Another budget mirror lens, 900/8 this time

[Alan WF]

Let me give you an example from my field. When my engineers can place optics in a cylindrical barrel, they can achieve 10-15 micron alignment between the elements. When they cannot, they typically achieve 50-100 micron alignment. And our optomechanics are hand-built, we do metrology and iterate the pieces bases on that metrology, and they are then coddled and (hopefully!) not subjected to the rough treatment served to photographic lenses.

10 microns is pretty impressive, but 100 microns is just 0.1mm -- a tolerance I can hold with 3D-printed parts on a well-tuned printer.

It’s 100 microns over 1 meter in aluminum with multiple parts fastened together with screws.

Regards,

Alan

 

[Alan WF]

Chiefspeigler | Optical Ed's (opticaleds.com)

8 inch F/7 CHIEF group build - ATM, Optics and DIY Forum - Cloudy Nights

Doesn’t this use a parabolic mirror? I also suspect the adequately corrected field is too small to be useful in a photographic lens.

Now all the factors I described show historically why TCT designs like the Schiefspeigler have been limited to f/25 or f/27.

Spherical aberration and field curvature, probably.

Regards,

Alan

 

[3dpan]

So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?

Do mirror lenses focus by changing the distance between the primary and secondary mirrors?

If so, that would explain why they are so difficult to focus. In a Cassegrain telescope, the displacement of the focal plane is a large multiple of the change in the distance between the primary and secondary. The precise factor depends on the optical prescription, but 10-20 is common in my experience. So, if you move the secondary or the primary by 10 microns, the focal plane can shift by 100-200 microns.

If so, then a helicoid focuser, which would give a 1:1 relation between lens displacement and focal plane displacement, might be a great help.

Regards,

Alan

It seems common for mirror lenses to alter the spacing between primary and secondary mirrors. I'm quite happy to be wrong though.

An exception I can think of is the Zeiss Mirotar 500mm f4.5, see pic below.

Zeiss Mirotar 500mm f4.5

There may be others in the big mirror range, eg Sigma 500/4, Nikon 500/5, Yashinon 500/5, Singer 300/1.8,

Ultra-rare Singer 300/1.8
Note the focusing helicoids,
https://www.ebay.com/itm/295031157673?hash=item44b13a33a9:g:~~AAAOSwOiNiovdH

Singer 300mm f1.8

Or, just as rare, Yashinon 500/5

Yashinon 500mm f5

 

[ProfHankD]

Let me give you an example from my field. When my engineers can place optics in a cylindrical barrel, they can achieve 10-15 micron alignment between the elements. When they cannot, they typically achieve 50-100 micron alignment. And our optomechanics are hand-built, we do metrology and iterate the pieces bases on that metrology, and they are then coddled and (hopefully!) not subjected to the rough treatment served to photographic lenses.

10 microns is pretty impressive, but 100 microns is just 0.1mm -- a tolerance I can hold with 3D-printed parts on a well-tuned printer.

It’s 100 microns over 1 meter in aluminum with multiple parts fastened together with screws.

I assumed as much -- telescopes tend to have large optics. 100 microns for 3D printed parts with well-tuned commodity printers is only if it fits on the printer bed, which for my largest printer is just a 370mm circle -- so about 0.37 meters. I suspect that number might surprise you more than your number surprises me. It also might surprise you to know that I have 3D-printed some special lens mounts for instruments being made by the department of Physics and Astronomy here.

My experience with tight-tolerance manufacture began in the late 1970s, when I learned to program and use a Bridgeport Series I CNC mill. I made quite a few metal parts accurate to about 1/1000" (about 25 microns) back then... so I have long appreciated how exponentially harder it is to go from 100 to 10 micron tolerances. BTW, my current little home CNC couldn't really do 10 micron no matter how small the part.

Anyway, my point is that 100 micron tolerances are pretty easy. High-quality injection-molded plastics typically can hit 50 micron tolerance (and Lego claims they typically do much better than that). So, if that tolerance would suffice for placement of camera lens mirrors, it really doesn't add any per-unit cost. BTW, tight-tolerance molded plastics are very common in modern lenses, and have gotten good enough so that many not-very-cheap lenses no longer have shimming or other adjustments (as many LensRentals tear-downs have revealed).

