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The Secret of the Broken Element: A Canon RF 100-500mm f4.7-7.1 Teardown

Image courtesy Joey Miller.

In ancient times, the IS unit in Canon EF lenses was physically locked down, so the unit didn’t make rattling noises when the lens wasn’t powered up. Canon decided the lockdown was no longer necessary in modern times, so some RF lenses, like the Canon RF 100-500mm f/4.5-7.1, rattled when the lens wasn’t mounted to the camera. And the people heard the noise and were sore afraid. And they worried about this on interweb forums and such.

Me being me, I said, that’s silly, it will be fine, don’t worry. So, of course, something bad happened. We had several of these lenses come back from rental with a cracked internal element. In every case, it seemed to have happened during shipping. If you’ve ever rented from us, you know how we pack. Nothing should break in shipping. And so, we were sore afraid.

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In every case, when we looked through the lens, the crack moved when we shook the lens. So, I assumed the cracks were in the IS unit since it moves when you shake the lens. As usually happens when I assume, I was wrong. But it took a complete teardown (and I do mean complete) to figure that out.

So, Junior Tear Down Scouts, grab your screwdriver and follow along as we tear down our 100-500 lenses!!! (Editor’s note: No, do not do that. Roger’s being ironic because he thinks it’s amusing.)

The Easy Stuff First

All we know about the inside of this lens is from an online lens diagram. There are eight groups with 20 elements. We think the second group from the front is the IS group.

Image from Canon USA.

So we thought we’d start disassembly from the front.

 

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The filter barrel comes off after removing a few screws—good news for those of you who break filter barrels or lens hood tabs. There’s a nice thick ring of sealing felt underneath there, too.

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There’s an impressive nine screws around the front group.

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But three of them hold on to a weather sealing ring. This had the foam we showed you before, and the undersurface was coated in sticky adhesive as a secondary dust trap.

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The other six screws were there to hold the front group in place; there were no adjusting shims for tilt, and no centering mechanism in the front element, so it’s easy to remove.

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For those of you following along with the lens diagram above, this first group has three elements.

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Now we can look down at the front of the IS group, where we had thought our crack was. Without the front element bending light rays, the crack moves less when we shake it, although it still appears to move. So we’re not sure if it’s the rearmost element in the IS group or not in the IS unit at all. In other words, we’re now a little confused.

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There was no apparent way to remove that light baffle, and taking the zoom rubber off showed a LOT of cams and screws, some of which were adjusting collars. Since we weren’t sure what was connected to what, and there was no obvious way to take the zoom barrel off, we left well enough alone.

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Like the cowards we are, we fled around to the back of the lens, where we knew how to take stuff apart. The usual rear removal: screws out, baffle out, bayonet off, weather seal off.

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There’s a spacer that lifts off next. As best we could tell, this is not sized, so it’s not acting as a variable optical spacer. (Canon sometimes uses variable-sized spacers instead of shims. This lens had neither.)

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Then we disconnect a bunch of flexes and remove the PCB.

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Which exposes all the screws holding the back lens barrels in place.

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The programmable barrel comes off first; you can see the optical markings on the inside.

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Underneath are the optical sensors (near the forceps) that tell the camera that the barrel is being rotated. To the right is one of the IS sensors.

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Removing a set of 6 screws let us slide off the entire rear barrel, which was a wonderful thing.

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Remember all those flexes going into the PCB? With some brands, we have to trace out and disconnect each of them individually. With this modular construction, most of them are contained within this barrel and just slide right off. I get accused every so often of being a Canon fanboy (or Sony, or Sigma, etc.). I’m not, but I’ll readily admit I’m a Canon lens construction fan; these are a pleasure to work on compared to most brands.

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The smooth tighten ring slides right off. Every external barrel joint has a ton of thick sealing/friction foam, which is what we expect to see from a lens in this price range.

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Underneath the barrel, we start to get a look at how the optical elements move around.

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And it quickly became apparent that just about everything moves in an impressively complex fashion.

