Canon Japan Files for a 400mm Mirror Lens Patent

Canon Japan is brewing up a mirror lens project for APS-C DSLR cameras.

According to a tip from the CanonWatch blog, Canon Japan has very recently filed a patent application for a 400mm f/5.6 mirror lens. But for the uninitiated, it’s worth knowing first what exactly this optical system is to get a clue on why could Canon Japan be gearing up to make such lens.

According to the Wikipedia information cited by CanonWatch regarding the mirror lens, also known as reflex lens or catadioptric lens, they have “some form of cassegrain design which greatly reduces the physical length of the optical assembly, partly by folding the optical path, but mostly through the telephoto effect of the convex secondary mirror which multiplies the focal length many times (up to 4 to 5 times).” This means they make for shorter and more compact telephoto lenses with focal lengths of 250mm and above, sometimes even beyond 1000mm. “Moreover, chromatic aberration, a major problem with long refractive lenses, and off-axis aberration, a major problem with reflective telescopes, is almost completely eliminated by the catadioptric system, making the image they produce suitable to fill the large focal plane of a camera.”

The mirror lens, however, comes with some drawbacks. Because it has a central obstruction to accommodate a secondary mirror, it cannot use an adjustable diaphragm for controlling light transmission. With this, the lens’ F-stop value is “fixed to the overall designed focal ratio of the optical system (the diameter of the primary mirror divided into the focal length).” A Neutral Density filter placed on the front or rear of the lens is the usual way to adjust the exposure. Lastly, this lens also creates a noticeable ring-like or doughnut-shaped bokeh in the contrasty, out-of-focus areas of the picture, which is often found distracting.

As for the Canon Japan mirror lens, it’s reported to come with an electrochromic ND filter for APS-C DSLR cameras. CanonWatch provided the following details for Patent JP-A-2017-219642:

Focal length: 400 mm

F No.: 5.6

2ω: 1.96

Image height: 13.66 mm

Back focus: 33.07 mm

Lens length: 247.73 mm

*OC in the diagram is an electrochromic ND filter

Could it be that Canon Japan is gearing to market mirror lenses as a cheaper, quirky alternative to telephoto lenses for APS-C DSLRs? Let’s hope to find out in the coming months.