Cherries and lemons (1 Viewer)

MKinHK · Sep 2, 2019

I have been planning to buy a new scope for the first time in more than a decade and, as I've read the insights by various experts and other scope users, it seems that the issue of product quality - finding the cherry among them lemons - is a critical issue in finding the right scope and choosing between the different brands.

The aim of starting this thread is to explore whether the collective expertise of BF members might make it possible to improve the average buyer's chances of coming home with a cherry instead of a lemon.

There are two aspects to this:

1. How to help the non-expert scope buyer - say a relative or friend living far away - to find a cherry among the lemons through

a) research b) at the store c) relatively straightforward testing at home d) returning an obvious lemon.

I could imagine this coming together as a simple handbook of tips which scope buyers could use - similar to the extremely useful recommendations for settings of bridge cameras on the Canon and Sony forums.

2. Develop a database that could over time reflect the proportion of cherries and lemons from each manufacturer as a way to encourage the manufacturers to pay closer attention to product quality than the threads I've been reading suggest they apparently do now.

There is also one important question that would help to make this process fairer for the scope manufacturers and not become list of criticisms or a splitting of ever finer hairs of expert opinion- and that is to understand the size of the gap between the cherry and the lemon.

Is it something that only the truly expert would notice with exhaustive testing, or are there flaws/shortcomings that could be noticed and avoided by a reasonably careful birder using the scope in a range of typical birding conditions?

Many thanks in advance for any thoughts.

Cheers
Mike

etudiant · Sep 2, 2019

Think this would be a highly desirable development.
Roger Cicala of Lens Rentals (https://www.lensrentals.com/blog/) has done splendid work developing measures of optical performance and product uniformity for camera lenses. His work has helped manufacturers improve the quality of their products.
Unfortunately there is no comparable service for renting binoculars and scopes, so it is much harder to measure sample variability. However, as MKinHK suggests, even a simple 'lemon spotter' test would be very helpful.
Of course, the ideal would be for the manufacturers to provide a performance measurement sheet or some documented minimum standard. Thus far however, none seem willing to expose themselves in that fashion, perhaps because the production is still artisanal with all the variability that implies.

Binastro · Sep 2, 2019

With binoculars, I used to test 3, 6 or 12 individual binoculars one after the other in store using ball bearings left across the road if the sun was shining.
But I also just tried to read any text across the road in a shop window for instance.

Collimation problems quickly made themselves apparent just by looking, so I rejected these.
I could probably see a reject in thirty seconds.
Mechanical problems are also quickly apparent.

As the binoculars I was dealing with were lower price, but still good optics I either bought one or sometimes three if these were all well above average.

With scopes this is somewhat easier as two barrel alignment is not a problem. But there can still be poor alignment in one scope barrel.

I would think that reading fine text at a distance and a star/ ball bearing test would rather quickly reveal a good scope. But test at least three and preferably six.

The difference in lower price instruments is large.
I suppose expensive optics less.

Real cherries are I think rare, except with new military stuff where it is fairly common.

B.

Binastro · Sep 2, 2019

Regarding post 3.

If a birdwatcher is using a Kowa or Swarovski spotting scope at 30x or 40x, I don't think many people would see a difference between a good scope and a cherry scope.
It is only above 60x, and say at 100x or 150x that a cherry scope is needed.

With camera lenses from good makers, good results are achieved if one only views images on a computer screen.
It is only with big enlargements that cherry lenses are needed.

It is mainly the optics enthusiasts who need the best optics.
With low power use, most birdwatchers will probably be happy with an average good make scope.

B.

Binastro · Sep 2, 2019

Of 2,000 optics tested, lenses, telescopes and binoculars, I think that 2.5% were cherries, approx. 30% lemons and the rest somewhere in between from very good to very bad.

I have tested rather few spotting scopes, perhaps 50, and very few top quality spotting scopes.
The two Questars are very good but not cherries. The 77mm Apo Televid may be very good, but the magnification is too low to know. Same with Optolyth Fluoride? 80mm.