I would still be a bit worried about thermal expansion because many plastics move even more than aluminum does with heating/cooling...

 

[3dpan]

However, the most common version listed is effectively a helicoid-adjustable macro spacer, from one lens mount to another.

Yes, I've seen those… and have a few in my junk box that I haven't really put to use, yet.

Can you remember how many turns of the helicoid to get from one end of the travel to the other ?

 

[Jan Steinman]

However, the most common version listed is effectively a helicoid-adjustable macro spacer, from one lens mount to another.

Yes, I've seen those… and have a few in my junk box that I haven't really put to use, yet.

The most promising option looks like… starting with a C/Y macro tube, removing the C/Y male bayonet and replacing it with a m42 male flange.

Ha! That's what I get caught up in all the time.

I use cheap reversing adaptors to mount stuff. 58mm works well. Use a belt sander to grind the filter threads off, and you have a nice, flat mounting surface.

You can use the belt sander to take the M42 threads off the helicoid, too — I've done it! Just make sure you stuff a rag in there, so the whole thing doesn't get filled up with metal dust.

Then, just drill and tap some holes, and Bob's Your Uncle!

 

[Jan Steinman]

So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?

Do mirror lenses focus by changing the distance between the primary and secondary mirrors?

Most appear to do so, but at least the Sigma XQ 500/4 has a tiny helicoid on the rear of the lens that is used to focus.

Note that focusing by changing the mirror separation also changes the focal length. The OM Zuiko 500/8 is only about 400mm at closest focus.

 

[Jan Steinman]

However, the most common version listed is effectively a helicoid-adjustable macro spacer, from one lens mount to another.

Yes, I've seen those… and have a few in my junk box that I haven't really put to use, yet.

Can you remember how many turns of the helicoid to get from one end of the travel to the other ?

Seems it was quite a few… it's inaccessible at the moment, buried in a 20' shipping container in a box labeled: "Camera bits, too late to classify…"

 

[Eggplantt]

Chiefspeigler | Optical Ed's (opticaleds.com)

8 inch F/7 CHIEF group build - ATM, Optics and DIY Forum - Cloudy Nights

Doesn’t this use a parabolic mirror? I also suspect the adequately corrected field is too small to be useful in a photographic lens.

Well one of the described benefits is

1. Wide well corrected field off axis performance is similar to a Newtonian. CHief correctors contribute very little to the off-axis aberrations.

Newtonians require a simple corrector usually for imaging. Not sure about this. I can ask.

Now all the factors I described show historically why TCT designs like the Schiefspeigler have been limited to f/25 or f/27.

Spherical aberration and field curvature, probably.

Regards,

Alan

Well no, I linked something which describes main aberrations of TCTs. It's the off-axis ones such as astigmatism and coma:

With coma and astigmatism being in different proportions throughout the field of the two mirrors, due to their different tilt angles (that of the secondary is commonly in excess of double the primary's tilt angle), the final image in a simple TCT, after the center-field aberration is minimized, has only partially corrected field - unless of very small relative aperture - with uneven distribution of off-axis aberrations. Uneven compensation of astigmatism can, and most often does induce image tilt. Even the very field center is a compromise: since it is impossible to generate with the secondary's tilt the exact proportion of astigmatism and coma that are induced by primary's tilt, some level of residual aberration remains present.

These consequences of the uneven aberration match of two different far off-axis field segments limit simple two-mirror TCTs - and most more complex tilted systems as well - to small relative apertures and smallish aperture diameters.

Two-mirror tilted component telescopes (telescope-optics.net)

 

[Alan WF]

Chiefspeigler | Optical Ed's (opticaleds.com)

8 inch F/7 CHIEF group build - ATM, Optics and DIY Forum - Cloudy Nights

Doesn’t this use a parabolic mirror? I also suspect the adequately corrected field is too small to be useful in a photographic lens.

Well one of the described benefits is

1. Wide well corrected field off axis performance is similar to a Newtonian. CHief correctors contribute very little to the off-axis aberrations.

Newtonians require a simple corrector usually for imaging. Not sure about this. I can ask.

OK, but my impression is that Newtonians do not have adequately large corrected fields.

Now all the factors I described show historically why TCT designs like the Schiefspeigler have been limited to f/25 or f/27.

