Cracked and confused: Lensrentals tears down the Canon RF 100-500mm lens to locate cracked glass
posted Friday, January 22, 2021 at 1:00 PM EST
Roger Cicala and the Lensrentals team ships and receives a lot of gear. Some cameras and lenses will inevitably break with regular use and more often due to accidents. Damage during shipping is rare, and if you've rented from Lensrentals, you'll understand why. They're conscientious. However, several copies of the new Canon RF 100-500mm f/4.5-7.1 lenses have returned to Lensrentals with a cracked internal lens element. In each case, it appears the crack happened during shipping. Eager to get to the bottom of the issue, they went to work taking apart one of the damaged lenses to hopefully figure out what went wrong.
With every broken lens, when they looked through the lens and shook it, the cracked element moved. This led to the assumption that the cracks were in the image stabilization unit. If you've held a recent IS-equipped lens, you might have noticed that when the lens is not in use, there's a bit of noise generated when you move the lens around. This rattle, while concerning, is often completely normal and fine. However, Lensrentals wondered if the floating IS unit could be the culprit for the broken glass. To find out if this assumption was true, the lens required a complete teardown.
As is often the case, the assumption proved wrong. However, to learn this, many screws must be unscrewed, and many tiny, precise components must be marked and removed. As you might imagine, a 100-500mm zoom lens with 20 elements across 8 groups has very complicated internals. As an aside, even if you're not interested in this lens, if you're into lenses in general, you've got to check out the full teardown because it's fascinating.
Starting at the front of the lens, the team removed the filter barrel – a simple task, which is great because it's an important piece to be able to replace – and got to work removing glass easily. Of course, it required removing more screws. The first group removed contained 3 of the 20 total lens elements. At this point, looking through the rest of the lens reveals little. The frontmost glass is now the front of the IS group. When shaking the lens, the crack moves less, but it's not clear where it is. As Roger Cicala writes, 'In other words, we're now a little confused.' There are many elements in lenses, and they refract images, making it very difficult to locate a broken element.
At this point, Lensrentals moved to the back of the lens to try their luck there. After removing screws, the baffle, the bayonet, and the weather seal, it's time to get into the lens. There are many electronics, flexes, and cables in modern lenses. The lens must be able to communicate with the camera for autofocus and image stabilization to function.
Skipping ahead, the way Canon has designed and constructed this lens is cool. The front barrel extends to zoom, and it moves along size heavy-duty rollers with sliding cams. Each barrel roller is sized for smooth movement with no extra play, which is great for the user but requires extra care and precision for someone disassembling a lens with hopes of putting it back together. Cicala remarks that the components are well constructed, which is great news for anyone spending nearly $3,000 on a pro-level zoom lens.
At this point, the team has made it to the lens's inner barrel, where the remaining 17 lens elements reside. It's now possible to remove the IS unit, the suspected location of the crack. No crack. The IS unit is very robust, complete with heavy-duty interlocking plastic shells, multiple screws, and tension springs. If the crack isn't here, where is it?
After additional disassembly, which revealed more robust, well-constructed components and excellent engineering, the location of the crack was discovered…mostly. The cracked glass is in a group of elements all contained within a single assembly, which Cicala believes is likely its own repair part for Canon. If true, instead of taking apart this assembly, the entire piece and its glass would be replaced as a single component by Canon. Not content to know just the general group of glass containing a broken piece, they pressed onward.
The cracked element is the glass directly behind the aperture assembly. It's a thin singlet and is the forward focusing lens element. It may also be a close-focus compensating element, says Cicala, but he's not certain. He is certain that based on the lens's construction, there's no way that something collided with the glass inside the lens. So how did it break?
Well, it's difficult to say how the lenses cracked. What is clear is that the lens is robust and well-built. Lensrentals has been in discussion with Canon, and a team is investigating the issue. To read Cicala's theories about what's going on and what the Lensrentals team learned from disassembling the RF 100-500mm lens, head on over to Lensrentals.