The problem with most spotting scopes is that the magnification with the supplied eyepieces is too low to know if they are top quality or not.
Star tests show a lot but not the finest resolution.

B.

lewis20126 · Sep 2, 2019

I'm not an optics obsessive, but a keen birder - just bought a Kowa 883 with the 'new' zoom eyepiece - very happy with that. Alternative for me was the Swaro 95mm modular but put off by connection issues for modular system reported by heavy users. Optically very similar.

cheers, alan

jring · Sep 2, 2019

Hi,

while I have certainly looked through quite a bit less telescopes than Binastro (maybe 1/10 or so) and actually star tested even less (especially spotting scopes not in my possession with a count of one - a Leica 82 which belonged to a friend from the astro club but didn't really cut it and thus had to go...), I agree that the average birder will not notice the difference between fair and cherry during daylight and at the usual magnifications offered by the manufacturers eyepiece lineup.

As for star testing, the usual rule for astro scopes is to use an eyepiece with a focal length equal to the objective focal ration or less in mm. With magnification = objective focal length / eyepiece focal length and focal ratio = objective focal length / objective aperture we get that the magnification must equal or higher than the objective aperture in mm.

This means we have different degrees of close, but not quite there for most scopes - with some exceptions of right on target like some small aperture high mag options like Kowa 603/613 or Nikon Fieldscope 60ED @60x.

But even with a bit less magnification than optimal a star test will show blatant problems with the optics - and those are the only ones which will matter unless you want to go beyond the nominal range of 60x or maybe 75x by using an extender or alternative eyepieces (or have a well above average eyesight).

Unfortunately a real star test means you have to observe a bright star in good enough seeing for the needed magnification, which implies having the scope overnight and on a clear day with at least ok seeing (for the small apertures of spotting scopes).

One can use an artificial star (basically a very small and round hole in an opaque piece of material illuminated by a bright light source - ideally with a diffusor) but unlike a real star which is at an (almost) infinite distance, placing an artificial one that far away proves difficult. Anything closer will make the star test show some degree of spherical aberration (one of the many optical defects it can show), but for typical spotters 20-30m should be ok. Also the air should be fairly calm between scope and the artificial star which means warm and sunny days are not a good idea and when done inside a large enough room, it should ideally be unheated.

Lastly, there have been quite a few books written on the theory and correct interpretation star test results (with H.R. Suiter: Star Testing Astronomical Telescopes being the canonical one), for the purpose of ruling out a lemon of a spotting scope, diffraction patterns showing more or less concentric circles a little bit inside and outside of focus are acceptable.

For example I just star-tested an old Kowa 613 which was a total mess when I got it used (had bad case of pinched optics - probably s.b. heard the objective rattle a bit and nicely tightened up that objective cell) but after loosening up the retaining ring a bit I'm left with a scope which is perfectly usable at 60x (unless compared side by side to a really good example) and shows about 1/3 lambda of astigmatism and the usual bit of undercorrection in a star test - which is a decidedly so-so result and if it was an astro scope, I would try to rotate the elements in order to minimize the astigmatism, but for a spotter it's actually ok.

Joachim

etudiant · Sep 3, 2019

jring said:
Hi,

while I have certainly looked through quite a bit less telescopes than Binastro (maybe 1/10 or so) and actually star tested even less (especially spotting scopes not in my possession with a count of one - a Leica 82 which belonged to a friend from the astro club but didn't really cut it and thus had to go...), I agree that the average birder will not notice the difference between fair and cherry during daylight and at the usual magnifications offered by the manufacturers eyepiece lineup.

As for star testing, the usual rule for astro scopes is to use an eyepiece with a focal length equal to the objective focal ration or less in mm. With magnification = objective focal length / eyepiece focal length and focal ratio = objective focal length / objective aperture we get that the magnification must equal or higher than the objective aperture in mm.

This means we have different degrees of close, but not quite there for most scopes - with some exceptions of right on target like some small aperture high mag options like Kowa 603/613 or Nikon Fieldscope 60ED @60x.

