# Patent: Canon 400mm f/5.6 Mirror (Catadioptric) Lens



## Canon Rumors Guy (Dec 14, 2017)

```
A patent application showing a new mirror lens design from Canon is confusing to say the least. I figured the days of the catadioptric lens were well behind us, but here we are.</p>
<p><span class="notranslate"><strong>Specification from the patent:</strong></span></p>
<ul>
<li><span class="notranslate">Focal length: 400mm</span></li>
<li><span class="notranslate">Aperture: f/5.6</span></li>
<li><span class="notranslate">2 ω: 1.96</span></li>
<li><span class="notranslate">Image height: 13.66mm</span></li>
<li><span class="notranslate">Back focus: 33.07mm</span></li>
<li><span class="notranslate">Lens length: 247.73mm</span></li>
</ul>
<p>The <a href="http://www.northlight-images.co.uk/canon-ef-lenses-rumours-and-news/">image below from Northlight</a> shows the light path through the lens. The light path is in red, and the mirrors are outlined in blue and labelled.</p>


<blockquote><p>The blue bits are mirrors. The light comes in through L1, bounces off M1/M2 and through a hole in M1 back to the camera.</p></blockquote>
<p><a href="http://www.canonrumors.com/forum/index.php?topic=34076.msg698929#msg698929">neuroanatomist on the forum</a> thinks this could be an EF-M mount lens. You can view the patent at the Japanese Patent Office <a href="https://www.j-platpat.inpit.go.jp/web/PU/JPA_H29219642/E2F2B7A25E23CBF08049D831B95527EF">here</a>.</p>
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## neuroanatomist (Dec 14, 2017)

An EF-M lens, it seems (from the image height and back focus distance).


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## Antono Refa (Dec 14, 2017)

neuroanatomist said:


> An EF-M lens, it seems (from the image height and back focus distance).



First thing I noticed was it's for APS-C sensor with shorter back focus.

I'm not sure it's small enough to be produced for EOS-M bodies, as it's about as long (or short) as the 400mm DO lenses. It would probably be lighter, though.


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## Mt Spokane Photography (Dec 14, 2017)

What the patent actually claims is that it can vary the effective aperture by dimming the light as it basses thru the lens elements. (LCD?). As many know, it is not possible to put a variable aperture in a mirror lens, so ND elements must be used if f/8 is not sufficient. They have rear drop in lens holders to do that.

"The imaging system which was caused conventionally, especially used the reflection system as the method of overall-length shortening of a telephoto lens is proposed. However, when a physical diaphragm is placed as problem peculiar to a reflection system, at the time of a small diaphragm, the light volume of an image periphery is reduced substantially, or may be intercepted altogether. In recent years, needs to make animation correspondence also in a digital camera are growing, and it has become an important element to have a mechanism in which light volume is adjusted. On the other hand, it is not a physical diaphragm and there is a means to change voltage and to adjust light volume. 
[0003] 
The example with above means to change voltage to a catadioptric system and to adjust light volume to it is disclosed (Patent document 1). It has a bending reflection member and the example which carried out the luminous energy regulating means owner is disclosed"

The invention could be applied to a mirror lens for any lens mount.

I was hoping they had solved the bokeh issues with mirror lenses, but thats not the case.


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## Mt Spokane Photography (Dec 14, 2017)

Mt Spokane Photography said:


> What the patent actually claims is that it can vary the effective aperture by dimming the light as it basses thru the lens elements. (LCD?). As many know, it is not possible to put a variable aperture in a mirror lens, so ND elements must be used if f/8 is not sufficient. They have rear drop in lens holders to do that.
> 
> "The imaging system which was caused conventionally, especially used the reflection system as the method of overall-length shortening of a telephoto lens is proposed. However, when a physical diaphragm is placed as problem peculiar to a reflection system, at the time of a small diaphragm, the light volume of an image periphery is reduced substantially, or may be intercepted altogether. In recent years, needs to make animation correspondence also in a digital camera are growing, and it has become an important element to have a mechanism in which light volume is adjusted. On the other hand, it is not a physical diaphragm and there is a means to change voltage and to adjust light volume.
> [0003]
> ...



Don't look for this to show up soon in your store, mirror lenses are not popular enough to warrant the Canon high prices.


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## andrei1989 (Dec 14, 2017)

Antono Refa said:


> neuroanatomist said:
> 
> 
> > An EF-M lens, it seems (from the image height and back focus distance).
> ...



eh? how do you figure it's the same length as a 400 DO?

samyang make a 300mm f/6.3 mirror lens for mirrorless cameras (cool stuff if you miss your dslr ;D ) which is 74mm in length. the DO equivalent for that focal length would be...nikon's 300mm f4 PF which is 147mm long so by that (flawed?) logic this canon one should be around 100mm in length


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## KeithBreazeal (Dec 14, 2017)

But it will have awesome donut bokeh!


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## jolyonralph (Dec 14, 2017)

Mt Spokane Photography said:


> I was hoping they had solved the bokeh issues with mirror lenses, but thats not the case.



Issues? What's wrong with donuts?


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## neuroanatomist (Dec 14, 2017)

andrei1989 said:


> Antono Refa said:
> 
> 
> > neuroanatomist said:
> ...