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A couple of things are worth noting. The front barrel moves along six (as opposed to three in many extending barrel zooms) heavy-duty rollers with some added straight sliding cams (not visible in images), preventing barrel sag. The rear group moves forward into the barrel at full extension, but it’s a separate movement distance, it’s not just dragging along for the ride with the extending barrel.

A closeup of those extending barrel rollers: heavy screw through heavy brass bearing with a thick nylon roller. When we took them out we found they were sized, so that each has no play but still moves smoothly. But that also means we had to keep track of which bearing went in which slot when we took them out. You don’t have to be OCD to take lenses apart, but . . . oh, wait, yeah you do have to be OCD to take lenses apart.

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This is why nice things cost more; a lesser lens has three same-size small nylon bushings over screws. Here’s a close-up of one of those bearings.

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Go have a cup of coffee now while Aaron takes out and marks the positions of a half dozen rollers, three guide bars, and another half dozen assorted screws. After he takes those out we can remove the zoom barrel.

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We can move around the zoom and focusing barrels, letting us look at deeper sets of roller bearings (blue lines) and adjustable eccentric collars (red lines). Everything is oversized and robust.

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Removing a few more screws lets us take off the extending barrel.

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Now for the Harder Part

Time-wise, we’re about halfway done; we’re down to the inner barrel which contains all of the optics excluding the front element that we took out an hour ago. The IS unit flex is running up to the IS unit.

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With the barrel off we can get a clearer look at the IS unit, but still aren’t certain if the crack is in one of the rearmost elements of the IS group or somewhere behind it. It still seems to move when the IS unit moves.

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Near the rear element is an interesting thing we haven’t seen before. The rear baffle is suspended on three springs, letting it move with a little give as the rear element moves. I think that’s the purpose, at least. We’ve noticed more and more tensioning springs in Canon lenses the last few years, which suggests they originally thought it was a good idea, found out they were correct and increased usage. This lens has over a dozen springs.

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The IS unit has optical adjustments, so those have to be marked for position, then we can remove the fixed screws and adjustment collars.

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Once out, it’s apparent the IS unit is not the location of the broken glass. It’s apparent this is a pretty robust unit. Optically it’s pretty strong; you can get an idea in the image below, looking through the glass at the gridlines on the table. That is why the crack behind it appeared to move when the IS unit moved.

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In older lenses, we sometimes saw IS units that were encased in a ‘cage’ of plastic bars, which broke sometimes. This is not that at all, it’s heavy-duty interlocking plastic shells with multiple screws and tension springs. We could (OK, we did) shake the heck out of it. It just rattled a bit, but there was nothing but solidness here.

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Looking through the lens now, we could see the broken element was not right behind the IS unit, it was further down, so more disassembly was needed. The rear group seemed pretty accessible so we went there next.

Removing a few screws let us take out the spring-loaded baffle assembly. It’s a complex little bit of engineering for a baffle.

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With that out of the way, the rear element can be removed.

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After this, there don’t seem to be any more elements volunteering to leave the barrel singly, they’re all nested in there together. It might be worth revisiting the lens diagram now; at least that’s what we did. We’ve removed the front group, IS unit, and rear element. I’ve drawn a blue box around what’s left in the barrel.

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So, go have another cup of coffee (I did) while Aaron spends 20 minutes marking and removing screws, cams, rollers, and sliders. All of which were heavy-duty robust things as we would expect them to be in a lens like this.

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After which, the Inner Barrel of All the Good Stuff comes sliding out of the cam barrel. We still haven’t removed any glass, just taken off another layer of mechanics.

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This inner area is an impressive and intense piece of engineering. You can see the two linear focusing motors (notice they are at different levels, because each drives a different element), along with adjustable collars, various flexes, and lots more tensioning springs. I told you Canon was liking them some springs these days.

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The cracked element is obviously within this assembly but we still can’t be certain which element it is, other than it’s neither the frontmost nor the rearmost.

