Jan Steinman
Senior Member
Never held one. My info source on plastic in the Solid Cat is, "I think I read it somewhere." I'm obviously wrong!
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Another budget mirror lens, 900/8 this time
- Thread starter 3dpan
- Start date
Jan Steinman
Senior Member
Yea, I would happily trade my 900/8 for it.
I'm always willing to trade less-exotic useless junk for more-exotic useless junk!
Alan WF
Senior Member
Mirror lenses are essentially Cassegrain telescopes, and that’s my day job. I’d note that building a fast[*], wide-field[**] Cassegrain telescope is challenging. I’d also note that the secondary obscuration typically gets larger at faster focal ratios, and this tends to reduce the T-stop and contrast. So, no, I don’t think you’re going to see this.
Regards,
Alan
[*] f/4 is fast for a telescope.
[**] Wide-field here means the size of the image circle compared to the diameter of the aperture, rather than the size of the image circle compared to the focal length.
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ProfHankD
Veteran Member
Alan,
I get what you're saying there, but what I don't understand is why we haven't seen mirror lenses using the kind of offset mirrors you see in binoculars -- that way, you wouldn't have the center obscured and could even have a variable aperture.
I have experimented with using an offset Waterhouse stop on a conventional mirror lens to get around the doughnut bokeh, and it works, but is of course rather dark because the biggest aperture you can fit there is usually several stops down from the lens wide open.
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Alan WF
Senior Member
You mean taking a mirror lens, stopping it down with an off-axis circular aperture between the edge of the primary and the edge of the secondary, and then “throwing away” all of the optics that are not illuminated?
Reflecting solar telescope, such as DKIST, do this.
There are a number of challenges. One if that you’re using off-axis optics, so anything that is not spherical will be expensive to polish. In a small mirror lens, you might get away with spherical primary and secondary mirrors, but the corrector plate at the entrance to the telescope will be off-axis and aspherical. In larger telescopes, the mirrors are typically off-axis and aspherical too. Optomechanics (mirror/lens support) and alignment are also more difficult.
It’s something we only do when we need very high contrast (e.g., solar telescopes[*]) combined with large aperture (because at small apertures it’s easier to get high contrast with refractors).
Regards,
Alan
[*] Solar telescopes need high contrast because they are looking for low-contrast features in a field that is filled with a very bright source. Night-time telescopes are looking for relatively bright features in a field that is dark. More or less.
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ProfHankD
Veteran Member
Essentially, yes. Although there are other ways to fold the light path with an offset. I would think one could even do fairly well using flat mirrors to offset-fold a light path for a purely refractive lens design. In fact, light doesn't much mind crossing its own path, so I can imagine folding the light path in complex patterns to dramatically reduce the absolute size of a lens.
Telescopes aren't really bothered by the linear length of the optical path compared to camera lenses, which need to be more portable. Doesn't an offset structure sound like a potentially worthwhile way to make a fast camera lens that has a long focal length (but technically isn't a telephoto)?
Alan WF
Senior Member
This would be a challenge to align and then keep aligned the face of rough treatment. I know, because we do this in astronomical instruments in order to keep them compact, and it is not at all free. Putting lenses in a coaxially in a cylinder is much easier for manufacture, alignment, and robustness.
I’d also note that the engineering plastics used in modern lenses are very light, so I don’t know if you’d save much weight by making a more compact structure. I’d guess that most of the weight is in the optics. That said, the relative mass of the structure might be more important in extreme telephotos such as 800 mm f/5.6.
The only consumer optics I know that use something like this are monoculars and binoculars. However, in these the prism isn’t so much to reduce the length but rather to give an erect image.
For the alignment and robustness reasons, I think a telephoto lens (converging initial group and diverging second group) is probably a superior solution to folding a long lens with mirrors or prisms.
We try to keep telescopes compact for several reasons: a long telescope has support and vibration issues and requires a larger and more expensive enclosure.
Regards,
Alan
I think that the issue with Mirror Lenses is a similar perception issue to that which is commonly applied to small camera bodies:
Small cameras are not considered to be as good as larger camera bodies - so they should be cheaper ... right? So manufacturers make them as cheaper cameras to suit and it becomes a self fulfilling prophesy and small cameras are not as good as larger cameras so they become hard to sell to anyone but entry level users.
Swap language to Mirror Lenses - easy to criticise - generally are not considered good enough (for reasons that we all understand) so they have to be cheap to attract. Because only cheap Mirror Lenses sell well that is what is usually made and we have another self-fulfilling prophesy.
