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
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.
Canon has some interesting patents when it comes to mirror lenses.
It would be great for some, if not most, to come to fruition.
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.
The back focus would allow for an EF version which could use a drop-in filter adapter,
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.