility for any damage done trying to follow this guide, my recommendation is always to have an experienced technician do the work!

Before any disassembly, I check all the settings and give the camera an inspection to look for potential issues. Flange depth and ground glass focus are checked, the movement is turned over, the current draw is measured, the optics are assessed, the door seal is examined, the magazine lock checked etc. (There's nothing worse than putting a camera back together only to find something damaged or way out of tolerance and not knowing whether it might have been a pre-existing condition.)

The first thing I do is check the camera focal flange depth. This is the distance from lens mount flange to film plane, and determines whether the focus marks on a fitted lens will be accurate and whether the lens will reach infinity. The Arri FFD is nominally 52.00mm, but on IICs and many other models it is physically set to 51.97 - 51.98mm to account for film flex and emulsion depth. With a 35mm wide, very flat  steel backing plate in the gate and the mirror inched out of the way, I measure each port with a gauge.

Checking the ground glass focus is the next important assessment. This is to check that the ground glass is seated at the correct depth so that a focussed image reflected off the mirror will perfectly match the image formed at the film plane. Professional technicians do this with an autocollimator and an accurate test lens. A less reliable method is to use a medium focal length lens such as a 50mm and check the sharpness of an object at infinity. This method presupposes that the lens is accurately collimated.

Unlike every other camera that Arri later manufactured the 35II series have very little actual adjustment opportunities. Most of the critical settings are pinned or machined to the correct tolerances, if they have gone out of tolerance through wear or damage they generally need parts replaced. The good thing about this is that after disassembly the parts should go back together exactly as they were before, without the need to re-calibrate settings. It's always a good idea to double check things after reassembly though, in case something has been put back incorrectly, or some contamination has altered a critical distance.

The gate should be regularly cleaned with a soft cloth and any emulsion deposits removed with an orange stick. For closer inspection, the gate can be removed, but before removing it the movement must be inched over so that the claw is fully retracted and in the middle of its travel, as pictured. It should line up with a scooped out recess in the right-side rail of the gate. This is very important to avoid damaging the claw.

The 2 screws holding the retaining clips should be undone and the clips slid down (or the single clip removed in earlier models).

Now the gate can be lifted up from the side and carefully slid out past the claw.

The gate removed. On the left side is the spring-loaded side-rail which exerts side pressure on the film and helps to prevent gate weave. It should be checked to make sure it moves freely. During an overhaul I perform other checks that require specific tools, which will be covered later in this post.

Ordinarily the focal flange depth would be double checked after the gate is fitted back, to make sure nothing has changed. All it takes is a hair or speck of grit under the gate seats to throw it off.

The turret should be checked for any axial play that may throw off the FFD. To remove the turret, the centre screw is undone (there may be a spacer washer beneath) and the turret is gently twisted and pulled out. There are 3 spring rollers around the turret edge that will fly out when the turret is pulled out a certain amount, so cover the edge with a cloth to catch them. Be careful, they can pop out with great speed.

With the turret removed, the spring rollers can be checked to make sure they compress freely into their holes, and the lens port lock mechanisms can be checked. A fitted lens should lock in without any axial play, and early lenses that rotate within the mount as they focus should rotate smoothly. A light smear of grease in the mount can assist with these types of lenses.

The sliding surfaces, including the centre stud, should be cleaned and relubricated. I use a thick Arri grease called Catenera KSB but it's not too critical, as long as the grease doesn't migrate inside or separate over time.
Re-fitting the turret can be a bit tricky, with each spring roller needing to be depressed with a screwdriver blade before the turret can be pushed all the way in.

Without needing to remove the mirror shutter or any of the drive chain the claw arm can be removed for better cleaning and access to the cardiod cam. First the film guide needs to be removed. It is held by 2 small screws in the magazine port and one large screw at the bottom.

Pull the guide out through the mag port.

Undo the rear screw at the claw arm pivot.

