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Leica and chromatic aberration

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Old Wednesday 26th September 2018, 14:05   #1
adhoc
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Leica and chromatic aberration

A post in another thread [Link] reported 2 days ago: "...[The Leica Ultravid-Plus 7x42] did betray their own weakness the other day though. I was watching ravens circle above a building in a completely grey, but bright sky. Those poor ravens were encased in a green fog of chromatic aberration..."

An experienced member responded: "...[Such a situation] will test any binocular for CA. Focus has to be precise..."

Comments on this will be much appreciated. What compromise/s will Leica have to make if their optical design remedies this CA?
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Old Wednesday 26th September 2018, 14:32   #2
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Originally Posted by adhoc View Post
A post in another thread [Link] reported 2 days ago: "...[The Leica Ultravid-Plus 7x42] did betray their own weakness the other day though. I was watching ravens circle above a building in a completely grey, but bright sky. Those poor ravens were encased in a green fog of chromatic aberration..."

An experienced member responded: "...[Such a situation] will test any binocular for CA. Focus has to be precise..."

Comments on this will be much appreciated. What compromise/s will Leica have to make if their optical design remedies this CA?

I made that comment but I have no idea what Leica could do to remedy CA under the condition described by the OP. The binocular I was using in my example had a lagging focus problem in one tube that contributed to the bad CA I saw. Once I realized that and played around with the focuser and loosened it up the CA became less intrusive but it was still there.

I almost never see CA in normal use with any of my binoculars and I use an 8x42 Ultravid Blackline often. And I don't look for it. As you know the Ultravid Blacklines still have Leica's original Ultravid coatings.

Bob
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Old Wednesday 26th September 2018, 14:46   #3
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My memory may be faulty and I would value correction if this is so, but I am sure I read that shorter focal lengths lead to greater CA. Leica has usually been a champion of the compact bino and even Noctivid is for example well shorter than a Zeiss FL at 150mm for the Leica and 173mm for the Zeiss, and nobody ever complained the FL was too long.

If so then the CA and the level of correction applied to it must be what Leica thinks is the right balance between CA/Compact Length/Cost. Other brands have their own opinions of course. Troubadoris never complains about CA in her Ultravid 8x32 HD.

Lee
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Old Wednesday 26th September 2018, 15:20   #4
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For a 8X42, the SF is indeed the longest on the market. I personally do not mind the length, with the new rumors of the SF 8X32, it will be curious on what others think of it if it is bigger than the FL 8X32, no doubt it will be. I remember when members criticized the EDG 8X32 as being too big for an 8X32, it still blows away many 8X32s on the market today with respect to CA and glare.

P.S. leica makes a great glass, perhaps it is the vision of the persons who keep harping on CA. If Leica does not work for an individual, go with another brand, there are many today to choose from.

Andy W.

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Old Wednesday 26th September 2018, 15:31   #5
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The first thing to do is to try to figure out what exact aberration is involved. A little closer examination should pin down what kind of CA is being observed. A fringe that's green rather than purple just about eliminates longitudinal CA.

If the green color completely surrounds a centered object it might be sphero-chromatism. If a slight shift in focus eliminates the color or changes the color to a red/magenta then that's probably what it is.

If the object is moved off-axis is the green fringe then limited to one just side of the object (either facing the center or facing the field edge) with a red/magenta fringe on the other side of the object? If yes, then it's latitudinal CA.

All three aberrations are there in all binoculars, but the last two are more likely to be easily visible, especially latitudinal (or lateral) CA. All eyepieces have some off-axis lateral color and some objective designs with wide air spaces (like the space in front of the focusing element in binoculars) seem to be prone to it. Both lateral color and sphero-chromatism could be visible in the same binocular.

Not too much can be gleaned from a modern binocular's physical length. The 42/56mm FLs length came from the AK prisms, not the focal ratio. The SF is long because it uses a different objective design (fixed doublet with positive focusing lens, just like the old Swaro EL) compared to all the other alpha roofs (fixed triplet with negative focusing lens). That design probably does require extra physical length just to reach the same optical focal ratio and aberration level as the others.

