Canon RF 400mm f2.8 and 600mm f4 Rumored With Novel 1.5x Built-In Converter

Canon is reportedly developing new RF 400mm f2.8 L and RF 600mm f4 L lenses with a built-in teleconverter of a different design than anything currently on the market — and an unpublished tip points specifically to an RF 400mm f2.8 L IS 1.5x, a lens that would deliver roughly 600mm of reach in a single optic.

The report comes from Canon Rumors, and everything in it remains unconfirmed by Canon. No announcement has been made.

Why the lenses have slipped

According to the rumor report, both lenses have been delayed for some time because of parts shortages. Circuit boards and lens materials are the specific bottlenecks cited, and Canon may not be fully transparent about the cause. Canon's latest financial report does acknowledge circuit board shortages, which could affect new product availability even after products are announced.

Canon still had one more announcement planned for 2026: the RF 90mm f2.8 IS STM Macro and a new f2 zoom. More information is expected "very soon," including details on a forthcoming supertelephoto zoom.

The launch window for the new supertelephotos was set for before the end of the year. That now looks unlikely.

What makes the built-in converter different?

The report suggests Canon's built-in teleconverters will differ from what other manufacturers produce, though the nature of that difference is unclear. A patent suggested a drop-in teleconverter filter, but the publication notes that probably is not the answer — the patent was more concerned with space inside the lens barrel.

The most concrete detail is the unpublished tip: an RF 400mm f2.8 L IS 1.5x. The math matters here. Current built-in converters are 1.4x, costing about one stop of light. A 1.5x converter costs about 1.17 stops, turning an f4 optic into roughly f4.2. In practice, a photographer would carry one lens covering both 400mm f2.8 and approximately 600mm.

The publication cautions the specification may not be a certainty, since lens specs are routinely rounded — a 50mm f1.4 may in fact be a 48mm f1.47.

Who buys this lens?

The target market is straightforward: wildlife, bird and sports photographers who need long reach without giving up wide apertures. Owners of older EF-mount supertelephotos are the obvious upgrade candidates, since the current RF versions were originally designed for the EF mount. Any working photographer who wants two focal lengths in one body would also qualify.

Reader reaction to the rumor has been positive. One commenter called the 400mm f2.8 with a 1.5x converter "a smart option," arguing photographers will value the focal length gain over the aperture change. Another said the older lenses look dated and that wildlife shooters who prefer fast primes have felt overlooked.

Owners of the EF 400mm II and 600mm II said the existing RF versions alone were not tempting — but built-in converters would make the upgrade worthwhile. Another reader said simply that they have been waiting for this lens.

The light-loss dispute

Not everyone accepts the reported figures at face value. One commenter questioned whether the report actually knew about the 1.17-stop loss or was assuming it. Others responded that the number follows basic optics: the loss is twice the base-2 logarithm of the magnification, yielding roughly 0.97 stops at 1.4x, 1.17 stops at 1.5x and 2 stops at 2x.

A separate, unverified claim circulated in the discussion: YouTube creator Jan Wegener asserts Canon will not release any supertelephoto lens in 2026 or 2027. Readers hoped that claim is wrong.

What does this change for working shooters?

For now, nothing is firm. Canon has announced nothing, and the part shortages documented in its own financial reporting cast doubt on any near-term ship date. Photographers weighing a supertelephoto purchase in the coming months should treat these lenses as rumors, not roadmap items — and watch whether Canon's "very soon" announcements materialize before year-end.