This diagram of the light path through Canon's newly-patented 400mm catadioptric lens shows how the mirrors help to 'fold' the light path and decrease the size of the lens.

A new Canon lens patent out of Japan has been raising eyebrows around the photo community this week. The patent describes a 400mm F5.6 lens, which wouldn't necessarily be newsworthy... except that it's a catadioptric lens (also known as a 'mirror' or 'reflex' lens).

Catadioptric lenses went 'out of style' so-to-speak many years ago, but for a time they offered economical and compact alternatives to standard long telephoto lenses. The optical design of these lenses use mirrors to both 'fold' the optical path and magnify the image coming in, allowing for a far more compact design.

Take, for example, this Vivitar Series 1 600mm F8 catadioptric lens:

Vivitar Series 1 600mm f/8 Solid Catadioptric Lens | Photo by pointnshoot (CC BY-SA 2.0)

Other advantages of a catadioptric lens design includes very nearly eliminating chromatic aberration and off-axis aberration; but, of course, this kind of lens design doesn't come without its drawbacks.

There are two main issues, both of them caused by the central mirrors causing an obstruction in the middle of the lens.

The first of these is that you can't use a standard diaphragm aperture system, a problem this Canon patent seems to 'solve' by using a variable density ‘electrochromic’ filter to 'stop down' the lens—although this will obviously not have any impact on depth of field.

The second problem is the donut-shaped bokeh produced by catadioptric lenses:

Donut Bokeh Example | Photo by Hustvedt (CC BY-SA 2.0)

In the end, it seems manufacturers (or consumers) decided that the drawbacks of catadioptric lenses were not worth the ultra-compact design. But as more and more photographers seek to lighten their kit, maybe Canon sees an opportunity to bring the 'mirror lens' back into public consciousness.