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Old Wednesday 26th September 2018, 18:08   #6
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Might it not be the case that Leica bins, compared to the other alpha brands, do NOT show significantly higher levels of CA in purely TECHNICAL terms, but that existing chromatic aberrations are just more VISIBLE because of Leica’s superior (?) contrast?
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Old Wednesday 26th September 2018, 18:13   #7
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Thanks all, especially Henry yet again, for the information so far. A bit there to digest!

Lightbender, do you reckon that in that same view the Kowa Genesis and Meopta HDs would have shown CA like that or at all? They are said to be best at controlling it, and contrast in the latter is I gather comparable to Leica. (In that situation the effective aperture, in the Ultravid 7x42 and in those, will be diminished by the user's pupil to the same.)

Last edited by adhoc : Thursday 27th September 2018 at 05:37.
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Old Wednesday 26th September 2018, 20:05   #8
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Hello All,
This is my quote.
I really appreciate Henry Link's comment here describing the different types of chromatic aberration.
The Ravens were in the center of the field and they appeared to be completely surrounded in a green haze. I am not sure if I shifted the focus to try to eliminate the problem. I will try to recreate the observation and get back to you.
-thanks, q
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Old Wednesday 26th September 2018, 22:11   #9
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There is magenta involved.
Attached are two copies of the same photo that I took through my binoculars. The first is an overview (resized to fit birdforum size constraints, otherwise exactly as seen by my phone) and the second is was generated by zooming in to the overview on my computer and screenshotting the raven.
This photo was taken by putting my binoculars on a tripod (looking through my kitchen window) and holding the lens of the phone camera where one's pupil would be if looking through the binoculars with their eyes. Any clarity issues are introduced by my shoddy camera work as I focused precisely with my eyes before snapping photos.
This is a very cooperative Raven in a Tamarack (pretty rare tree here) about 40-50 yards away.
-thanks, q
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Old Wednesday 26th September 2018, 23:20   #10
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Classic lateral color. It may be possible to minimize it with careful IPD setting and trying to keep the eye pupils as well centered on the exit pupils as hand holding allows.

All binoculars will show some of this. It's highly variable with distance from the field center and pupil alignment, so comparisons really need be made with both binoculars trained on exactly the same target, placed exactly the same degree off-axis and under exactly the same lighting conditions.

Here are some photos of the target I use for the purpose. The binoculars mentioned are the Zeiss 8x56 FL and 8x54 HT.

https://www.birdforum.net/showpost.p...61&postcount=4

Henry

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Old Wednesday 26th September 2018, 23:48   #11
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https://www.birdforum.net/showpost.p...9&postcount=14
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Old Thursday 27th September 2018, 01:14   #12
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Originally Posted by dries1 View Post
For a 8X42, the SF is indeed the longest on the market. I personally do not mind the length, with the new rumors of the SF 8X32, it will be curious on what others think of it if it is bigger than the FL 8X32, no doubt it will be. I remember when members criticized the EDG 8X32 as being too big for an 8X32, it still blows away many 8X32s on the market today with respect to CA and glare.

P.S. leica makes a great glass, perhaps it is the vision of the persons who keep harping on CA. If Leica does not work for an individual, go with another brand, there are many today to choose from.

Andy W.
Bless you, Andy; you may someday qualify for curmudgeon first class. Taking your direction a little further, the whole problem may be solved by adding a few $$$$$$ to the equation. These companies are trying to keep their doors open, and they can't do that by catering to the few but vocal nit-noids

Cheers,

Bill
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Old Thursday 27th September 2018, 02:37   #13
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Originally Posted by Lightbender View Post
Might it not be the case that Leica bins, compared to the other alpha brands, do NOT show significantly higher levels of CA in purely TECHNICAL terms, but that existing chromatic aberrations are just more VISIBLE because of Leica’s superior (?) contrast?
Ha! No.

[Said as one who owns and appreciates a number of Leica binoculars, as well as many others for comparison].