But even with a bit less magnification than optimal a star test will show blatant problems with the optics - and those are the only ones which will matter unless you want to go beyond the nominal range of 60x or maybe 75x by using an extender or alternative eyepieces (or have a well above average eyesight).

Unfortunately a real star test means you have to observe a bright star in good enough seeing for the needed magnification, which implies having the scope overnight and on a clear day with at least ok seeing (for the small apertures of spotting scopes).

One can use an artificial star (basically a very small and round hole in an opaque piece of material illuminated by a bright light source - ideally with a diffusor) but unlike a real star which is at an (almost) infinite distance, placing an artificial one that far away proves difficult. Anything closer will make the star test show some degree of spherical aberration (one of the many optical defects it can show), but for typical spotters 20-30m should be ok. Also the air should be fairly calm between scope and the artificial star which means warm and sunny days are not a good idea and when done inside a large enough room, it should ideally be unheated.

Lastly, there have been quite a few books written on the theory and correct interpretation star test results (with H.R. Suiter: Star Testing Astronomical Telescopes being the canonical one), for the purpose of ruling out a lemon of a spotting scope, diffraction patterns showing more or less concentric circles a little bit inside and outside of focus are acceptable.

For example I just star-tested an old Kowa 613 which was a total mess when I got it used (had bad case of pinched optics - probably s.b. heard the objective rattle a bit and nicely tightened up that objective cell) but after loosening up the retaining ring a bit I'm left with a scope which is perfectly usable at 60x (unless compared side by side to a really good example) and shows about 1/3 lambda of astigmatism and the usual bit of undercorrection in a star test - which is a decidedly so-so result and if it was an astro scope, I would try to rotate the elements in order to minimize the astigmatism, but for a spotter it's actually ok.

Joachim

In short, it does not matter much, even a very imperfect glass will still be amply adequate for birding use. That does though somewhat undercut the logic of paying heavily for alpha glass.

jring · Sep 3, 2019

etudiant said:
In short, it does not matter much, even a very imperfect glass will still be amply adequate for birding use. That does though somewhat undercut the logic of paying heavily for alpha glass.

Hi,

well 1/3 lambda of astigmatism might even be barely diffraction limited (the usual minimum standard for astro optics which is usually guaranteed and met most of the time) but far from very imperfect.

It seems the usually quoted 1/4 lambda criterion is actually valid for spherical aberration only and other aberrations have a smaller effect and thus a slightly larger limit.

see https://www.cloudynights.com/topic/53607-what-are-diffraction-limited-standards/?p=697860

Joachim

MKinHK · Sep 3, 2019

Many thanks for all the comments

Etudiant's comment in post 8 reminded me that it was Henry Link's review of a cherry Nikon Monarch 82ED that outperformed the Kowa and Swaro alphas https://www.birdforum.net/showthread.php?t=377440 really prompted me to write this post. I'm veering towards getting the Kowa 883 when I next visit Japan, where its very reasonably priced in the BIC Camera chain, and has extra tax discounts if you're buying as a visitor to Japan, but the thought of a significantly cheaper scope performing as well or better really had me wondering.

Realistically a one-time scope buyer will only find a cherry by luck if they are only 2.5% of the stock. And maybe that doesn't matter for birding. So the question then becomes how many really acidic lemons are there out there, and what are the steps for avoiding those?

Cheers
Mike

Binastro · Sep 3, 2019

There are really acidic lemons, especially secondhand where they have been disassembled by a non optical person and reassembled.

The new Vivitar Series 1 600mm f/8 solid Cat cleanly resolved epsilon Lyrae both components with clear sky between on a Slik 88 photo tripod at 180x using a 3x teleconverter and monocular converter.
This is less than 2.5 arcseconds double stars in the garden. Real stars.

A secondhand one was a complete useless lemon that had been interfered with.

An Optomax 500mm f/8 ordinary mirror lens just separated the two pairs but with temperature effects.

