Canon Patents Another Extreme Lens Design

Canon Patents Another Extreme Lens Design

Ever since the arrival of the RF mount, Canon has shown they are willing to push the boundaries of lens design to new levels. It seems they don't plan to slow down, with two new patents emerging, including one for a 300mm f/2.3 mirror lens with autofocus. 

Mirror lenses achieved some level of popularity several decades ago by providing an affordable way for photographers to enjoy supertelephoto focal lengths in a relatively small package. They work by using internal mirrors in the lens to allow for a longer focal length by bending the light rays back on themselves several times before they are sent to the sensor. Typically, these lenses were manual focus designs with slow apertures, normally something like 500mm f/8. The other drawback was that due to the mirror in the center of the lens, out-of-focus elements had a very distinct ring shape, which could be rather distracting. 

On the other hand, Canon's newly patented lenses buck the narrow aperture and manual focus trend, as both feature autofocus, with one being a 300mm f/2.3 design and the other a 400mm f/5 design. The 300mm option is particularly eye-catching, as even professional 300mm lenses normally max out at f/2.8. There is still a second mirror in the center of the optical path, meaning such a lens will still have ring-shaped bokeh, but it is quite interesting to see such designs emerge. 

Of course, just because a patent is filed doesn't mean the product will make it to the market, but it is nice to see Canon experimenting either way. 

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Alex Cooke is a Cleveland-based portrait, events, and landscape photographer. He holds an M.S. in Applied Mathematics and a doctorate in Music Composition. He is also an avid equestrian.

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With a fast mirror lens, the lack of an adjustable aperture becomes a problem. You might not want to always shoot at ƒ/2.3.

Many cameras are shipping with 1/16000th, and 1/32000th electronic shutters, so that's one solution.

Perhaps I'm not reading the schematic properly, but it appears to me that they are using rear-surface mirrors, which would seem to make controlling chromatic aberration more difficult. Most mirror lenses use front-surface mirrors.