Use of mirror lenses with MFT

AllanG

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With the possible development of mirror lenses for Canon (https://petapixel.com/2021/05/15/canon-might-be-planning-super-cheap-super-telephoto-lenses/) I've always find myself interested in this type of lens for MFT bodies. I've heard all the pros and cons of such lenses but since I do a lot of astronomy using mirror telescopes I think a lot of this is based on cheap and badly made lenses.
I had the opportunity to buy a Samyang 300mm f6.3 mirror lens a while back. Knowing Samyang do make good lenses I tried it at the Brisbane Tennis and I was quite happy with the result.
I would be interested to see other people's experience.

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ac12

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I used a Nikon 500/8 reflex/mirror to shoot tennis on my APS-C camera. I NEEDED the reach to shoot out across five courts, to the sixth court. The 70-300 was not long enough.
The Nikon has a wonderfully smooth and light focus ring.
BUT, follow focusing a fast moving subject (tennis) is REALLY HARD, primarily because the DoF is shallow, so turn the focus ring too much or not enough and subject is OOF. :confused:
For sports, I would REALLY like an autofocus mirror lens.

As for the Canon mirror lenses in the link, it shows IS, which is cool, but we have IBIS on the Olympus, so we can already stabilize the old non IS lenses. Though for these LONG lenses Optical IS may work better than IBIS.
What I did not see, but expect, is that the lens has autofocus capability. THAT would be an advancement. :)

Personal opinion without having tested it.
I think the low contract complaint of the old reflex lenses is due to the lack of a decent lens hood. The "hood" on my Nikon 500/8 is only about an INCH long. How is that shallow hood going to shade the front element? It can't. So you loose contrast, due to off angle light entering the lens.
I think with a longer hood, you will gain some contrast.
 
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Interesting that the photo in the linked article was a Maksutov design, but the diagrams were all Schmidt designs.

In "Maks," as they are affectionately called by astronomers, a central spot in the front element is silvered, whereas in Schmidt Cassagrain Telescopes (SCTs), there is a separate mirror assembly in the centre, which typically must be adjusted for alignment (collimated).

My favourite mirror lens is the Olympus OM Zuiko 500mm ƒ/8 Reflex, which is reputedly one of the best mirrors ever made — as well as being tiny and lightweight. It is both sharp and contrasty, neither of which mirrors are known for.

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Another lovely mirror is the Sigma XQ 500mm ƒ/4, but it is a beast, at nearly 3kg. It's decently sharp in the centre, but not as contrasty as the Olympus.
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I have about 5-6 other mirrors, but none of them have the image quality of these two.
 

Baenwort

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Why do none of them have AF? Is it technical to do with phase detection AF not supporting the design?

Couldn't contrast detection still work for them?

Or is it to do with the mass to he moved meaning AF would require impractical motors?
 
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Why do none of them have AF?
I'm guessing it has more to do with momentum than anything else.

Putting AF in a mirror lens would require a different sort of mechanism than typical refracting lenses have.

The "mass being moved" does not necessarily have much to do with it. There are mirrors that move the front element (like Olympus), ones that move the rear element (like the Sigma 500/4), and ones that move the primary mirror.

I have a Bausch and Lomb and a Celestron that have a knob sticking out the back that you turn that seems to move the primary mirror.

I don't see why any of these mechanisms should have any trouble with either PDAF or CDAF.

One consideration may be that a lot of refracting AF lenses seem to use internal focusing these days, and it seems there is plenty of room inside the lens to stick a tiny motor. A problem with mirrors is that their space is "used up" because the primary mirror in back is as wide as the front element. They could extend the cylinder behind the primary mirror to house the motor and electronics, but it's just not the way things are done on "normal" lenses, I guess.
 

ivanbae07

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Why do none of them have AF? Is it technical to do with phase detection AF not supporting the design?

Couldn't contrast detection still work for them?

Or is it to do with the mass to he moved meaning AF would require impractical motors?
really? none of them? i've found one, never tried it, but many people rave about it: konica minolta / sony 500mm f8 af reflex.
fun facts: this is the first time i hear about 'olympus om zuiko 500mm ƒ8 reflex' being 'reputedly one of the best mirrors ever made'... and oh boy, it is hard to search the iq's comparison between mirror / reflex lens...
 
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PakkyT

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The "hood" on my Nikon 500/8 is only about an INCH long. How is that shallow hood going to shade the front element? It can't. So you loose contrast, due to off angle light entering the lens.

You could certainly test that out easily enough. Just roll up some cardboard or a black piece of construction paper and "snoot" the lens and see how it does.


Why do none of them have AF? Is it technical to do with phase detection AF not supporting the design?

My own guess is that mirror lenses were mostly meant to give a lot of reach very cheaply and compactly. And most were developed back when AF wasn't really a thing. Now they are still meant to be cheap and light and are often made by Chinese and other third party vendors looking to churn out a bunch of them fast and easy across a bunch of mounts for not much more than a hundred bucks. I don't think there would be much of a market for an AF faster aperture mirror lens that was $300+.
 

Brownie

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really? none of them? i've found one, never tried it, but many people rave about it: konica minolta / sony 500mm f8 af reflex.
fun facts: this is the first time i hear about 'olympus om zuiko 500mm ƒ8 reflex' being 'reputedly one of the best mirrors ever made'... and oh boy, it is hard to search the iq's comparison between mirror / reflex lens...
I've been thinking about a Minolta 500/8 for some time. I could use it on my Maxxum/Sony/Panny cameras. Interesting thing is that the A-Mounts have the focus motor in the body, thus this lens is autofocus when mounted as such. It's rated pretty decent at Dyxum. Scale is 'out of 5'

overall:4.49
sharpness:4.35
color:4.37
build:4.62
distortion:4.80
flare control:4.32
You could certainly test that out easily enough. Just roll up some cardboard or a black piece of construction paper and "snoot" the lens and see how it does.
Could probably also get an idea by using your hand as a shade to 'extend' the hood on the light side.
 

ac12

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Why do none of them have AF? Is it technical to do with phase detection AF not supporting the design?