eh? read the patent...



Canon Rumors said:


> Lens length: 247.73mm



He figures that because the 400/4 DO II lens measures *233mm* in physical length (which corresponds to an optical formula length of 276mm), and patent we're discussing specifically states the optical formula length for the patent is 250mm, which if it's an EF-M lens corresponds to a physical length of *232mm*.


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## KeithBreazeal (Dec 14, 2017)

Variable LCD ND filter. This is an adapter demonstrated but once perfected, could be part of an internal design by lens manufacturers. 

https://vimeo.com/139312965


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## Don Haines (Dec 14, 2017)

KeithBreazeal said:


> But it will have awesome donut bokeh!



mmmmmmmmmm...... donuts.......

This could be an interesting lens, particularly for food photography!


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## Mistral75 (Dec 14, 2017)

neuroanatomist said:


> An EF-M lens, it seems (from the image height and back focus distance).



There are five examples in the patent request. All of them are APS-C lenses.


two 400mm f/5.6, one with a back focus distance of 33.07mm, the other one with a back focus distance of 45.65mm
two 300mm f/4.2, one with a back focus distance of 36.18mm, the other one with a back focus distance of 35.11mm
an odd 400-520 mm f/6,7-9.9 zoom lens with a back focus distance of 6.61mm; this one is definitely not an SLR lens.


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## andrei1989 (Dec 14, 2017)

my bad. i somehow missed that line
carry on


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## 9VIII (Dec 14, 2017)

My Samyang 800mm f8 lens is incredibly compact for what it is.
That said, image quality is slightly worse than the equivalent frame cropped out of the 400f5.6L so the two lenses are roughly equivalent, but the Mirror lens is WAY cheaper.

The biggest problems with the Samyang are a slight loss of colour saturation and the lack of autofocus.

If Canon can make a decent 400mm mirror lens (putting the mirror back in Mirrorless) I'm sure it would be an excellent product.


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## magarity (Dec 15, 2017)

Which part justified a patent? It's not like mirror telescopes are something new. The LCD for ND?


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## neuroanatomist (Dec 15, 2017)

magarity said:


> Which part justified a patent? It's not like mirror telescopes are something new. The LCD for ND?



The optical formula. Canon had a 35/1.4L, yet they patented the MkII design...


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## neonlight (Dec 16, 2017)

I thought people had given up on donuts. What would be impressive is an off-axis mirror that does not have donut bokehs. Might look a little strange on a camera with the primary axis offset from the sensor. And something bigger than f/5.6.


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## AvTvM (Dec 18, 2017)

I doubt this will come to life. Donut bokeh combined with huge form factor = DOA. 

No idea why the patent thing is so long ... 120 to 150mm physical length of lens should be more than sufficient for a crop (APS-C) catadioptric 400/5.6 ... definitely one of the weirder Canon patents I've seen in some time. 

Canon would be better off dedicating a bit of R&D effort towards a nice EF-M 85/2.4 IS STM lens instead of wasting time and money on useless exotics.


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## GMCPhotographics (Dec 19, 2017)

It's nice to see that Canon are still considering re-inventing old ideas and designs.
What if Canon could build a small, cheap and light 400mm f5.6 lens with descent sharpness and contrast (usually the problem with catadioptric lens design). I think it could be a strong seller for some photographic niches. 

Just because most of these types of lenses are made by dodgy Chinese clones of ancient designs and poor manufacturing tolerances shouldn't prejudice our opinion of Canon entering the Frey and making a well designed and thought out lens. Auto aperture, auto focus...IS are all options on this lens and it could easily be shorter than a 135mm (although a lot fatter) and maybe lighter. It could easily become a small, cheap and light walkabout '400.


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## AvTvM (Dec 19, 2017)

sorry, but catadioptric lenses for (general photography) are stillborn technology, no matter how "hi-quality" optical formula and implementation are. Law of optics are still in effect, even for Canon.


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## Michael Clark (Dec 20, 2017)

Mt Spokane Photography said:


> What the patent actually claims is that it can vary the effective aperture by dimming the light as it basses thru the lens elements. (LCD?). As many know, it is not possible to put a variable aperture in a mirror lens, so ND elements must be used if f/8 is not sufficient. They have rear drop in lens holders to do that.
> 
> "The imaging system which was caused conventionally, especially used the reflection system as the method of overall-length shortening of a telephoto lens is proposed. However, when a physical diaphragm is placed as problem peculiar to a reflection system, at the time of a small diaphragm, the light volume of an image periphery is reduced substantially, or may be intercepted altogether. In recent years, needs to make animation correspondence also in a digital camera are growing, and it has become an important element to have a mechanism in which light volume is adjusted. On the other hand, it is not a physical diaphragm and there is a means to change voltage and to adjust light volume.
> [0003]
> ...



You've never heard of or seen the Ohnar 300/5.6 Mirror (also sold under other names such as Hanimex, Makinon, Panagor, etc.), have you?


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## Michael Clark (Dec 20, 2017)

Don Haines said:


> KeithBreazeal said:
> 
> 
> > But it will have awesome donut bokeh!
> ...




This might gain them a niche in the police surveillance market as well...


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