Spherical aberration and field curvature, probably.

Regards,

Alan

Well no, I linked something which describes main aberrations of TCTs. It's the off-axis ones such as astigmatism and coma:

OK. Thanks.

Regards,

Alan

 

[ProfHankD]

So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?

Do mirror lenses focus by changing the distance between the primary and secondary mirrors?

Definitely common. I believe all the cheap ones do it that way.

If so, that would explain why they are so difficult to focus. In a Cassegrain telescope, the displacement of the focal plane is a large multiple of the change in the distance between the primary and secondary. The precise factor depends on the optical prescription, but 10-20 is common in my experience. So, if you move the secondary or the primary by 10 microns, the focal plane can shift by 100-200 microns.

If so, then a helicoid focuser, which would give a 1:1 relation between lens displacement and focal plane displacement, might be a great help.

For what it's worth, I can stick a short extension tube on my Samyang 500mm f/6.3 and still hit infinity! In fact, I use to use it that way all the time because it gave no penalty and I could thus focus a bit closer. It hadn't occurred to me that what you just described above must be happening. I had figured it was 2-3X, because, well, bounce length is multiplied that way, but thinking about it, it has to be more than 2-3X more dramatic change in focus per mm moved...

Yet another good argument for something like the LM-EA7 to tweak focus.

 

[Alan WF]

For what it's worth, I can stick a short extension tube on my Samyang 500mm f/6.3 and still hit infinity! In fact, I use to use it that way all the time because it gave no penalty and I could thus focus a bit closer.

Since you’re using the mirrors far from their design configuration, you’re almost certainly increasing the aberrations.

Regards,

Alan

 

[ProfHankD]

For what it's worth, I can stick a short extension tube on my Samyang 500mm f/6.3 and still hit infinity! In fact, I use to use it that way all the time because it gave no penalty and I could thus focus a bit closer.

Since you’re using the mirrors far from their design configuration, you’re almost certainly increasing the aberrations.

That makes sense, but in practice, I don't see it.

Put another way, the difference is well within the focus overshoot past infinity on the lens, so it's probably no worse than using the lens in extreme temperature situations.

Here 's my 2018 review of the Samyang lens, with some images. There really aren't a lot of aberrations from this lens ever. It's just really hard to nail focus. Here's one example close-up from that lens:

 

[Jan Steinman]

I've been slowly unpacking our 20' sea can, and guess what I found today?

If anyone wants this lens, send me offers. I'll surely part with it for less than you can pay for it anywhere else!

Very lightly used. In fact, you've all probably seen the only photos I ever took with it! So it hasn't suffered any photon damage.

I added an after-market snap-on cap to replace the screw-on cap that came with it. Make a nice offer, and I can probably find some sort of case to go with it. (It came new wrapped in plastic bubble-wrap.)

(BTW: given that I thoroughly trashed this lens in this thread, this is a joke, folks! I consider a lens this bad to be collectable, and would never consider selling it, especially in a group where sales are not allowed! I'm sure I'll be buried with this thing! My estate wouldn't even be able to sell it in an estate auction!)

 

[3dpan]

So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?

Do mirror lenses focus by changing the distance between the primary and secondary mirrors?

Definitely common. I believe all the cheap ones do it that way.

If so, that would explain why they are so difficult to focus. In a Cassegrain telescope, the displacement of the focal plane is a large multiple of the change in the distance between the primary and secondary. The precise factor depends on the optical prescription, but 10-20 is common in my experience. So, if you move the secondary or the primary by 10 microns, the focal plane can shift by 100-200 microns.

If so, then a helicoid focuser, which would give a 1:1 relation between lens displacement and focal plane displacement, might be a great help.

For what it's worth, I can stick a short extension tube on my Samyang 500mm f/6.3 and still hit infinity! In fact, I use to use it that way all the time because it gave no penalty and I could thus focus a bit closer. It hadn't occurred to me that what you just described above must be happening. I had figured it was 2-3X, because, well, bounce length is multiplied that way, but thinking about it, it has to be more than 2-3X more dramatic change in focus per mm moved...

Yet another good argument for something like the LM-EA7 to tweak focus.

Your idea of a short extension tube on a mirror lens inspired me to try that with my Mirotar 500/8.