Maybe if we were willing to pay more we could have better quality small camera bodies and better Mirror Lenses. But I could not see these perceptions changing any time real soon.
3dpan
Leading Member
With the Covid related shipping limitations US is not such a great market now, shipping is plus expensive.
Though in fairness shipping from NZ is also now even more expensive because limited to International Courier Post.
Best place to buy from is China (via eBay) for anything made in China. Seems to arrive in NZ by container load and then processed separately at an NZ facility (and that can add a week or two to delivery times). But at least shipping is cheap, often free.
Second best is probably Japan, for quality and variety of used photo goods. Occasional problem with Japan Post not shipping to NZ, and then it's hyper-expensive DHL or similar.
For new Oly items it pays to shop locally. New are often cheaper than even used from Japan. Possibly because of poor turnover means new stock is old prices. Especially our poor exchange rate with the USD.
e.g. my E-M1 III/12-40mm kit was cheaper ($2289NZ, and thats about $1200US) than anywhere else I could find in the world, even used. (taking into account shipping too).
That's right, I paid $1380 US including shipping and local taxes, for a brand-new E-M1 III with 12-40mm PRO lens, with Olympus (Austalasia) warranty,.
Anyway, getting of-topic.
But that's why the Jintu 900/8 is so attractive. Made in China, low Chinese manufacture costs, and very low shipping cost. ($1.48 US to NZ).
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Jan Steinman
Senior Member
Don't want to speak for Hank, but I think he meant using prisms, like binoculars do. With porro prisms, you could even have a straight-looking lens, rather than the lumpy look that roof prism binoculars have.
Prisms are just mirrors. So it would be a refractor with a folded light path, and you wouldn't get "free" low CA like you do with true focused-mirror lenses.
Jan Steinman
Senior Member
3dpan
Leading Member
Yeah, well. You could call it a soft-focus lens, and use it for portraits.
ProfHankD
Veteran Member
Please do speak on this...
My basic point is that there are multiple ways that "mirrors" can be used to make a long-focal-length lens small, light, and fast without forcing mirror bokeh and lack of aperture control. I just don't understand why we haven't seen any of them. Well, actually, that's not quite true: I once saw an old Bushnell monocular camera lens on eBay, but haven't really seen it anywhere else so don't know what was inside of its obviously mirror-containing optics.
Jan Steinman
Senior Member
Great for shooting older people… "What wrinkles?"Yeah, well. You could call it a soft-focus lens, and use it for portraits.And the lovely, "painterly" quality of its fuzzy images!
YGWYPF!
3dpan
Leading Member
Yep, and from a safe "Covid" distance. :-DGreat for shooting older people… "What wrinkles?"Yeah, well. You could call it a soft-focus lens, and use it for portraits.And the lovely, "painterly" quality of its fuzzy images!
YGWYPF!
3dpan
Leading Member
Just from observation and experience it seems that the focus throw on mirror lenses can be a real problem with obtaining sharp focus.
For example, and from my mirror lens collection, focus throws are,
(i) Olympus reflex 500/8, ~ 180 degrees
(ii) Yashica ML 500/8, ~ 180 "
(iii) Vivitar 600/8 solid Cat, ~180 "
(iv) Vivitar 450/4.5 solid Cat, ~ 270 "
(iv) Zeiss Mirotar 500/8, ~ 270 "
(v) Rubinar 300/4.5, ~ 270 "
(vi) Rubinar 500/8, ~ 360 "
(vii) Tokina RMC 500/8, ~ 360 "
(viii) Celestron C90 500/5.6 mirror lens, 720 degrees
And of course, more to topic,
(ix) Jintu 900/8, ~ 180 degrees (from pictures of the lens)
My personal experience is that focus throws of 180 and 270 degrees make it very difficult to fine tune the focus.
Note I'm referring to focal lengths around 500mm or so.
At 360 degrees or more, much easier. Though the Celestron with 720 degrees can be a mission going from near to far.
Now since all these lenses are designed for 35mm full frame, and since I am using m4/3 crop sensor, I need to add a lens adapter to fit the lens to my camera.
So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?
For example, and from my mirror lens collection, focus throws are,
(i) Olympus reflex 500/8, ~ 180 degrees
(ii) Yashica ML 500/8, ~ 180 "
(iii) Vivitar 600/8 solid Cat, ~180 "
(iv) Vivitar 450/4.5 solid Cat, ~ 270 "
(iv) Zeiss Mirotar 500/8, ~ 270 "
(v) Rubinar 300/4.5, ~ 270 "
(vi) Rubinar 500/8, ~ 360 "
(vii) Tokina RMC 500/8, ~ 360 "
(viii) Celestron C90 500/5.6 mirror lens, 720 degrees
And of course, more to topic,
(ix) Jintu 900/8, ~ 180 degrees (from pictures of the lens)
My personal experience is that focus throws of 180 and 270 degrees make it very difficult to fine tune the focus.