--AP
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Old Thursday 27th September 2018, 05:54   #14
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"What compromise/s will Leica have to make if their optical design remedies this CA?"

Leica beat the drum over their supposed introduction of fluoride glass in the objectives of the Ultravid HD, yet an improvement in lateral CA was not visible to the great majority of users reporting to this forum. To Leica's advertising department's credit, they only promised (judging from the drawings, showing an object with (before) and then without (after) a surrounding purple haze) an improvement in longitudinal CA, which as Henry points out, is mostly invisible in binoculars in the first place. This logically leads to the conclusion that they must somehow "compromise" the design of their eyepieces in order to attain the improvement the OP speaks of.

Still and yet. The noteworthy, and in many reviewers' opinions disastrous, flaw of the original Zeiss Victory was lateral CA. This was merrily remedied in the Victory FL, which actually seems to contain bona fide second rate fluoride glass in the objective, and greatest thanks to Henry for the tests and analysis leading to that conclusion. The Zeiss FL still represents the standard in lateral CA correction, with the possible exception of the Kowa Genesis, which may use actual freakin' fluorite crystal in the objectives!, supporting the general idea of lateral CA coming primarily from air spaced objectives.

So is Leica fibbing on the use of fluoride glass in the objectives, or perhaps using relatively poor formulations of such? This would be my guess, considering that Zeiss's FL fix did not change their classic field correction, and therefore probably did not change their eyepiece design. On the other hand, as Henry points out, the Leica lateral CA could originate in the eyepieces, and their objectives are now wonderful and nobody can see it. This would not be my guess, from the rather specious evidence that the Zeiss Victory (original and FL) and Leica Ultravid (original and HD) field corrections are not very different.

To make a long story short (too late now), it beats me, but Leica CA doesn't bother me much in the first place, so I don't really care, emotionally. Only as a hard cold nitnoid.

RonH

Last edited by ronh : Thursday 27th September 2018 at 06:06. Reason: Covering up fact that I don't know what I'm talking about.
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Old Thursday 27th September 2018, 06:25   #15
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Quincy88, thanks. Not trying to tell you what you saw, but I wonder if it might it be possible that the CA type in your view described earlier and the image shown are different due to different focusing. If you had focused closer in that view than in this photo then that may have been sphero-chromatism.

Henry, your response to that will also be valued, of course. Also, Leica and CA has been a subject on here (and elsewhere) for years. I think we can assume that it is the same type or types of CA in all recent Leica models. BTW what I have noted in Leica is lateral CA. That was not in normal/"natural" use but in a deliberate attempt to evoke it! Now, if you can get ahold of a Leica, test and report on its CA in this forum I am sure I will not be the only one who will greatly appreciate it.

RonH, as I am about to send that in I see your post. Good to hear, and learn, from you after--unless I have not been alert--what seems ages. Might I ask you to expand on something there: Do you think Leica CA will not "bother" you "much" in the raven situation of the OP?

Mostly I am trying to suss out whether Leica might perfect their binoculars with CA also tackled further in a new generation of models. I want a smallish 7x. Nikon will probably replace the EDG line. They might still have a 7x and that might be small enough. Do I wait for these things or splash out on the present Ultravid or go for the Opticron 7x...

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Old Thursday 27th September 2018, 06:41   #16
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Adhoc,
You are right. I am not convinced that what I saw today was the same phenomenon that I originally observed. I reported it faithfully in my monarch 7 review. What I posted today was what I was able to document when nuance was requested, but I actually didn’t understand that nuance at the time of the initial observation. I distinctly remember seeing a green cloud near field center and it was on the other side of the bird.
For example, the photo I posted shows a SLIGHT green hue on the left side of the bird and a strong purple hue on the right. What I remember from the other day was a pronounced green cloud on the bottom right of a bird in flight.
-as you were, q
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Old Thursday 27th September 2018, 11:31   #17
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Originally Posted by henry link View Post

If the object is moved off-axis is the green fringe then limited to one just side of the object (either facing the center or facing the field edge) with a red/magenta fringe on the other side of the object? If yes, then it's latitudinal CA.
Henry,

The green/blue fringe can hardly face the field edge.
Refraction is stronger for light of short wavelengths, for example blue, which means that the blue/green fringe will typically face the center with the purple/red fringe on the other side towards the field edge. Perhaps you were thinking of a set with over-corrected lateral CA---I have never come across such a set, maybe you have.

