Eclair ACL flicker (technical)

[Gregg MacPherson]

Attempting to gather any technical ideas about ACL flicker here. The prompt was some recent informal posts in the Fleamarket thread. I'll offer some introductory ideas then paste those...

ACL Flicker. Introductory...what causes it?

1) Is it generic to ACL or is it a fixable fault? Are ther multiple causes?
2) On both N16 and some of the S16 conversions, is flicker caused by)?
   - Mirror off position due to wider aperture?
   - Mirror off position due to timing.
   - Mirror off position due to link length.
3) Stray light from the mirror? The mirror face or the edge? Can that light be    controlled with black paint?
4) Uneven friction in a rotational cycle? Hoping Paul can explain that...
5) Mechanical connection between the motor and claw control shaft, the rubber coupling (20).
6) Resonance.
7) Light leaks in the body casting, such as from S16 machining errors.

Can careful examination of the negative or scan show what the flicker is in the frame. Is it a dark area or a bright area?

I have yet to understand points 4) and 5). I get that small angular accelerations (uneven angular velocity during a rotation) could cause the mirror to have lead/lag with its position, but I don't know more. Hoping that Paul does. On the rubber coupling, Dirk DeJonghe once commented that replacing that was a common fix for ACL flicker back in the day.

I wonder if uneven torque from the motor, cushioned by the rubber coupling, would give these small angular accelerations. The torque could pulse within one revolution, or within a cycle over two revolutions, and some resonance is possible with the natural mode shapes and frequencies of the mirror vibrations. This could lead to the mirror being a tiny bit out of position at the wrong time. I'm guessing a bit, but instinctively I look for resonance between the motor and the significant elements in the movement. The mirror, with its 2 motor revs (2 frames) per cycle and long arm feels likely.

If the variation in angular velocity was frame by frame then this could also account for variation in density, flicker.

The ACL mirror movement. 
I'm going to look at the ACL mirror movement. Back later..

 

Edited October 11, 2023 by Gregg MacPherson
added more words

[Gregg MacPherson]

The recent post on ACL flicker from the Fleamarket thread....
Or you can go there https://cinematography.com/index.php?/forums/topic/91893-eclair-flea-market/page/4/#comment-583224

steven jackson
Posted 15 hours ago (10th Oct..?)
I'm going to sell a super16 ACL body that according to Paul at Visual Products, was modified by him, but for some reason was never fully finished...meaning that it does not have a VP ground glass, but a standard ground glass.  It's just the body, no motor or viewfinder, so would suit someone that already has an ACL and wants to swap over a few parts to have a super16 camera.  I've shot film with this camera on numerous occasions and the results are good but under certain situations, mostly backlit sunlight, I get flicker, which was easily dealt with in DaVinci.  I won't be asking a fortune for it.

aapo lettinen
Posted 13 hours ago
  15 hours ago, steven jackson said:
I've shot film with this camera on numerous occasions and the results are good but under certain situations, mostly backlit sunlight, I get flicker, which was easily dealt with in DaVinci.  I won't be asking a fortune for it.

I had a discuss with Heikki about this weird flickering of ACLs under certain backlit conditions and I guessed it would be caused the slightly asymmetric movement of the mirror which causes every other frame to receive just couple of % more exposure than the other. Heikki sent me some test footage and I confirmed that it is indeed exactly every other frame being slightly different exposure than the other so it has to be caused by the slightly asymmetric movement of the mirror. I don't know if it can be corrected by timing the mirror differently but all cameras don't seem to do it so I guess it can

Heikki Repo
Posted 13 hours ago
The backlit sunlight flicker is a different issue compared to the one we discussed Aapo - it's a feature present on all ACLs due to the way the mirror works in this camera. It's not a bug.