Note I'm referring to focal lengths around 500mm or so.
At 360 degrees or more, much easier. Though the Celestron with 720 degrees can be a mission going from near to far.
Now since all these lenses are designed for 35mm full frame, and since I am using m4/3 crop sensor, I need to add a lens adapter to fit the lens to my camera.
So, my question, why not add a helicoid focuser instead of the normal lens adapter, and use that to fine tune the focus ?
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Alan WF
Senior Member
Thank you both for these questions!
I don’t think standard porro prisms would do much to shorten a long focal-length refractor.
However, let’s see what we can do:
First, let’s consider a simple 300 mm doublet (so no aperture control or higher-order correction) and fold the 300 mm into 3 x 100 mm by using prisms/mirrors or whatever. The two folding surfaces would be separated by about 100 mm, so would have to be two separate elements for cost and weight reasons. And, roughly, we would need to 45+ mm diameter prisms/mirrors with good optical quality; they won’t be cheap. That said, for a simple doublet there are no alignment issues, so this would work, but it comes at the cost of the two folding elements.
Second, let’s improve this by adding a field-flatter, an element close to the detector that acts to correct the field curvature of the doublet. The alignment tolerances on a field flattener are typically very generous, so there is no issue here. Again, this would work, but at the cost of the two folding elements.
Third, let’s suppose we want to correct higher-order aberrations and form an image of the pupil, so we have somewhere to place the pupil stop. This requires more optics, probably after the second mirror. And now we have to maintain good alignment between the fore optics and the pupil/corrective optics. And maintaining good alignment of two prisms/mirrors that are 100 mm apart is not trivial. Can it be done? Certainly. Does it have a cost in terms of engineering complexity? Certainly.
So, in summary:
- One can fold a simple doublet or a field-flattened doublet for the cost of two mirrors/prisms. However, simple doublets are the cheapest telephotos, so probably the additional cost of the folding elements makes this unattractive.
- For more complex designs, in addition one needs to design optomechanics to maintain alignment of the elements in the different sections of the folded path,. I strongly suspect that this additional complication makes such designs uncompetitive with conventional telephoto designs.
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.
Regards,
Alan
That might be worth a try. Especially with a helicoid adapter that had a very fine throw. My comment is that normally near infinity a hiccup whist turning the focus ring is the difference between in and out of focus. Needs a micrometer adjustment. Vernier scale anybody?
A longer throw would mean a tiny bit more room at the infinity end and lost more twisting and turning everywhere else. As these lenses are not really at their best near infinity it might be most useful on the better Mirror Lenses.
ProfHankD
Veteran Member
As I said earlier, I think focus touchiness is one of the biggest causes of unsharp mirror lens images -- and something like the TechArt Pro LM-EA7 is the best answer I've found.
Focus throw on that autofocus adapter is nowhere near enough to autofocus most mirror lenses, but it's perfect for automatically tweaking an approximate manual focus. Just two issues:
- There is a weight limit on what the adapter can move. Most cheap mirror lenses are OK, but lots of older/"better" ones are way over the weight limit. Although TechArt has never recommend use with heavy lenses, the camera body isn't over the weight limit, so provided you support everything by the lens, the adapter can move the body to focus...
- The adapter is tuned to use main-sensor PDAF, not CDAF, and that gets less effective as the lens aperture gets slower. It's pretty snappy at f/5.6, but gets increasingly hit-or-miss at apertures slower than that. By f/8, it's struggling a little on my A7RII. Incidentally, you'll definitely want to configure the camera so a separate button activates autofocus rather than having the shutter button do it, because that way at least you can still take a shot when the adapter fails to autofocus.
BTW, this makes a HUGE difference. For example, manually focusing my Samyang 500mm f/6.3, it seems much less sharp than my Minolta 500mm f/8 AF Reflex. However, on the LM-EA7, the Samyang might actually be a hair sharper than the Minolta! In truth, I suspect they're both pretty good and the difference is entirely down to how accurately the autofocus adjusts; I'm using the Minolta on an LA-EA4, which means a single PDAF point active using the SLT and separate PDAF sensor in the adapter rather than the main sensor, so minor misalignments are likely.
3dpan
Leading Member
Unfortunately for me TechArt don't seem to include m4/3 in their product range.
Otherwise a superb idea.
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