Photographs kindly provided by Lensrentals

Roger Cicala, founder of Lensrentals, has torn apart the 400mm F2.8L IS III to see what Canon has done to remove 2.2lbs of weight from the third-generation super telephoto lens.

It's only been a few weeks since Cicala shared Lensrentals 50mm F1.2L RF lens teardown, but as he hinted at in that teardown, it's time for Canon's much larger larger lens to get the signature treatment.

'We tend to not publish tear-downs of super-telephoto lenses. They’re big and heavy, always well made and well engineered, like you would expect in something that costs as much as a used car,' says Cicala in the teardown post. 'But when the new 400mm f/2.8 lenses were released we decided it was finally time to publish a teardown of one. Largely, because they are amazingly light (you see what I did there? largely light?) and we were really interested in how they did that.'

Cicala also says he's interested in doing a direct comparison between Canon's latest 400mm F2.8 lens and its Sony counterpart. So it's safe to say a Sony 400mm F2.8 teardown is just around the corner as well.

The small spring-loaded mechanism used to help lock the lens in place while rotating inside its collar.

The first order of business for Cicala and team was to remove the lens collar. While it might seem like a small detail for such a massive — and expensive — lens, the collar itself is quite complicated in its own right and often one of the first things that can break, due to an internal clicking mechanism that helps lock the lens into place at 90-degree intervals while rotating it on a tripod. As expected, the tripod collar assembly was solid and fairly straightforward — good news for photographers who prefer a more DIY approach to fixing smaller issues like a stuck collar tripod.

From there, it was onto the drop-in filter, which was probably the easiest part of the lens to remove. Cicala says 'we really, really, really like [the drop-in filter] Because instead of being a proprietary $200 filter, it’s a holder that you can attach any 52mm filter to.'

With the easy stuff out of the way, now it was onto the lens itself. Without giving away all of Lensrentals findings, it's worth pointing out a few details discovered during the teardown.

One of the adjustable screws can be seen in the internal structure of the lens (surrounded by red)

Starting with build quality, Cicala says that despite being light, the 'inner, weight-bearing barrel is very sturdy magnesium alloy.' It was at this point in the teardown Cicala noticed two adjustable elements integrated into the lens. The first is a set of screws found just in front of the rear element group, while the other was a set of adjustable collars where the two halves of the lens are joined together.

Regarding the image above, Cicala says 'a couple of interesting things are in this image [...] First, you see the two barrel halves are joined without shimming and held tightly together with 12 large screws – it’s a very strong joint with interlocking pieces. Also, as we saw with the RF 50mm f1.2 teardown, there are springs going down to the focusing ring, we assumed to put tension on a ball bearing ring for a smooth focusing feel in this electronically focused lens. We assumed wrong.'

A little deeper into the build, Cicala and team come across the image stabilization unit. Cicala notes the unit is roughly 2cm thick and full of various electromagnets, springs, sensor inputs and optical elements.

A look inside the intricate image stabilization unit.

In his 50mm F1.2L RF teardown, Cicala said the USM motor used by Canon in its fast prime is the exact same one used in the 400mm F2.8L IS III. Sure enough, that's been confirmed again with this teardown, complete with the tension spring that continues to leave Cicala and team bamboozled.

A profile view of the ultrasonic motor in all its glory.

'This is really interesting information, that the same motor and electronic focusing system is used in both lenses,' says Cicala in the blog post. 'Obviously, there is a business advantage in using the same subsystem in several lenses. But putting the same electronic focusing system in this EF lens makes me think that going forward Canon lenses may have a lot of internal similarity in either RF or EF mount. Converting the Canon 400mm f/2.8 to an RF wouldn’t be quite as simple as a different rear element and an RF bayonet mount, of course. The additional electronics for the RF Control Ring would have to be stuffed in here and probably some optical tweaks made, but the core structure could be very similar.'

Further disassembly of the USM section also revealed it's the section of the lens that contains the aperture assembly. From there, it was onto the front optical elements of the lens, some of which were held in place by another adjustable element, which included screws to account for centering and tilting.

The aperture assembly shows all nine aperture blades in action.

After a little more work, the focus ring comes off and bears the various springs and more adjustable components. Cicala and team come up with a theory or two as to why the tension spring is there and what it does, but a definitive purpose is still unknown. Cicala says 'one day we’ll spend 4 or 5 hours playing with them and figure it out. But don’t tell Canon that; they get aggravated when we do void-the-warranty stuff.'

In the conclusion section titled "So What Did We Learn Today,' Cicala shares a few of his final thoughts regarding the teardown. The first thing Cicala mentions is the movement of the optical elements inside the glass. Canon said when it announced the 400mm F2.8L IS III that it had moved elements more towards the mount of the lens to create a better center of gravity and that proved to be the case. Also, Cicala says that 'Despite being far lighter, it’s still very solidly built with a very strong frame, tons of big screws holding everything together, and no weak joints that we can see [...] It’s what we expected; Canon’s been building super telephotos for a long time.'

The USM is also elaborated on. Cicala says it's a very likely possibility Canon will continue to use this motor for all electrically focused lenses, or at least in lenses with a hefty amount of glass in the focusing element.

Overall, it appears as though the 400mm F2.8L IS III is roughly what Cicala was expecting from Canon. A well-built lens with plenty of adjustments in a tightly packaged frame. As interesting of a teardown as this was though, Cicala says the more interesting teardown will be the Sony 400mm F2.8 G lens. 'Hopefully, it is awesome and for once the Sonyfans* won’t line up 6 deep to explain how what I find is wrong again.'

To see the full teardown in all its glory (and to discover the footnotes on what Sonyfans* means), head over to the Lensrentals blog and take in every bit of detail.