A 300mm f/2.8 and 400mm f/4 in one prime lens? Canon might follow Nikon's built-in teleconverter super-telephoto trend

Looks like Canon is preparing one of its most versatile professional telephoto lenses yet. A newly published patent suggests Canon is developing an RF 300mm f/2.8 L IS USM with a built-in 1.4x teleconverter, allowing photographers to switch instantly between 300mm f/2.8 and around 400mm f/4 – giving them two lenses in one body.

If this sounds familiar, it's because Nikon already did it first, with its Z-mount super-telephoto primes: the Z 400mm f/2.8 TC VR S (extending to 560mm f/4) and Z 600mm f/4 TC VR S (extending to 840mm f/5.6). Canon's approach could be even more compact – and aimed at a broader range of sports and wildlife photographers.

That said, Canon isn't entirely new to built-in teleconverters. It previously released the EF 200-400mm f/4L IS USM Extender 1.4x, a professional zoom lens with a built-in teleconverter – but unlike the new patent, that lens wasn't a prime, so this would mark a significant step forward for Canon's RF prime lineup.

Canon's 300mm f/2.6 built-in extender patent

The patent, publication number P2026004766, published by J-PlatPat (Japan Platform for Patent Information) on January 15, 2026, describes an optical system with a removable internal extender group (EXT) positioned near the rear of the lens. When engaged, this group increases the focal length by approx. 1.4x, without changing the lens's overall length.

In real-world terms, this means you get 300mm f/2.8 for speed and low light, and 400mm f/4 at the flick of a switch – without removing the lens, exposing the sensor, or breaking weather sealing.

Unlike external teleconverters, a built-in extender allows instant switching between focal lengths, maintaining balance, AF (autofocus) performance, and shooting momentum.

Canon's diagrams also show a modern super-telephoto layout, with the lens's weight centered toward the rear for better balance when handheld or mounted on a monopod.

Where it fits – and how it compares

Canon currently lacks an RF-mount 300mm f/2.8, making a telephoto prime with built-in teleconverter a fantastic choice for professionals. Meanwhile, Nikon's built-in TC lenses start at 400mm f/2.8, but tend to be large, heavy, and very expensive (the 600mm f/4 retails for over US$16,000).

By starting at 300mm, Canon could offer better portability, stronger low-light performance, and greater versatility. This lens would sit neatly alongside Canon's RF 100-300mm f/2.8L in both size and focal range.

As always, a patent doesn't guarantee a product. But Nikon's success with built-in teleconverter primes shows that this concept is both viable and highly valued by professionals.

If Canon brings this lens to market, it could become one of the most practical and desirable telephoto primes in the RF system, offering flexibility for sports, wildlife, and fast-action photography.

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