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I suspect that this entire assembly is a single part in the part catalog; that the repair center just replaces it as one piece if anything breaks in there. But we want to know what broke and perhaps find out why it broke, so we’re going to take this a bit further than the usual disassembly.

So Aaron started marking adjustable collars to remove the foremost group, the one in front of the aperture assembly.

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This comes out easily enough. As the lens diagram above shows, it’s a fairly thick group containing three elements.

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Finally, we can see that the cracked element is the one right behind the aperture assembly. That’s a thin singlet that we can tell by moving the front focusing motor manually is also the forward focusing element. (It may be a close focus compensating element, I don’t know. Hopefully one of the lens designers will let us know in the comments.)

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There’s no possibility that this element could physically contact the one we just removed. The aperture assembly separates them and the aperture is soft and flexible, so that couldn’t have caused damage. We decided we should open up the AF assembly and see what could have happened in there.

So off comes the aperture assembly,

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the linear focusing motors,

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and the complicated set of flexes that contain the focusing position sensors.

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The rest of the elements are removed through the back. Removing a few screws and cams let us take out the large biconvex singlet.

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Next, some back caps have to be removed to expose the rods (there are two sets of two rods) that the focusing elements slide on.

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Then the rear focusing group can be removed after it’s detached from the rear focusing motor.

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You can see the positioning sensor strip that is read by the sensors on that flex we just removed.

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There are still some adjustment collars, springs, and cams to take out that hold the center group in place.

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This is another chunky group containing four elements.

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There is no indication that the cracked element ever impacted here; the glass and surrounding mount have no impact marks. We later measured things and it’s physically impossible for the focusing element to impact this group.

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After that, the forward focusing element, the one that is cracked, can now be disconnected from its motor and removed. Again, the element was properly attached to its motor and sliding bars. Experimenting made it clear that there was no way the element could impact anything inside the barrel.

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No question this is the cracked element. Roger’s Rule of Broken Parts: the hardest to get to part is the one that’s broken.

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So What Did We Learn Today?

Well, for the 116th time, I learned don’t make assumptions. For the 243rd time, I learned that a lens is full of elements that refract images so looking through either end into the middle is a fool’s errand. Why do some lenses have lots of dust inside? Mostly because some front elements magnify dust. Why did this crack seem to move with the IS unit? Because the IS unit refracted the image of the crack and made it seem to move with the IS unit.

We also learned that the crack occurred in the front focusing group, a fairly thin, singlet. I’m no materials physicist, but I think thin elements are more likely to crack than thick ones. But there I go, assuming again. We did carefully check the movement of each element in the inner barrel with the focus motors off, etc. and there is no possibility this element impacts any other element. There doesn’t seem to be any hard stop in its travel that could cause a shock with movement.

So why did several of these crack during shipping? I have no idea. My first thought, given that it’s winter, was perhaps temperature shock, moving from sub-zero trucks to warm indoors or something. But I’ve asked several people more knowledgeable than I and none think that’s a possibility. The ones that cracked are all early copies from a similar serial number range, perhaps there were some flawed elements early on.

Maybe it’s just a statistical anomaly; we have a lot of copies and stuff happens. Or maybe it’s something we do or something with shipping. Nobody else has reported this. It’s worth looking into further, there are a number of things we’ve noticed before anyone else just because we have a lot of gear and a lot of repairs and inspections. But it may be an oddity that never happens again.

In any case, we’ve forwarded all of our data and broken lenses to Canon. Canon is always proactive about investigating these things and one of the few companies willing to publicly say when they actually have a problem. They have assigned a team to look into it, we’ve already given them some more information they’ve requested, and they’ll figure out if there’s a problem or not.

The rest of the teardown is what we expect from an RF L series lens. It’s filled with very robust construction, neatly and clearly laid out in a modular manner. It’s a very well-built and sturdy lens with cutting edge technology. It will be very easy to do simple repairs like filter barrel and front element replacement. It will probably be expensive to do major repairs if I’m right and that central barrel is a single part.