So I added a 6mm ext tube to the m4/3 camera and then attached the Mirotar complete with the 26mm C/Y - m4/3 adapter.

Different results from your Samyang,
I found infinity shrank to about 50 metres, but basically no effect on minimum focus = 3.5m.
On that basis it seems like a good idea to buy a standard C/Y helicoid focuser (26mm to 40mm extension) and use it as a fine-focuser. It won't have any effect at infinity with the helicoid wound back to 26mm, but increasing effect as the subject gets closer, and with the helicoid progressively wound out.

 

[3dpan]

So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?

Do mirror lenses focus by changing the distance between the primary and secondary mirrors?

Definitely common. I believe all the cheap ones do it that way.

If so, that would explain why they are so difficult to focus. In a Cassegrain telescope, the displacement of the focal plane is a large multiple of the change in the distance between the primary and secondary. The precise factor depends on the optical prescription, but 10-20 is common in my experience. So, if you move the secondary or the primary by 10 microns, the focal plane can shift by 100-200 microns.

If so, then a helicoid focuser, which would give a 1:1 relation between lens displacement and focal plane displacement, might be a great help.

For what it's worth, I can stick a short extension tube on my Samyang 500mm f/6.3 and still hit infinity! In fact, I use to use it that way all the time because it gave no penalty and I could thus focus a bit closer. It hadn't occurred to me that what you just described above must be happening. I had figured it was 2-3X, because, well, bounce length is multiplied that way, but thinking about it, it has to be more than 2-3X more dramatic change in focus per mm moved...

Yet another good argument for something like the LM-EA7 to tweak focus.

Your idea of a short extension tube on a mirror lens inspired me to try that with my Mirotar 500/8.

So I added a 6mm ext tube to the m4/3 camera and then attached the Mirotar complete with the 26mm C/Y - m4/3 adapter.

Different results from your Samyang,
I found infinity shrank to about 50 metres, but basically no effect on minimum focus = 3.5m.
On that basis it seems like a good idea to buy a standard C/Y helicoid focuser (26mm to 40mm extension) and use it as a fine-focuser. It won't have any effect at infinity with the helicoid wound back to 26mm, but increasing effect as the subject gets closer, and with the helicoid progressively wound out.

Just an update on helicoid focusers.

I have ordered an M42-M42 helicoid, and also a C/Y-m4/3 helicoid, to play with.

Probably a week or two for delivery from China.

Watch this space.

 

[Tom Caldwell]

My 900/8.0 mirror lens arrived today. As suspected a no-name lens in a Kelda box identified the manufacturer of these apparent wonders. I already had a Kelda-boxed 500/6.3 mirror lens that is about the worst mirror lens that i have owned. Like this one its focus rotation is very tight - in fact it give the feeling that the lubrication has been slapped on when the lens was assembled and not "rotatated-in" as it varies in tightness between extremes - it might settle down in hot weather - that is: if I ever use it enough.

The box illustration shows the same grip as the more expensive "German version" which completes the identification circle. There is only one lens of this type and it is made by Kelda.

Kelda is a Chinese brand of cheap and cheerful lenses, mostly they are just cheap. But I have one of their lenses as a 85mm standard construction - comes in EF mount and MF - which is an unusual combination but it is not a terrible lens. Their 135mm variant of this lens is fairly forgettable though.

So why did I buy this lens after its not so good reception on this thread. I had been pre-warned.

Well as a bit of a straight faced comic I could hardly resist the thought of this lens adapted to the M4/3 GM5 and playing tourist around our harbour-front. A sort "up your kilt kit tourist dslr" and of course you don't have to show anyone the images you have caught .... Unless you are a real clown and have carefully made some "specimens" with another lens beforehand. But that would be truly naughty and dishonest - I would not do that.

It was not that expensive and maybe I might get lucky - I don't have any other lens with this huge reach.

The T2 adapter to EF has a quite large cut-out for the locking pin not helped by my EF-M4/3 not having flange depth guides and being a fairly loose fit. I have better focal reduction adapters that I will try later. Even focal reduced the reach is huge.

Of course I have only roughly tested the lens so far - all GM5, no IBIS nor lens IS in sight.

