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    Patent: Canon is actively developing fast mirror lenses

    By 74 Comments1 Min Read Canon Patents
    300mm F2.3 canon patent 728x649 - Patent: Canon is actively developing fast mirror lenses

    Mirror lenses have a bad wrap, as most of the implementations out there are for long and slow lenses, this patent discovered by asobinet shoes that Canon has been working on fast aperture mirror lenses.

    Whether these ever make it to market remains to be seen.

    Canon 300mm f/2.4 Mirror Lens

    • Focal length:298.89mm
    • F-number: 2.28
    • Half angle of view: 4.14°
    • Height: 21.64mm
    • Length Overall: 177.20mm
    • Back Focus: 65.95mm

    Canon 400mm f/5 Mirror Lens

    • Focal length: 408.04mm
    • F-number: 5.00
    • Half angle of view: 3.04°
    • Height: 21.64mm
    • Length Overall: 189.17mm
    • Back Focus: 47.69mm
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    74 comments

    1. This almost seems too niche to ever hit the market, but this would be an interesting piece of gear.
      Especially if they have decent autofocus, which is not exactly common in mirror lenses, these might make for a very portable solution.

      Poor contrast often associated with mirror design will probably be either a challenge to design around, or an accepted shortcoming of the lens.
      Are there any other limitations? These could not be fixed aperture, right? Otherwise 300mm f/2.4 would have ridiculously small DOF. The only other non-fixed aperture mirror lens I found was an ancient Ohnar 300/5.6 Mirror.
    6. These would be pretty crazy and could change the industry,
      Canon has some interesting patents when it comes to mirror lenses.
      It would be great for some, if not most, to come to fruition.
    7. For anyone interested, there is an excellent overview article over at OpticalLimits.

      Long reach and fast aperture make it sound like this would be a great and possibly lightweight choice for birding, but as soon as you have a close nervous background such as branches, stalks, or grass, the donut bokeh can really punishing.

      As fast as these lenses are, most situations could probably be worked around if you either go fully open or stopped way down.
    8. Currently, next to me, on my Canon 7D Mk2, a large 1000mm MTo 10/1000 is sitting on a top of a Canon EF 1.4x MkIII extender. The image quality with this extender is almost identical as without it, so as the noise level on the f14 aperture. This manual focus 1400mm mirror lens is a quite good toy for the long distance aircraft spotting from my 8th floor balcony. I just need an IBIS and I would be 100% happy :)
    9. As fast as these lenses are, most situations could probably be worked around if you either go fully open or stopped way down.
      I guess the 300 f/2.2 would need an adjustable aperture to be practical.
    10. I guess the 300 f/2.2 would need an adjustable aperture to be practical.
      Given that it has a huge mirror covering the center of the front element, an adjustable aperture won't be practical against donut bokeh.
    11. Given that it has a huge mirror covering the center of the front element, an adjustable aperture won't be practical against donut bokeh.
      A rear drop-in filter might do the trick but I highly doubt such a lens would get one.
      The back focus would allow for an EF version which could use a drop-in filter adapter,
    12. Currently, next to me, on my Canon 7D Mk2, a large 1000mm MTo 10/1000 is sitting on a top of a Canon EF 1.4x MkIII extender. The image quality with this extender is almost identical as without it, so as the noise level on the f14 aperture. This manual focus 1400mm mirror lens is a quite good toy for the long distance aircraft spotting from my 8th floor balcony. I just need an IBIS and I would be 100% happy :)
      An R7 or R10 with an RF 800 f/11 would make you 100% happy!
    13. Are there any other limitations? These could not be fixed aperture, right? Otherwise 300mm f/2.4 would have ridiculously small DOF. The only other non-fixed aperture mirror lens I found was an ancient Ohnar 300/5.6 Mirror.
      The biggest problem with mirrors is the bizarre bokeh: instead of a disk, it's a donut: a circle with a smaller central circle sharply cut out.

      You could lower transmissivity with a kind of typical aperture, but while you'd make the outer circle smaller, the inner empty circle would stay the same diameter and DOF would be poorer than you're used to for a given transmission. And bokeh would still be horrible unless you're a big fan of the letter O.
      Now, that's based on mirror lenses typically having a mirror reflect back to a second, small mirror, suspended in front of the first (which is what is blocking the center of the highlight circles). That second mirror probably doesn't HAVE to be blocking the first; maybe they could put it over to the side. But I'm not sure if such a lens could focus and I don't even see them doing this for telescopes where focus really IS always at infinity.
    15. The biggest problem with mirrors is the bizarre bokeh: instead of a disk, it's a donut: a circle with a smaller central circle sharply cut out.

      You could lower transmissivity with a kind of typical aperture, but while you'd make the outer circle smaller, the inner empty circle would stay the same diameter and DOF would be poorer than you're used to for a given transmission. And bokeh would still be horrible unless you're a big fan of the letter O.
      Now, that's based on mirror lenses typically having a mirror reflect back to a second, small mirror, suspended in front of the first (which is what is blocking the center of the highlight circles). That second mirror probably doesn't HAVE to be blocking the first; maybe they could put it over to the side. But I'm not sure if such a lens could focus and I don't even see them doing this for telescopes where focus really IS always at infinity.
      No doubt about bokeh quality, that is definitely one aspect putting these lenses in a niche market — at least based on the information we have at this point, which is far from the entire picture.
    18. An R7 or R10 with an RF 800 f/11 would make you 100% happy!
      Regarding to the RF 800 f/11, my EF100-400 Mk2 with the MkIII 2x extender has equal focal length at the long end, without any significant IQ difference. So I do not see any benefitial step up from the RF800 f/11 to my current EF lineup. And for me, from the 1400-2000mm is the regulary used focal length, not the 800mm. The 1400-2000mm is the right tool for the contrail spotting, but the noise level is quite high because the lack of IS. The 100-400 and the extenders are for the airshows, where I have to shoot on both the close taxiing aircraft throught out the whole demo box airspace, with the smae equipment. That's why I do not see any relevant purpose for the fix 800mm f/11 lens.
    19. They keep releasing F4 and F5 lenses while their competitors release F1.2 and F 1.8. Will just continue using my EF lenses with the adspter booster. They work.
      Interesting. Care to enlighten us as to who makes an f/1.2 or f/1.8 300mm lens?

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