 

Roger Cicala and Aaron Closz

Lensrentals.com

January, 2021

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

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  • I see a lot of discussion about CTE in the comments. Based on my own extensive experience, I think you should be focusing on three completely different letters: UPS. (Showing myself out now…)

  • Brandon Dube

    The material for a drinking glass and making lenses are orders of magnitude apart.

  • Brandon Dube

    When I say “stiffness” I am referring to Young’s modulus.

    No, poorly annealed glass would not interfere with its grinding or its polishing. The slurry removes essentially all heat during the process, which is itself very gentle. Even high speed grinding of optics is like 1/10th the removal rate of “ordinary” grinding of metal.

  • martian0815@hotmail.com

    Glass is generally very brittle and cracks in a conchoidal way, which can be seen by closely looking at the photos of the crack from both sides of the lens.

    Furthermore, you can see so-called ‘beach marks’ in many places, these show the progress of the crack tip over time. Therefore tracing back beach marks can serve to locate the crack origin. Based on this the crack appears to have started at the outer periphery of the lens where it is in contact with the surrounding holder, then the crack quickly grew across the entire lens outer surface to the opposite side, in parallel also growing laterally into the depth of the glass. In particular the crack growth into the depth of the glass shows macroscopic beach marks, which indicate how the crack surface gradually progressed from one beach mark to the next.

    As the crack origin is located at the lens periphery, there could have been a small damage/scratch, e.g. from impact during manufacturing or assembly, which caused a local stress intensification and subsequent crack initiation and growth, when stressed thermally and/or mechanically.

    In line with Brandon Dube, I also suspect that improper annealing might have caused a high level of residual stress in the glass; when any otherwise acceptable thermal or mechanical stress is added, this combination may have caused the fracture.

  • Roger Cicala

    A lot of those are RF, Z, and L mounts, which for several reasons we didn’t get made for our bench yet. But I will be testing the ones we are able to test.

  • I was going to comment this, but didn’t get here fast enough.

    Looks like CTE mismatch indeed.

  • David Beall

    This was a great read! I’m very curious to find out what the issue is. Every time there’s a new teardown released, I immediately drop what I’m doing to read it haha, thanks for doing them!

  • Dave

    Thank you for your nice work!
    Are there any plans to do MTF measurements again? A lot of really nice lenses have been released last year…

  • Ciaran

    I’ve had drinking glasses spontaneously explode in my hand – shatter into hundreds of fragments with a loud pop. Clearly a lot of internal stress had been quenched in the glass and was suddenly released. It was generally a thermal shock that triggered the explosion.

    If such an effect can occur in optical glass, perhaps Roger’s suspicion that thermal stresses during shipping had something to do with the failure is not wrong.

  • Jim A.

    That was an impressively complex deconstruction. And another well written article. Thanks again for sharing your findings with the rest of the world. The first thing I did after looking at this article was go look thru my 100-500 and see if it was clear! Mine’s all good for now. I hope your info helps the fine folks at Canon work out what happened and I also hope they share that with you. Even if you can’t tell us what they found because of an NDA or a gentleman’s agreement, the fact that they share the findings means they value your input, and I’ll bet they do.

  • Trevor Overman

    A couple of thoughts on stiffness. The stiffness of a part is determined by both the modulus of elasticity and the moment of inertia of the part. I would assume the plastic part is very stiff so that it would hold the element in an extremely precise location even with a lot of force applied from the focusing motors.

    You mentioned a bad annealing, but wouldn’t that cause the element to not be able to be ground accurately? I would think a bad annealing would be caught during testing of the lens.

    Thanks for giving your thoughts on this. It is an interesting discussion. I could be way off base. I have never designed lenses, just pavement.

  • Roger Cicala

    It is winter and they ship in unheated trucks, but I was told (don’t know firsthand) that even -30F was acceptable.

  • Charles Simonds

    Is there any reason to think that the lenses in question got unusually cold (< -10 F) in use or shipping? Some materials, even steel, become brittle in cold. I could not find the operating/non-operating temperature spec for the lens. I expect that a lot of your rental stock gets exposed to temperature a lot colder than in Canon's spec what ever it is.