The claw arm can now be slid off the cardiod cam. It and the cam can be cleaned and fresh grease applied to all the working surfaces. To completely dismantle the movement and access the bearings and secondary cam underneath however, the entire drive chain needs to be removed.

To remove the side cover in order to access the drive chain the inching knob must be first removed. It is held with a screw pin that also has a lock nut on the other side, although sometimes this nut is missing.

Take care to note any spacer shims (they look like thin washers) under the knob as it is removed. The original ones should stay under the side cover, but later additions with possibly smaller outside diameters might stick to the bottom of the knob. The spacers are critical in controlling any lateral play in the movement.

Now the 4 screws holding the side cover can be undone.

The side cover should be gently removed from the rear first. If it feels stuck, use a thin wedge to slowly pry apart the castings rather than force it and potentially damage the mirror with a sudden release.

Lift the rear until the inching knob shaft has cleared its hole, then pull the side cover away and forward to slide over the 45 degree angle of the mirror shutter. The mirror is made of glass and will easily scratch, chip or fracture if the cover is removed without great care.

There is a gasket over the tachometer that will create some spongy resistance as the cover is first lifted. If the camera has a pilot light impulse generator socket (used with a blimp to indicate the camera is running) the side cover will be attached by wires going to the tachometer. These should be desoldered.

Some IICs may have pilotone outputs, buckle switches and bloop lights like this one. The bloop lamp needs to be removed to get the side cover off.
With the cover removed, watch again for any shims over the bearing where the inching knob shaft protrudes. This shaft runs directly through to the cardiod cam of the movement.

The drive chain is now visible, with the central vertical drive assembly transferring torque from the motor at the camera base to the shutter mirror, tachometer and movement driveshaft all at once. The large, brown fibre intermediate gear at top, which drives the magazine via a smaller brass gear beneath it, runs off the movement driveshaft.

Most of the gears are accessible at this point without removing much more than the brown intermediate gear. They can be cleaned with a toothbrush and a mild degreasing agent and some fresh grease applied.  Avoid trying to clean bearings in situ, it just risks pushing any contamination inside.  For a full overhaul, where parts are ultrasonically cleaned, bearings checked and so on, the drive chain needs to be pulled down, starting with removal of the mirror shutter assembly.

To remove the mirror shutter assembly, the 2 pins either side of the shutter bearing housing need to be pulled out with pliers. These very accurately position the mirror with respect to both ground glass framing and focus. Avoid jiggling the pins and potentially opening up the holes. Be extremely careful of the protruding parts of the mirror.
Once the pins are removed, the 4 screws can be undone and the mirror shutter assembly carefully removed from the main camera casting.
The mirror shutter is a "bow-tie" or "butterfly" shape, designed to balance the centrifugal forces and help prevent vibration. It spins at half the speed of the movement, turning 180° for each pull-down cycle, so the shutter angle is therefore double the angle of each cut-out segment.

Contrary to much information online and in reference books that suggests a 180° shutter, most IICs will have a fixed 165° shutter angle. The variable shutter model also opened up to 165°. Some cameras may have had a 172.8° shutter fitted later to deal with flicker from HMI or fluorescent lighting in 50 Hz countries at 24 fps. In terms of exposure 165° or 180° makes very little difference (1/52 sec instead of 1/48 at 24fps), but mechanically a 180° bow-tie shutter would not be able to properly cover the moving film during the entire pulldown phase. A 180° shutter was introduced with the IIA in 1954, but problems with travel ghost caused Arri to quietly reduce the angle to 165° when the IIB was introduced in 1958, a change that remained for the IIC. Arri felt it unnecessary (and perhaps embarrassing) to change the description in the manual, so it remains described as 180°.

The vertical drive assembly still can't be removed until the intermediate gear is taken off, by undoing the 4 (or 3 in earlier models) screws around the centre one.

Now the pins positioning the vertical drive assembly can be removed, and the 4 holding screws undone.

Removing the assembly.

The tachometer should be left alone.