The flicker that's present in the footage I sent to Aapo on the other hand is present on every other frame regardless of the lighting situation. That one is due to very slight higher friction in the movement and has been generally noted by Visual Products to be the difficult to solve cause of flicker in some ACLs. In order to solve that one one has to be able to make the movement smooth without any bumps in friction.

steven jackson
Posted 12 hours ago (edited)
Paul Scaglione from Visual Products did tell me this too.  Is this flickering from your newly converted ACL from Les??

Duncan Brown
Posted 11 hours ago
Wasn't there a whole other cause of flicker that had to do with light sneaking in around the recentered lens mount on a S16 converted camera?  I'm sure I read that somewhere around here.    But it sounds like you are talking about potential flicker on even a stock ACL.

Heikki Repo
Posted 9 hours ago
  12 hours ago, steven jackson said:
Is this flickering from your newly converted ACL from Les??

No, this is on one of the ACLs I have serviced. Still haven't had time to finish tests on the newly converted one...

 

[Dom Jaeger]

Hi Gregg,

From working on a few ACLs over the last years I think the flickering could be caused by a few different factors.

One is situations where the mirror covers the gate aperture just before the shutter closes, or lingers just after the shutter opens. Because the mirror oscillates to one side and then the other, you have two seperate cycles where this can occur, and often it will only happen during one of these, causing slight exposure variances.

Both the timing of the shutter to the mirror, and the mirror arm oscillation being centred relative to the gate need to be accurately set to make sure that no part of the mirror ever crosses in front of the gate aperture while the shutter is still open.

S16 conversions both widen the gate aperture (meaning there is more aperture width to cover) and offset it relative to the mirror oscillation. The solution is to use a larger shutter (ie one with a smaller angle). I have seen Bosher conversions where he does not replace the shutter, but only adjusts the timing so that the mirror clears during one direction of its oscillation but not the other.

I think there is also the possibility for light to bounce off the mirror or mirror arm edge when there is strong frontal light, and the edges are not fully blacked.

If there is movement stiffness causing intermittent tight spots it could also cause tiny differences in exposure. Because the mirror essentially oscillates once for every two shutter rotations there is the possibility of a tight spot happening every second shutter rotation.

[Gregg MacPherson]

Hey Dom,

Your ideas and experience with it seem in line with my intuited suspicions. I was wondering about resonance between motor torque variation and the natural vibration behaviour of mechanical components, probably the mirror. Now I wonder if the cyclic variation in friction could also contribute to resonance.  I don't have an academic background that lets me explore this directly with  math, but maybe we can take a simple approach to remove the potential contributions. 

I'm going to ask Andrzej at AZ Spectrum about possible uneven torque from the motor. Actually, various people will understand the motors well enough to give some ideas on that. Maybe the motor is sometimes a contributor and some times not. 

There was another possible cause of flicker that I forgot to include. The machining to the camera body for some S16 conversions had made a tiny hole. Heikki had one of those cameras and made some documentation about it. We should find that and link to it or paste it in.

I corrected a stuff up in my first post above. Now the relative frequency of the mirror and motor is clear. If motor (at 24fps) is 24Hz, the mirror is 12Hz.

Gregg.

 

 

[Scott Bullock]

4 hours ago, Dom Jaeger said:

S16 conversions both widen the gate aperture (meaning there is more aperture width to cover) and offset it relative to the mirror oscillation. The solution is to use a larger shutter (ie one with a smaller angle). I have seen Bosher conversions where he does not replace the shutter, but only adjusts the timing so that the mirror clears during one direction of its oscillation but not the other.

This is my experience with the ACL also. I recall Bernie O'Doherty telling me about this when he converted an ACL for me many, many years ago. The shutter angle on the camera had to be modified to 144 degrees from its factory 175 degrees in order to cover the entire S16 frame when closed.