  • Jalan Lee

    Did Aaron end up with some “extra” parts after putting it back together… that’s what always happens to me…

  • Roger Cicala

    Aaron put it all back together. Canon would not have liked getting 13 baggies full of parts.

  • I’ll second that!

  • boeck hannes

    just reading this gives me a headache, makes my palms sweat and my heart pound. hope you could ship it canon without reassembly.

  • Wow guys, the first answers by Brandon Trevor and DV appear to be the most logical. In another lifetime I was a Camera tech back in the 70s. This is the first teardown I have watched and all I can say is OMG. I have been photographing birds with a Olympus E-M1X body, and an older 4/3rds 300mm F/2.8 that I purchased, (like new) from Japan a couple of years ago. Upon initial inspection of the lens when I received it the elements appeared to be perfect. I have since noticed what appears to be a wipe mark or dried condensation droplets on one side of what I think is one of the elements in the front element group. (Hmmm). It definitely isn’t fungus! I now think the lens I received was a Olympus recondition when they were closing out the 4/3rds lens line. I was going to attempt to dismantle the front group and see if I could clean it off, but since it does not appear to have any affect on the images, and after seeing this article, (rolls eyes), I think I will be tempting fate and will leave well enough alone.

  • Marc P.

    Roger, i wrote it on DPR. It looks like a crack produced by tension inside the Lens Element over Time, the way it is being pressed inside this Metal Part. Even glas (elements) do themically react, expand, and vice versa, into microscopic small units. That might be the case here….a tension-based crack.

    (edit) We don’t know, with how much force that Glas Element is being put & being hold into this specific Lens Part, means -force. Only Canon does.

  • 100 points for the Photoshopped Hardy Boys book cover ?

  • Brandon Dube

    If it’s fiber reinforced it might be 1/10th the stiffness of glass, but it would also be much more likely to yield than the glass. If it’s not reinforced it will have O(1/100) the stiffness of glass.

  • Brandon Dube

    Optical glass is very pure, ppm for ordinary optical glass and ~100ppb for high purity. Internal stress, not impurity, would be the problem.

  • Roger Cicala

    Thanks Brandon. I didn’t want to speculate about things I know little about (well, no more than usual), but the size of that fracture made me wonder about a glass batch issue.

  • Roger Cicala

    I did not but several of our techs did, they said the pictures were blurry.

  • Slepý Slepá

    Did you try to take a picture with the lens? If so, can you post the result?

  • Trevor Overman

    I wonder if the plastic housing of this focus group might be to blame. Normally plastic has a much higher CTE than glass. The fact that everything is made very robust could be part of the problem. If the cold made the plastic shrink, and the plastic is a large strong part, then maybe it put enough stress on the glass that when the element was bounced around during shipping it cracked.

    I doubt it was a single issue that caused failure as that would most likely have been considered during design of the lens, but they may not have tested the resistance to cold and vibration a the same time.

  • DV

    Came to post this too. Thermal expansion/contraction amongst different materials with perhaps some impurities thrown in.

    Basically the inverse of the John Hancock building problem (where the glass windows popped out and fell to the ground due to differing contraction rates). The framework was too loose instead of too tight. Since the glass in the lens has nowhere to go… it breaks instead of popping out.

  • Brandon Dube

    The lens that broke is “UD,” low dispersion glasses usually have bad thermal properties (large CTE). It could be mechanically overconstrained, and it broke because glass is stiff and doesn’t bend; if no one bends someone breaks.

    The “blow out” inside instead of a clean break might be a material problem from bad/improper annealing. A stress fracture would be likely to be a clean break (test it – wack some of those front groups in the buckets with a hammer). Maybe one of Canon’s glass suppliers let out a bit of a bad batch due to Covid.

  • You two are the Hercule Poirot of lens mysteries. Thanks for another lovely teardown.

  • Károly Zieber

    Professional content again, reading it was a pleasure!

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