To avoid lemons set up say 4 good spotting scopes of any make but say 4 Kowas or Swarovskis, if they have them, and read small text in a large unheated school sports hall for instance.
A real lemon will be easily seen.

Or have a known really good scope and compare the intended purchase.

B.

henry link · Sep 4, 2019

MKinHK said:
Many thanks for all the comments

Etudiant's comment in post 8 reminded me that it was Henry Link's review of a cherry Nikon Monarch 82ED that outperformed the Kowa and Swaro alphas https://www.birdforum.net/showthread.php?t=377440 really prompted me to write this post. I'm veering towards getting the Kowa 883 when I next visit Japan, where its very reasonably priced in the BIC Camera chain, and has extra tax discounts if you're buying as a visitor to Japan, but the thought of a significantly cheaper scope performing as well or better really had me wondering.

Realistically a one-time scope buyer will only find a cherry by luck if they are only 2.5% of the stock. And maybe that doesn't matter for birding. So the question then becomes how many really acidic lemons are there out there, and what are the steps for avoiding those?

Cheers
Mike

Two basic tests (or in a pinch only one) are all that's really needed to determine the quality of a scope's objective lens: a high magnification star test and a resolution measurement using something like the USAF 1951 Resolution Test Pattern. The star test alone is good enough if there is no way to set up an accurate resolution test. I usually bring an artificial star with me to a store, but I've improvised many artificial stars in sunlight, like shiny trim on cars, porcelain insulators on power poles, etc. For revealing defects use the highest magnification possible, a 1-0.5 exit pupil is best. I usually bring a 3x booster to use behind the scope eyepiece if possible. Don't defocus more than about 3-5 diffraction rings and learn to recognize the appearance of the usual suspects: spherical aberration, astigmatism, coma, pinching and a poorly made roof prism. I don't know any way to avoid first doing a little research into star testing online so you know what you are looking for.

I wouldn't want to guess at the percentage, but there are certainly enough acidic lemons to make a prudent person not just assume or hope for the best. This is especially true for the "alpha" scopes. Who wants to spent $3,000 to $5000 on a scope with optics more characteristic of an average sub $1000 scope? Personally, I won't accept much less than cherry if the price is very high, especially since I know from the Nikon Monarch that it's possible to make a scope with nearly perfect optics for $1,600.

As for how many birders will notice a scope with mediocre to bad optics, in my experience there is almost no bottom to what can be ignored or excused away. A birding friend of mine used an old Kowa scope with a cracked prism for twenty years and just assumed the image was supposed to be somewhat fuzzy at 20x and completely useless at any higher magnification. I've encouraged people to return outright lemons under warranty only to find them happily using the same scope a few years later.

MKinHK · Sep 6, 2019

Many thanks for these additional insights Binastro and Henry.

Now I need to figure out how to do a star test inside the Haneda airport terminal - I guess I will practice with my battered Zeiss 85 and thread-stripped but optically sound Leica 77 APO before I go.

Cheers
Mike

jring · Sep 6, 2019

MKinHK said:
Many thanks for these additional insights Binastro and Henry.

Now I need to figure out how to do a star test inside the Haneda airport terminal - I guess I will practice with my battered Zeiss 85 and thread-stripped but optically sound Leica 77 APO before I go.

Cheers
Mike

Hi,

an airport terminal should be at least large enough, although not sure if aircon will make for bad seeing.

You can make an artificial star like this:

- take a piece of alumuminum foil and place on a glass surface
- take a pin and very gently perforate the foil - so that a very tiny hole is visible (best seen backlit with a flashlight).
- put foil in front of a led flashlight so only the tiny illuminated hole is visble and secure with tape.

This should be placed 20-30m from the spotting scope and should be observed at maximum magnification. You search for best focus and then defocus a little bit in each direction. You should ideally see the same diffraction pattern of concentric circles - in reality the patterns inside and outside of focus will differ. See the following link for some example images (the ones without obstruction) for how different problems will show:

https://www.telescope-optics.net/star_testing_telescope.htm

Joachim

Binastro · Sep 6, 2019

One should keep in mind airport security and explain what one is doing and why.