[Aapo Lettinen]

1 hour ago, Gregg MacPherson said:

Hey Dom,

Your ideas and experience with it seem in line with my intuited suspicions. I was wondering about resonance between motor torque variation and the natural vibration behaviour of mechanical components, probably the mirror. Now I wonder if the cyclic variation in friction could also contribute to resonance.  I don't have an academic background that lets me explore this directly with  math, but maybe we can take a simple approach to remove the potential contributions. 

I'm going to ask Andrzej at AZ Spectrum about possible uneven torque from the motor. Actually, various people will understand the motors well enough to give some ideas on that. Maybe the motor is sometimes a contributor and some times not.

to me it is pretty much impossible that a motor torque variation would cause exactly half fps fluctuation in exposure which only happens every other frame at all speeds and would even happen the same way with different motors, even if a motor has relatively slow speed update rate (much slower than 1/2 frame rate) and more poles than two (for example my current motors are 8 pole ones) . Inertia does not let mechanical devices behave like that (friction difference causing exactly the same result at exactly similar intervals even if the running speed is different) so to me it has to be something which is directly mechanically linked to run at exactly half the fps and is directly on the optical path and thus the mirror would be the most likely culprit.

theoretically the rubber coupling on the axle COULD cause some issues which happen at every revolution (every frame shot) but that would change too with framerates and the issue would shift and not stay exactly the same

[Aapo Lettinen]

the ACL camera has a clear design compromise on the mirror design so that the camera body can be very small and relatively lightweight. in the original use it was a very good choice but on the current indie use these cameras are used for it may sometimes matter more than the original designers would have expected.

if made differently (like having different relative size of the mirror and different swing distance so that the relative mirror speed when shadowing the gate would be as high as possible and the slow down would be phased on the very ends of the cycle well clear of the optical path instead of doing the slow down on the edges of the frame) it would likely have so much less flicker that it would be unnoticeable in almost every shooting situation

[Dom Jaeger]

37 minutes ago, aapo lettinen said:

to me it is pretty much impossible that a motor torque variation would cause exactly half fps fluctuation in exposure which only happens every other frame at all speeds and would even happen the same way with different motors, even if a motor has relatively slow speed update rate (much slower than 1/2 frame rate) and more poles than two (for example my current motors are 8 pole ones) . Inertia does not let mechanical devices behave like that

I agree that it’s very unlikely for this to be a cause, certainly not 1/2 fps variation. But if there is enough play in a system as well as tightness at a particular point it could in theory create very small vibration fluctuations. Whether that would be enough to create visible flicker is another question. 

[Gregg MacPherson]

Aapo,
How did you get the exactly half fps exposure idea from?

There may be several causes of flicker, some of them interacting. So our logic has to be extremely rigorous. For example, eliminating a motor as a cause in a particular case will not eliminate all motors as a potential cause in all cases.

Regarding the mechanical properties of the camera movement as a system. There is some elasticity in the behaviour and if a momentary angular impulse comes from the motor, smoothed and enabled by the rubber coupling, the mirror arm may respond. It may be a simple lag/lead response to the impulse. First thought is that the mirror arm is too stiff to contribute to this, or to have natural vibrational modes of low enough Hz or big enough amplitude.  

But a lag/lead effect may come from play in the mechanical system, likely the clearance in the link rod ball joints for the obvious example. The rubber drive coupler may not damp the motor impulses. It may make their effect worse, smoothed somewhat but delivered effectively to the mechanism.

It may help if we get a feel for the significance of tiny increments in critical parts of the system. I don't have an open mechanism at the moment, so I had to approximate to draw a schematic of the mirror with arm, link rod and "rim assembly" (83E). The rim assembly is at the end of the mag dive spindle assembly, with a ball for the link rod. So I approximated the effective radius of the mirror from the mirror support spindle (part 80). I made a schematic that allowed me to enquire about angles. If anyone can give me the dimensions of the mirror arm I can do this better.

Suppose the mirror intrudes almost 1mm into the corner of the aperture during exposure...this is approx a 2deg increment for the mirror arm, and a 16deg increment for the "rim assembly". If I knew the mirror arm dimensions I could say what the movement of the ball joint was to achieve that (about 0.18mm)...... 