Couldn't contrast detection still work for them?

Or is it to do with the mass to he moved meaning AF would require impractical motors?

Mirror lenses started back when there was no AF.
Then they went out of style.
Now what is sold are "cheap" lenses.

As was mentioned, Minolta did make an AF mirror lens. So it CAN be done.

On my Nikon Mirror, the focus ring is essentially the entire barrel of the lens, forward of the tripod collar. If you AF that, you would not have a place to hold the lens in your hand. So, the lens has to be designed to be able to AF. In the case of the Nikon mirror, a smaller focus ring, with a larger exposed barrel for you to hold.

As @Bytesmiths said, there are several basic designs that can be used.

As for the motor. It can be done. There are many ways to skin the cat.
  • I think the easiest is internally moving the secondary mirror, or the rear elements. Lowest mass to move.
  • I think the hardest one would be externally moving the camera mount, because of the mass of the attached camera. I would NOT use this option.
  • And several options in between.
  • Worst case is you do like the very early AF lenses. Put the motor in a box below the lens.
I would think that both PD and CD would work.
 

Enoch

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I honestly wonder if some of the downsides of mirror lenses could be mitigated digitally - even the donut bokeh. It would seem possible to get a program to identify donuts and "fill in" the holes, no? A lens like the recently rumored Canon RF 800mm f/5 IS would be very interesting to me, especially if some of the secondary concerns could be digitally, automatically fixed.
 

exakta

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I've heard all the pros and cons of such lenses but since I do a lot of astronomy using mirror telescopes I think a lot of this is based on cheap and badly made lenses.
The proper alignment of the mirors is critical for sharpness. This was the weakness of many of the Japanese third party mirrors that showed up in the mid 1960s.
 

fortwodriver

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Why do none of them have AF? Is it technical to do with phase detection AF not supporting the design?

Couldn't contrast detection still work for them?

Or is it to do with the mass to he moved meaning AF would require impractical motors?

Minolta made an AF mirror-tele in the late 80s. It stayed in their catalog right up until Sony took over, which would have been around 2006.
 

PeteS

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FWIW, I have a cheap mirror lens (Rokinon 300mm f6.3) that I have used on my Olympus Pen E-PL6. I was happy with many of the bird pictures I took with it, but I never found I could hand hold it or focus it without the digital 10x zoomed in focus aid. That made it pretty awkward to use. Most of the time it was used set up on a tripod pre-focused at one of my bird feeders. A quick fine tuning of the focus was still required.

I have not tried it on my OM-D E-M1 Mark III, but don't expect it would be much better. I have an M.Zuiko ED 75-300mm f4.8-6.7 II on order so I expect the Rokinon won't see much use in the future.
 
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I put my Nikon Reflex-Nikkor 500mm 1:8 lens on my G7 today.
Shot these pics in my backyard.
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Crow
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Mocking bird
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Machi

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It's interesting that there isn't really sharp photographic mirror lens.
Probably sharpest images which I ever saw from classic photo mirror lens were taken by old Tamron 500mm but even those are not comparable to the classic refractive lens.
Only really sharp images from mirror designs are from some astro telescopes and maybe from special (and very heavy) designs like Zeiss Mirotar 1000mm f/4.5.
 
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I put my Nikon Reflex-Nikkor 500mm 1:8 lens on my G7 today.
Shot these pics in my backyard.
Nice use of the "special" bokeh, there! I find it can either lend interest, or take it away.
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Cube Satellites, which are about the size of a roll of paper towels and are powered by four AAA batteries use fixed focus 800mm mirror lenses to capture 10m resolution images of the earth. A company called Planet has 120 of them in goespatial orbit and together they take a snapshot of our entire planet every day. The satellites are in an insecure low orbit and are designed to gradually fall out after 3+ years and burn up in the atmosphere to be replaced by newer birds with better capabilities. There are many companies using cube sats and there are hundreds of mirror lenses photographing the world and all of us in it every day.
 

ac12

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For some reason, I seem to be able to ignore the donuts.
I guess whenever I've done it, they were not so obtrusive, like Jan's pic.
Unlike John's last pic of the mocking bird with the large donuts.
 
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For some reason, I seem to be able to ignore the donuts.
If you can't avoid it, flaunt it!

Personally, I think there is a zone where it looks nasty, like John's otherwise excellent squirrel picture (pic #2). It looks like a snake orgy, and detracts considerably from the subject.

Without a diaphragm, there isn't much you can do to fix the situation, other than move. So when using a mirror, I look for situations where I can have the background far away from the subject.
 

ac12

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BTW, for those that have trouble understanding my posts about the length of the lens hood on the Nikon 500/8.
Look at the picture of John's set up.
That is the "hood" labeled HN-27. It is less than an inch long. On a 500mm lens! :confused:
I found a formula for hoods, which I can't find now, which says the 1 inch hood is actually correct. 😮

However, unlike my wife's astronomical mirror lens, there is a glass element at the front of the lens, to catch the light.
And the inside of the lens is not highly light absorbing. That ULTRA black paint would work great here.
 

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