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We have an interesting patent application (2026-004766) for you today. This is a patent for a Canon RF 300mm f/2.L IS USM. Now, as we have discussed previously, the 300mm f/2.8 is still noticeably missing, and while we lust for a Canon RF 300mm f2L IS USM, maybe the world needs a more affordable Canon RF 300mm f/2.8L IS USM instead. But this patent application expands on a general 300mm f/2.L IS USM and adds an Extender 1.4x.
This patent application shows off three different embodiments for a Canon RF 300mm f/2.8L. Nothing really uncommon about that, but what is uncommon is the last embodiment, which shows a complex two optical groups added into the optical path, presumably any of the 300mm f/2.8L embodiments would suffice since they all have that gap in the rear of the lens between the IS and element groups and the rear element.
Canon mentions in the patent application itself that this optical group, called (EXT), is the following;
A variable magnification subgroup EXT is inserted into the subsequent lens unit Br of the optical system of Embodiment 4 to increase the focal length by approximately 1.4 times. The variable magnification subgroup EXT is removable and is inserted into the subsequent lens unit Br of Embodiment 4 between the lens closest to the image and the cemented lens second from the image side. By inserting the variable magnification subgroup EXT, a longer focal length can be obtained without changing the overall lens length.
So yes, what we have here is a Canon RF 300mm f/2.8L IS USM Extender 1.4x, which, with a flip, allows you to change your 300mm f/2.8L IS USM to a 400mm f/4L IS USM in a matter of seconds without breaking your weather sealing.
Why Not a Zoom?
Now Canon does have zoom lenses that cover the 300mm, but they don't have any that cover 300 and above with speed, and any zoom that starts with 300mm at f/2.8 isn't going to be cheap, “lightweight” and relatively inexpensive. Consider that the 100-300mm f/2.8L is $10,600, compared to the release price of the Canon EF 300mm f/2.8L IS USM II at $ 7,000. We can only imagine that a zoom starting at 300mm f/2.8 would feature an astronomical price.
Oh, and it would maybe look something like this, though a little smaller.
So, maybe not the best idea, but it would look awesome, so I'm here for that, but it's not exactly practical. So the economics make more sense that if you want to do a fast zoom starting at 300mm, the answer is a teleextender.
So, back to this patent.
Canon RF 300mm f/2.8L IS USM EXTENDER 1.4x
Unlike most patents that I show with lenses, this one will obviously be a little different, because it has two embodiments. one with the optics inserted and one without.
This design is also amusingly teleextender-friendly, so if you wanted to add another teleextender behind the one included in the lens, you are welcome to do so. Now there is a change to lens length, but I'm presuming that's just a mathematical rounding error versus the movement of an element .01mm.
From the optical design, we can also see that, like modern Canon super telephotos, most of the weight from the elements is distributed to the center or rear of the lens for better balance, so this very much seems like a reasonable optical design for a 300mm super telephoto.
| Without Extender | With Extender | |
|---|---|---|
| Focal Length | 300.61 | 407.40 |
| F-Number | 2.91 | 4.12 |
| Half Angle Of View | 4.12 | 3.04 |
| Image Height | 21.64 | 21.64 |
| Lens Length | 296.71 | 296.72 |
| Back Focus Distance | 38.43 | 38.43 |
Mechanically, though, it will be a challenge to add a big extender housing on a relatively small lens, so this may be the only reason that this is not done. The Canon EF 200-400mm f/4L IS USM Extender 1.4x lens was 100mm longer than this lens design, so it had room for all the controls plus extender housing.
Closing Thoughts
This would be an interesting lens if Canon could shove everything into a 280mm metal package and bundle it up. The immediate ability to put a teleextender on the 300mm increases its usefulness significantly.
There have been a few super telephoto patents in the last little while, which may mean that Canon is getting serious about completing its lineup.
Another cool thing Canon could do with this patent application is actually create two Canon RF 300mm f2.8 models – one with the extender built in and the other without. Outside of the mechanical and optical assembly that gets dropped into the optical path, the two lenses would share most of the same features, except for cost and weight.
Just a Reminder!
With all patents and patent applications, I have to stress constantly – this is simply a look into Canon’s research; the only thing we can quantify accurately is that Canon is researching this. A patent application doesn’t mean they are going to release this in the next month, or even year, or even at all.
Why a 112mm front element? 300 / 2.8 = 107mm, but it’s really 300.6 / 2.91 = 103mm.