Remember the mirror arm assembly is as if on the surface of a cone, with the radius to the mirror measured orthogonally to the cone's central axis.

If I can find dimensions for the mirror arm assembly I will post a drawing.

Gregg.

[Aapo Lettinen]

1 minute ago, Gregg MacPherson said:

Aapo,
How did you get the exactly half fps exposure idea from?

I checked Heikki's scanned test footage with waveform monitor, advanced it frame by frame by carefully checking the waveform at the same time. One can very clearly see the exposure jumping couple of % up for every other frame resuming back in the intermittent frames and that was consistent on all different framerates tested without the pattern changing in any way. This type of method is much more sensitive than just trying to catch the flicker by eye and one can detect any flicker even if the footage seems fine by eye.

If a motor itself would cause that kind of thing it would need to happen for every other frame so for every other revolution of the motor, NOT on every revolution it rotates (motors work symmetrically in full single revolutions divided in steps symmetrically, in the case of my motors divided by 8). If a motor controller would be so faulty that it would create huge oscillation at 12Hz to the output it would sound so horrible that no one in their right mind could use it and additionally it would cause tons of resonance which would likely break something pretty quickly. With such a resonance being even worse at 50fps for example

[Gregg MacPherson]

Firstly, there may be generic characteristics of all motors that contribute to the flicker, but only show up when interacting with one or more other factors. Also, there may be characteristics of particular motors contributing to the problem.

I sent an email to Andrzej at AZ Spectrum. Hope he's not too busy to reply.

If there is an errant impulse or torque ripple (not sure of the best term)...I don't assume that its pattern cycles every two rev's. If it cycles every rev, I can  imagine ways that it couples with the inertial response of the camera mechanism, such that the lag/lead at the mirror misplaces the mirror at critical moments. There will be some elasticity in the mechanical system of the mechanism, but, the first obvious place to look would be the rubber drive coupling.

If errant impulses from the motors are eliminated as a cause, the rubber coupling could still be a factor, if other factors in the mechanical system interact to give angular accelerations that misplace the mirror, the rubber drive coupling will allow it, may even enhance it.

Gregg.

 

Edited October 11, 2023 by Gregg MacPherson
more words

[Gregg MacPherson]

8 hours ago, aapo lettinen said:

I checked Heikki's scanned test footage with waveform monitor, advanced it frame by frame by carefully checking the waveform at the same time. One can very clearly see the exposure jumping couple of % up for every other frame resuming back in the intermittent frames and that was consistent on all different framerates tested without the pattern changing in any way. ...

Was the variation in brightness just for part of the frame, or for the whole frame? If for part of the frame, then the shape, location  of that area may tell something about how it occured

[Aapo Lettinen]

1 hour ago, Gregg MacPherson said:

Was the variation in brightness just for part of the frame, or for the whole frame? If for part of the frame, then the shape, location  of that area may tell something about how it occured

it seemed to be pretty much the whole image, one could see the whole waveform "jumping" up and down when advancing frame by frame. It was Heikki's own material so I deleted it after the waveform tests but with only couple of % difference in exposure it would be pretty impossible to see any recognizable pattern in the exposure even with adding contrast. I can confirm though that it happened all the time, just was not apparent to the eye most of the time if there were no darker areas in the image where it is easier to spot. The waveform could detect it in all the material and it stayed pretty much the same.

The reason why I am extremely sceptical about the friction theory is because it is pretty much impossible for a friction based issue to stay the same in every frame, all the time, in different conditions and framerates and torque levels. So it can't be something happening "randomly" but rather has to be something built into the device. Because the motor's running parameters change continuously during operation it and changing a different motor to a camera (different technology motor, different parameters, all the variables different) would still result pretty similar flickering, it has to be something mechanically linked happening inside the camera.