The entire movement cam assembly plus the shaft and its gear can now be pushed out into the film chamber side. It is often a tight fit and may need to be tapped through with a soft mallet.

This particular shaft has been hit previously with something not so soft which has mushroomed the end, causing it to jam before passing through the bearing. With a micrometer I measure exactly where it is out of round and use a fine file to remove the bulge. It only needs to be a few hundredths of a mm oversize to jam in the bearing.

Tapping the assembly out with a small nylon-tipped mallet.

The cardiod cam assembly removed. Again, watch for spacer shims beneath the gear or left on the bearing that remains in the housing.
On earlier models the bearing between cam and gear is not enclosed, so watch for loose roller pins.

Be aware of the guide roller which may have come out with the cam or still be on the link arm beneath. The link arm (which creates the forward and reverse oscillation component of the claw movement) can be removed once the 2 screws holding the retaining bridge are undone.

The various components after cleaning, some ultrasonically. Bearings are checked for roughness and replaced or ultrasonically cleaned and repacked with grease. Pivots, guides and cams are checked for play, which will determine vertical steadiness, but replacement parts are scarce outside of donor bodies. Some assemblies may be difficult to dismantle if set screws are damaged or seized - it's probably prudent to leave them be rather than potentially damage anything with further attempts.

The mirror shutter assembly in particular should be treated with the utmost caution. Don't attempt to remove the mirror from it's hub, or the carefully calibrated flatness may be compromised, resulting in a jittery image in the viewfinder.  Try to avoid getting fingerprints on the mirror surface. If it requires cleaning, use lens cleaner.

The link arm refitted and lubricated.

The cam assembly lubricated and back in place, the claw arm being slid over the cardiod cam. It's important that the cam assembly (or more correctly its bearing) is properly seated before fitting the claw arm. To check if the cam is all the way home, fit the inching knob (along with all spacer shims) on the other side and make sure the screw pin lines up with its hole in the shaft.

When fitting the vertical drive assembly back, line up and insert the pins before tightening the screws. When fitting the mirror shutter next, the timing must be set. This means getting the gears to mesh such that when one wing of the mirror shutter is completely centred covering the gate, the claw is right in the middle of its travel. To check the timing, inch the camera and make sure that as the claw goes through its transporting film phase, the mirror is at all times covering the gate aperture. Once again, tighten the screws after the pins are fitted.

A more accurate method of checking a camera's timing is to lace some old film through the gate, and with the mirror out of the way, through the lens port mark a few consecutive frames with a sharpie. A scribble that extends into the corners is good. Then slowly inch the movement back and forth, watching to see if you can spot any motion of the film before the mirror shutter covers and uncovers the gate.

With the gate removed, I check for burrs on the pressure plate and film rails using a straight edge. Any burrs should be polished out. The shiny spot where paint has peeled on the pressure plate is cleaned and re-blacked.

The pressure plate spring pressure is measured, it should be around 180 gr.

The side rail is checked to make sure it moves freely, it's not gummed up and there are no film chips caught in the gap. The spring pressure should be around 90 gr.

The 4 shiny machined surfaces pictured are the seats for the gate and should be clean. If the FFD is not correct it could be a problem with the turret, the gate or these seats. The film plane should not only be accurate in its depth, but flat also, meaning that the depth should not deviate more than 0.02 mm from edge to edge.

Checking ground glass focus and FFD on an autocollimator, using an accurately collimated 28mm lens.

A simple method to check for accurate framing in the viewfinder is to simply look through the lens port and by inching the mirror back and forth, compare the groundglass marks to the gate aperture. For a more accurate check the camera is locked off close to a wall with a lens fitted. A light is shone through the gate (a 45 degree mirror in the gate can help) and the position of the actual gate aperture is marked on some paper taped to the wall.

This can then be checked against the framelines on the groundglass by inching the mirror into position and looking through the viewfinder. Since the groundglass holder is usually pinned to maintain the groundglass focus position, there is usually no framing adjustment possible on a IIC. If it is off it will be likely be due to the mirror angle or ground glass marks.