The infinity is close to 15 metres on the focus scale and the focus action is tighter than comfortable as already mentioned - add the sloppy fit adapter and it is hardly comfortable in use.

But a quick appraisal says it seems to be an improvement over the Kelda 500/6.3 lens (not that this would be hard). I think when I seriously try with the lens it will make images of a sort - reasonable enough perhaps. I am not going to be enrolled in the birder chapter of fame but this lens is not entirely a waste of time. More of a challenge to get the best out of it.

So if one's expectations are low enough we could not be disappointed.

Never had a 900mm lens before .... nor have I any ambitions to find the money to buy a proper one.

 

[3dpan]

My 900/8.0 mirror lens arrived today. As suspected a no-name lens in a Kelda box identified the manufacturer of these apparent wonders. I already had a Kelda-boxed 500/6.3 mirror lens that is about the worst mirror lens that i have owned. Like this one its focus rotation is very tight - in fact it give the feeling that the lubrication has been slapped on when the lens was assembled and not "rotatated-in" as it varies in tightness between extremes - it might settle down in hot weather - that is: if I ever use it enough.

The box illustration shows the same grip as the more expensive "German version" which completes the identification circle. There is only one lens of this type and it is made by Kelda.

Kelda is a Chinese brand of cheap and cheerful lenses, mostly they are just cheap. But I have one of their lenses as a 85mm standard construction - comes in EF mount and MF - which is an unusual combination but it is not a terrible lens. Their 135mm variant of this lens is fairly forgettable though.

So why did I buy this lens after its not so good reception on this thread. I had been pre-warned.

Well as a bit of a straight faced comic I could hardly resist the thought of this lens adapted to the M4/3 GM5 and playing tourist around our harbour-front. A sort "up your kilt kit tourist dslr" and of course you don't have to show anyone the images you have caught .... Unless you are a real clown and have carefully made some "specimens" with another lens beforehand. But that would be truly naughty and dishonest - I would not do that.

It was not that expensive and maybe I might get lucky - I don't have any other lens with this huge reach.

The T2 adapter to EF has a quite large cut-out for the locking pin not helped by my EF-M4/3 not having flange depth guides and being a fairly loose fit. I have better focal reduction adapters that I will try later. Even focal reduced the reach is huge.

Of course I have only roughly tested the lens so far - all GM5, no IBIS nor lens IS in sight.

The infinity is close to 15 metres on the focus scale and the focus action is tighter than comfortable as already mentioned - add the sloppy fit adapter and it is hardly comfortable in use.

But a quick appraisal says it seems to be an improvement over the Kelda 500/6.3 lens (not that this would be hard). I think when I seriously try with the lens it will make images of a sort - reasonable enough perhaps. I am not going to be enrolled in the birder chapter of fame but this lens is not entirely a waste of time. More of a challenge to get the best out of it.

So if one's expectations are low enough we could not be disappointed.

Never had a 900mm lens before .... nor have I any ambitions to find the money to buy a proper one.

Thanks Tom, for going to all that trouble.

Though you could say it is cheap entertainment, and valuable education.

My current interest is in a 1000mm f/11 Nikon reflex.
refer this current thread,

Nikkor 1000mm F11

Shot on a tripod with a Fujifilm X-E1 playing around in my sunroom. Was shooting the bluejays in a tree when the foxes appeared on another corner of my yard. I bought this lens on a whim to add to my expanding mirror lens collection. Lots of donuts on the first picture but I didn't mind.

www.dpreview.com

 

[Tom Caldwell]

Hmmm - you are obviously looking for a good mirror lens.

I have taken a few images (again) with my Tokina 300/6.3 specifically made for the M4/3 mount. Which has a good reach - nothing like the Nikon obviously - but it is a handy and quite tiny lens seeing that it is made fro M4/3.

Must get them off the camera and posted as samples.

 

[Jan Steinman]

Had a table at the Puget Sound Photographic Collectors Society swap meet last Saturday, and traded the 900/8 for half of this (without the Nikon):

Got home, rummaged through my junk box, and found a µ4/3rd mount for the rear, and wonders of wonders, an OM mount for the front!

I will give the Spiratone Macrotel 150/4.5 a test, but I'm not expecting much. Then, I'll put the OM Zuiko 135/4.5 macro on it…

--
Jan Steinman