It would be possible to make a measuring device to detect a flicker like this directly from the camera gate if running without magazine. the simplest method would be to use a properly tuned phototransistor or a small array of such transistors and then monitor the output signal with oscilloscope when running the camera at different speeds measuring the duty cycles of each exposure, adjusting the light source in front of the taking lens and changing a taking lens between traditional and telecentric so that the angle of the light rays in the mirrors path would be different which could affect the amount of flickering by altering the point where the mirror edge starts to shadow the still exposing frame in the gate.

Alternatively a video camera could be used which would be synced exactly to the camera's running speed (in practice both have to run at 25.00fps to ensure it is exact during entire process) and observe the mirror shadow patterns at slow speed.

Personally I just inched my camera manually and observed the mirror path... it was easy to observe that it is extremely difficult to try to make that kind of device moving exactly symmetrically and it can be easily seen how it can cause such issues especially if the light rays from the taking lens are coming in certain angles so that the mirror edge has possibility to shadow the frame more

[Aapo Lettinen]

btw, lowering the shutter angle dramatically should completely eliminate this kind of issue if lowered enough so that the mirror is always very far away from the optical path until shutter is completely closed.

this would result in limited performance of the camera system which is likely why it was not done in the first place. For a camera made for tv-documentaries and such broadcasting on analog TV, it is likely that no one could detect any couple of % flicker with any kind of instruments from the output stream or even the telecined version. Exposure, on the other had, mattered quite much with slow stocks so one would want all the shutter angle one could get no matter the cost ?

today when everything is file based, digitally shown and 4k or 5k scanned with great quality scanners, the small imperfections in the image will really start to show. everything just too clean and well done nowadays ?

[Gregg MacPherson]

Prefix this with the caution that what one discovers about the particular is not automatically applicable to the general or universal.?

We need to remind ourselves that the mechanism ideally would have constant angular velocity, so no angular acceleration, and the phases of the mirror movement (L/R) would be symmetrical. 

I think I have a plausible idea for how anomalous variation in friction could contribute to exposure variation over a two frame cycle. If we have bearing surfaces (bush and shaft) that are out of round on the mirror spindle or mag drive spindle, which both run at 1/2 the motor Hz, the friction peak would be every second frame. If this friction peak occurs when the mirror is in the left or right position, then the angular velocity on departure will be less than on the return stroke. So, a two frame cycle of exposure variation.

Could the drive shaft in the mag, also running at 1/2 the motor Hz, have a friction peak? It having ball bearings, I tend to exclude it.

We may be talking about small angular accelerations. Intuitively it seems like the inertial properties of the mechanical components will resist these angular accelerations, but the rubber drive coupler will enable them. So  oscillation is possible. Normal exposure time with 175deg shutter at 24fps is 175/360/24 = 0.02026 seconds.The variation in the exposure time to give a 2% change in brightness is 0.000405 seconds for a 175deg shutter.  Exposure time after that angular acceleration is 0.01986 seconds, and the fps=175/360/0.01986=24.47.

Oh, now I see where Aapo got the 1/2 fps variation idea from. He just gave 24fps an extra 2%. Well, it was late, and now I'm laughing at myself.

Another way to think about how the mirror assembly could cause angular accelerations of the shutter....Anything that allows a lack of symmetry in the two phases (L/R) of the mirror movement could cause angular accelerations in the mechanism.

Most of the scenarios I think of allow the possibility of resonance.

I'm thinking that at very low fps the inertial behaviour of the mechanism, the elastic behaviour of the rubber coupling and the likelihood of resonance will be much less. And this may be easy to test.

I sent an email to Andrzej at AZ Spectrum. He is very busy but had some interesting ideas. It's easiest just to paste it...