I am not sure if a new scope will necessarily be better than an old Zeiss 85 or Apo Televid 77.
Ideally one would just take the better of these and test it against an intended new purchase.

If the air is stable with constant temperatures even outside should be fine.

Regards,
B.

kabsetz · Sep 6, 2019

Actually, viewing in unstable air reveals quality differences between scopes providing one does enough back-and-forth between the scopes to make sure that seeing was not worse for one than the other.

A scope with closer to perfect optics will show a better image in poor seeing than a scope with less perfect optics, as the aberrations brought by the scope add to the disturbance caused by unstable air. This is one strong reason (although not widely understood by birders) why a low-aberration scope is important for birding, as birding is a hobby where one cannot just wait for perfect seeing. Unlike the stars or the planets, birds have their own mind about when to be visible and where.

- Kimmo

MKinHK · Sep 7, 2019

Thanks for this extra guidance on star testing Joachim, Binastro and Kimmo. I did think about bringing one of them with me, but given the state of my current scopes I would be profoundly shocked if a new 883 was not significantly better! I work at Hong Kong airport so your reminder about not upsetting security is very relevant right now, especially as the police are classifying laser pointers as lethal weapons during the demonstrations in HK right now!

I dug my old Leica 77APO out of the back of the draw to try it out and only then realised how trashed it was:

1. Dropped on object lens so lens cover is stuck
2. Multiple drops on zoom lens which was initially stuck and now rotates freely without gripping the connecting thread
3. In struggling to detach it I had to cut away an elastic band that had solidified on the join and in a ridiculous moment of comedy dropped it into the toilet, fogging the lens which remains fogged some 10 hours later
4. The thread for the tripod screw had ripped out the bottom of the base
5. In conclusion ... it is not worth bringing and deserves a discreet burial. It will probably be relieved!

The Zeiss 85 Diascope is in better condition, but has also been dropped on both ends at different times, including a chip in the glass(no filter) of the object lens and at least one drop on the eyecup-less zoom lens (which suggests it might have been dropped before that too!).

Air quality in Hong Kong is not conducive to using real stars, but there are plenty of distant lights at night. The Zeiss actually works OK at focusing on these, but maximum zoom revealed that at least one of the dents is affecting the optics and there seemed to be whole colonies of microscopic life on the lenses. It's perhaps amazing that it works at all. Well it does at low zoom, but struggles mightily as soon as I try to jack it up.

In writing this I'm coming to conclusion that I probably should not be trusted with a Toys'R'us rubber scope, let alone quality alpha glass. I'm pretty sure how I'd far if the scopes applied the cherry/lemon test to my scope ownership qualities, although to be fair there is a couple of decades of exciting birding history embodied in the wounds of both these two battered veterans!

Cheers
Mike

Binastro · Sep 7, 2019

Thanks Mike for the entertaining read.
The old Hong Kong airport approach was fun.

I presume that your scope has fungus.
However, I know that there were many experts in Hong Kong who regularly took optics apart for cleaning fungus, something not generally available in the U.K.

Zeiss say they won't deal with fungus, but they did a fine job on an old 15x60 F.O.C.

I also had dust mites going round in endless circles in two fresnel screens on Minolta cameras bought secondhand.
I put them in the freezer for 2 days three times, but still thought the creatures might wake up refreshed.
So I gave the cameras and lenses to a friend.
This was a mistake as the Minolta 250mm f/5.6 mirror lens on one might be nearing £1,000 now.

But I don't like creepy crawlies in optics as they might bring their friends or have children and grandchildren in their hundreds.

I had a Japanese 20x70 Porro with a spider in it.
Also Russian optics with completely unknown beings in them. At least creatures I have never seen before.
Spiders are common in astro scopes left outside. They don't affect the view.

Broadhurst Clarkson kept 6 common English house spiders on their staff to provide thread for their micrometer eyepieces.

Regards,
B.

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