After the gate is refitted, don't forget to slide the retaining clips back across before securing the screws.

Apart from cleaning the external glass surfaces of the eyepiece and viewfinder tube in the door, I would not recommend attempting to dismantle the optics for cleaning. If the door is a little loose, the locking latches can be removed and straightened.

An important adjustment is the take-up tension in the magazines. As film gets wound on to the core, the growing diameter means that the spindle needs to turn at a slower rate. Since it is driven at a constant rate, the spindle therefore needs to slip, which is achieved with a carefully calibrated take-up clutch. If the spindle slips too easily, the film will not be wound tightly onto the core, causing potential jams or scratching. If it is too stiff the spindle will pull too hard on the film coming out of the mag throat sprocket and possibly damage perfs or cause a jam.

An Arri tension gauge is used by technicians to check take-up tension, as pictured. If checking by hand, the spindle (fitted with a core) should slip without needing excessive force to hold it while the camera runs, but you should need more than 2 fingers to keep it from turning. It should slip smoothly, and with consistent force.

Arri made quite a few different magazine models over the years that can work on a IIC. This camera has an older 200 ft mag that uses a cotton belt with a spring tensioner to create the slipping force. It is not reversible, so only the take-up spindle is clutched. The feed tension, on the right, is created with a spring arm that applies pressure through a felt pad onto the underside of the spindle disk. I clean these surfaces and lightly lubricate the pad with Moly paste.

The take-up tension is adjusted by moving the tension arm spring to a different notch, as pictured.

On reversible magazines the clutches are enclosed friction assemblies on the back of the spindles. These are adjusted by turning the centre disk using a 2 prong wrench, clockwise to increase friction, anti-clockwise to reduce it. The friction assemblies use compressed felt rings that may need replacement if they are jerky or not responding to adjustment.

To remove the spindles the spool-holders need to be removed, by undoing the set screw as shown. Collapsible cores are similarly held by a set screw. When re-assembling, line the set screw up with the groove in the spindle shaft.

The plain bearings (basically just a bushing, left) or ball bearings (right) can be cleaned and relubricated with grease.

Be careful when removing the brass drive gear on earlier magazines as the gear itself contains many small balls as part of the bearing, which will dislodge and may fall out. It's worth ultrasonically cleaning and lubricating this bearing but be careful not to lose any balls.

The screw acts as the inner bearing race.

The throat contains 2 sprocket rollers which maintain the film loop. Sometimes film chips can be caught inside. 2 film thicknesses should fit through each channel. If there is resistance, the cover plate can be removed to inspect inside. I wouldn't advise completely disassembling the throat however.

With the camera re-assembled, the speed can be checked with a strobe gun and compared to the tacho. Sync motors can be checked. I also check the current draw again. This particular camera's consumption was reduced from 2 amps to 1 amp running without film.
The bearings in all IIC motors are ball bearings, if they need service they should either be replaced if damaged, or ultrasonically cleaned and repacked with grease.

Finally I do a scratch test, using fresh film. Ideally each magazine should be separately scratch tested. Only a few feet need to be run through each mag.

I mark the position of the gate and magazine on the film with a sharpie before removing it and inspecting it with a magnifying loupe for scratches. Depending on where a scratch begins relative to the sharpie marks I can work out what is causing the scratch.

A final steady test might be shot to determine how steady the movement is. This entails shooting a double exposure of a grid test chart, with the chart slightly twisted on the second pass. When watching the processed film, movement between the 2 grids will show how steady the camera is. Vertical unsteadiness will usually be due to play or wear in the movement, lateral unsteadiness is usually due to problems with the side rail spring. But other factors can contribute, such as dirt or emulsion build-up, out of tolerance pressure plate force, or incorrect loading (upper and lower loop sizes).

After such an overhaul, a IIC should run trouble free for some time, requiring only the 6 monthly movement lubrication and general maintenance such as gate cleaning and brushing out of the film chamber.