Wednesday, October 11, 2023
Hey Andrzej,
I'm helping some research on why ACLs sometimes give an image that flickers. Do you have any insights into this? I posted some ideas on the cinematography.com forum...
https://cinematography.com/index.php?/forums/topic/100490-eclair-acl-flicker-technical/#comment-583237
 
One possible cause is a ripple in the motor torque. The elasticity of the rubber drive coupling enables this impulse to be delivered to the mechanism, which has an inertial reaction.
 
So any thoughts? Do the motors normally have perfectly even torque during one revolution? Do particular motors have a defect? Imperfect positioning of the hall sensors?
 
Regards,
Gregg. 

His reply.....
Hey Gregg,

Thank you for your e-mail. Currently I'm very busy with HD video taps installations. The problem you described you can resolve by using 24 Hz crystal strobe light. For the camera speed 24fps you can see in the 24Hz strobe light any vibration, of the motor speed, shutter vibration with magazine attached with film or empty. Problem can be related to motor, camera mechanism, mag, movement of the film and also to the power cable, battery or power supply.

Best regards,
Andrzej 

Edited October 12, 2023 by Gregg MacPherson
formatting

[Aapo Lettinen]

1 hour ago, Gregg MacPherson said:

Oh, now I see where Aapo got the 1/2 fps variation idea from. He just gave 24fps an extra 2%. Well, it was late, and now I'm laughing at myself.

No, I was talking about full shot frame on the film negative. Every other frame had couple of % more exposure on the wavefor monitor.

I checked my camera again and now I see exactly what the issue is about. It is definitely the mirror and only the mirror.

The issue is that the pivot point of the mirror swing axis is not perfectly centered on the centerline of the optical axis and gate. When observing it very carefully, you can see that the swing axis is actually a tiny bit on the operator side of the gate (camera's left side). You can see this by checking where the mirror edge is located when the shutter is opening and closing: when the mirror is on the operator side, there is lots more room between the gate edge and the mirror edge, whereas when mirror is on the camera's right side, the mirror just barely tries to clear the gate edge when shutter is starting to open and can even shadow it a little bit if the taking lens would have certain output geometry of the light rays  to enhance the effect.

The asymmetrically centered swing axis means that the angular velocity of the mirror is lower when the shutter starts to open when the mirror is on the camera right side compared to the camera left side. This kind of variation in the mirror speed could cause even larger exposure variation between the adjacent frames and I am surprised that is is as small of a variation as it is.

So it is a design fault or design feature depending on how one wants to see it. The asymmetrical mirror swing axis helps to make the camera narower which makes it more compact and helps weight distribution too (when operating on the right shoulder like most people do, there is more of the camera's mass on the operator side so that it does not "twist" the camera as much and makes operating a little easier. I think though that the only reason why they made it asymmetrical is getting the camera body physically smaller. But it had this downside that the exposure is not exactly the same on adjacent frames and one can see it in certain conditions and lens choices may make the issue worse.

But it is clearly a design feature, there is nothing wrong with the camera. Mirror timing may be a little off of course to make it even worse but you can't remove the effect completely unless recentering the whole thing and machining the camera body to enable recentered mirror system to fit there

Edited October 12, 2023 by aapo lettinen

[Aapo Lettinen]

nothing wrong with the motors either because the flicker is not caused by them.

Converting the camera body to S16 which often requires changing the mirror movement path and/or shutter angle and alters the centering of the optical axis would cause changes to how the camera flickers because the mirror swing axis - optical centerline relationship, angular velocities and shutter clearance would all be changed at the same time.

this would mean that some S16 conversions would flicker more and some less. Don't know if it can be made completely flicker free but some people never complain about ACL flicker so I belive it can be made low enough level that people don't notice it anymore.

But as said it would require large mechanical changes to the camera body itself

[Gregg MacPherson]

9 hours ago, aapo lettinen said:

No, I was talking about full shot frame on the film negative. Every other frame had couple of % more exposure on the wavefor monitor.