Peter.

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Old Thursday 27th September 2018, 14:20   #18
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Hi Peter,

I guess this depends on whether the "object" is a black bird surrounded by bright sky or a white bird surrounded by a dark background or my test target's white strips of tape stuck on a black background. When I first cooked up the test target I thought of the white strips as the bright "objects" that provide the photons for the color fringes that stand out against the relatively light free black areas. Looked at that way the green/blue fringes in the examples I linked to face the field edges.

Henry
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Old Thursday 27th September 2018, 14:46   #19
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Ronh,

Happy to see a new post from you after a long time. The small cohort of real scientists here is all there is between the rest of us and the rabbit hole.

Henry
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Old Thursday 27th September 2018, 15:27   #20
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Hi Henry,

I does depend on the colors of the target and background.
For me the target was black on white background, whereas your test target was white on a black background. This reminds me of an old joke about a football match that was cancelled because the two teams were wearing identical jerseys, despite the fact that one team claimed their jerseys were white with black stripes, and the other team's jerseys were black with white stripes.

Peter.
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Old Thursday 27th September 2018, 15:35   #21
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The Zeiss FL still represents the standard in lateral CA correction, with the possible exception of the Kowa Genesis, which may use actual freakin' fluorite crystal in the objectives!, supporting the general idea of lateral CA coming primarily from air spaced objectives.
Hi Ron,

AFAIK Kowa only use fluorite crystal in their 55 mm and 88 mm scopes. The Genesis binoculars and some of their other scopes use so-called XD glass, probably containing fluorine ions.

I was at Photokina yesterday and Vixen were exhibiting a new 55 mm f5,5 fluorite apochromatic refractor. It's probably primarily intended for use with field flatteners as an astrograph and, IIRC has four elements and is the first Vixen fluorite refractor for decades. I asked the Japanese salesman/engineer if fluorite still offers advantages over FPL-53. He answered in the affirmative - I don't know.

Regards,
John (Russell)
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Old Thursday 27th September 2018, 16:01   #22
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John,

I'd be interested to know what the advantages of Fluorite are. I recall Roland Christen of Astro-Physics saying about ten years ago that he could make essentially the same scope, whether the material was Fluorite or FPL-53

I guess you would have mentioned any new Kowa spotting scope at Photokina? Where is that new 100mm TSN Fluorite?

Henry

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Old Thursday 27th September 2018, 17:06   #23
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Henry,

Well one theoretical advantage of fluorite is its homogeneity, but if it's only one of fifteen elements that is probably of no consequence. Disadvantages are its vulnerability to moisture, fragility and I believe a relatively high coefficient of thermal expansion.

I didn't ask about new scopes but there was a hint that there might be new 44 mm Genesis bins next year to address some customer issues (big eye cups?).

John
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Old Friday 28th September 2018, 06:18   #24
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Adhoc,
In a situation like that I would certainly have noticed lateral CA in a Leica. Any normal birder should have been irritated, but I would have probably thought "lateral CA, cool".

Tringa45,
Thanks for straightening me out on the Kowa Genesis objectives. I guess crystal would be wasted at 10.5x, but judging from most reviews, Genesis objectives must have some pretty good fluoride glass.

Hi Henry,
Golly, thanks, but if it's up to me the rabbit hole must be full of lost souls by now. I stlll look in most nights though, especially for contributions from you.


Ron
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Old Friday 28th September 2018, 08:43   #25
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Ron, I am just tripping round the rim of the rabbit hole and that is not cool :-( Maybe I should go back from these woods, relax, and wait for a report someday of a 7x that is sharp, bright, color-neutral, with no aberrations that bother, and no chromatic at all :-) (And smallish of course I insist.)

Last edited by adhoc : Friday 28th September 2018 at 09:39.
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