I checked my camera again and now I see exactly what the issue is about. It is definitely the mirror and only the mirror......

............The asymmetrically centered swing axis means that the angular velocity of the mirror is lower when the shutter starts to open when the mirror is on the camera right side compared to the camera left side. This kind of variation in the mirror speed could cause even larger exposure variation between the adjacent frames.............

I understood what you meant with the 2% change in exposure of the whole frame,  I looked at the increments of time and angle to achieve that and calculated a change in  shutter speed. It would be oscillating between 24 and 24.47Hz. Then realized that I could just scale the shutter freq by 2%. And this was humorous because I thought that's what you and Dom had done earlier. Simpler.

The "asymmetry" arising from the relationship of the mirror central axis and frame aperture is interesting to us both.?  I can't tell yet if the unmodified camera (N16) has that and how much. My cam body has the base disassembled, motor off and I can't easily inch it or open the body on a whim. But I can see in my schematic drawing the scale and nature of the asymmetry that comes with the S16 mod.

However, if we assume zero elasticity in the mechanical system, the "asymmetry'" of the mirror movement is hidden by the closed shutter. So lack of symmetry in the angular acceleration profile, L/R, is not relevant.  But, if we accept that the mech system is elastic, then the mirror axis  "asymmetry" may be a source of angular accelerations of the rotating elements, so the shutter. So examining whether some generic problem(s) underly the flicker with ACLs comes back to exploring a group of potential causes that may be acting, interacting in concert.

I think it is misleading to proclaim that the only possible cause of flicker is the mirror. (edit: I mean the asymmetry in the L/R mirror positions). ?

Instead, perhaps take the advice of Andrzej and explore with a crystal strobe. But lets not assume that what we discover on that camera will be true of all cameras.

Gregg.

 

Edited October 12, 2023 by Gregg MacPherson
more words.

[Gregg MacPherson]

It may help if we have a drawing to refer to. This one shows an approximation of the effective mirror arm radius in a schematic, The mirror is oscillating on the surface of a cone. This can be represented in 2D using the effective mirror radius, measured orthogonally from the actual mirror spindle to the mirror. The "rim assembly" on the mag drive spindle is then also a schematic, giving the correct rotational relationships to the mirror arm.

If I had the position of the mirror spindle axis relative to the lens axis I could put that in a drawing.  I need to know the horizontal offset of the mirror spindle to the lens axis, hence if there is a cant angle for the mirror arm when centered on the N16 frame. Not all S16 geometry will be the same, so it's better to start with N16.

 

[Aapo Lettinen]

50 minutes ago, Gregg MacPherson said:

The "asymmetry" arising from the relationship of the mirror central axis and frame aperture is interesting to us both.?  I can't tell yet if the unmodified camera (N16) has that and how much. My cam body has the base disassembled, motor off and I can't easily inch it or open the body on a whim. But I can see in my schematic drawing the scale and nature of the asymmetry that comes with the S16 mod.

However, if we assume zero elasticity in the mechanical system, the "asymmetry'" of the mirror movement is hidden by the closed shutter. So lack of symmetry in the angular acceleration profile, L/R, is not relevant.

Actually the edge of the mirror is so close to the opening shutter angle when the mirror is on the right side that it is easily possible for it to shadow the gate especially if the taking lens has certain characteristics which have potential enhancing the effect (large back element etc.)

UNMODIFIED N16 CAMERA, MIRROR ON THE RIGHT SIDE OF THE CAMERA:

 

THE SAME UNMODIFIED N16 CAMERA, the mirror position referenced to the moment where the shutter edge is on the center of the gate. See how much closer the mirror is to the gate edge when the mirror is on the camera right side:

COMPARED TO MIRROR ON THE CAMERA LEFT (operator) SIDE:

[Gregg MacPherson]

Thanks for posting those macro pics of the mirror. Now if Heikki measures the mirror axis position well have the good info.?

For simplicity I'm going to refer to my drawing above.

I get your point on the proximity of the mirror edge to the frame, with the exit of light from the rear elements of some lenses potentially hitting it. Do you think that light reflected from the edge of the mirror will raise the exposure evenly over the frame. Probably not. This is a problem peculiar to certain lenses perhaps.

If a mirror edge was masking a frame corner at position A or B, that should be obvious on the exposure, so that peculiar problem also has nothing to contribute to the elevated exposure over the whole frame (EEWF).

If the mirror positions L/R (A,B in drawing) are not symmetric about the aperture centre point, this is a function of link rod length. I always assumed that a new link rod was used for S16. I see that Les B also lists that on his S16 ACL mods. The link rods post ACL I are I think simple plastic. The ACL I had a metal one, some with clearance adjustable on the ball joints. Was metal abandoned for the high fps rates perhaps? An adjustable length metal link rod would be extremely useful.

Obviously, if new link rods are made for S16, this may be a source of error. I'll reserve thoughts about that until I can understand the problem more deeply.

The asymmetry in your pics is quite large. Interestingly, it favours the frame on camera L, where the S16 frame enlargement is. Is it possible that the shutter is one gear tooth out? I'll try and estimate the number of teeth from the parts drawing and calculate the angle increment. It may be too large to be a plausible cause. (once again we wish that I had one of Heikki's spare mechanisms sitting on my desk).   

Your mirror in position B looks clear of the frame corner by 2deg rotation, and a 180 tooth gear is not plausible, the vertical shaft has about 25 teeth. So mistiming I can't see as a cause of the asymmetry that you show.

In Aapo style, I will proclaim that link rod length is the sole possible cause of mirror position asymmetry. And if this asymmetry causes unfavourable dynamic coupling, resonance allowed by the rubber drive block, then I've solved it! Time for a cup of tea.

 Gregg.

PS: I'm getting the 9 pin connector replaced on my ACL II base v soon, then I can measure some things myself perhaps.

Edited October 12, 2023 by Gregg MacPherson

[Gregg MacPherson]

Adjustable mirror link rods

Adjustable length might be very useful if they can be light enough.  Ditto for adjustable ball cups. I have seen adjustable ball cup screws that went in axially, but I can't yet find a reference. An old, very faded,  ACL I manual shows adjustable ball cup screws (I think) that  screw in radially.

 

Versions of an adjustable length rod that could be made simply seem possible. Just need to make them light enough for 75fps, which may be a fatal problem. The solution to accurate link rod length may be to accept the plastic and find a way to machine a simple cup positioned super accurately. Or just buy one from Les B if he is makes them. He probably would make them in a batches, assuming he thought this was a solution to something. Here is my doodle on an adjustable length rod with (acetal) plastic cup halves. Better version may come quickly to mind..

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Edited October 19, 2023 by Gregg MacPherson
fixed some context

[Duncan Brown]

That adjustable end cap reminds me of the throttle linkages on my old TR-4 back in the day.  Though I think they had a fixed inner cup so that was just to set the friction...and the threaded rod between the links just set the distance between the links.

(Not sure why I bring this up, since TR-4 throttle linkages are far too large and heavy for this application!)

Duncan

[Gregg MacPherson]

But the ideas may be useful, so scale is not an issue. When I think of making an adjustable length link rod I assume using cheap chinese tap and dies, not cutting super fine threads on my old lathe. For RH thread at both ends,  the thread pitch increment to give a 1 deg shift in the mirror per turn would be about 0.1mm. The "tube" could be aluminum but the rod needs to be steel or titanium. Oh, unless it was bigger diam.

Gregg

[Boris Belay]

Coming late on this topic, but I would say that flicker on a standard 16 ACL is most likely due to a timing issue.  On a Super-16 modified camera, the shutter does noeed to be widened or flicker will appear under strong light conditions.

FYI (from Eclair maintenance docs ) :