-
Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.
Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
Chromatic Abberation definition (1 Viewer)
- Thread starter Justin
- Start date
I always recommend Paul van Walree's Optics Pages. These are oriented to photographers, but the section on CA is fully applicable to binoculars.
Quite understandable is
What is so special about the Zeiss Superachromat lenses?
This is from an issue of the newsletter Camera Lens News on the Zeiss web site. From a different issue, a more involved article is
Achromat, Apochromat, Superachromat - What is the Difference?
Happy reading!
Quite understandable is
What is so special about the Zeiss Superachromat lenses?
This is from an issue of the newsletter Camera Lens News on the Zeiss web site. From a different issue, a more involved article is
Achromat, Apochromat, Superachromat - What is the Difference?
Happy reading!
It is a colored halo usually around a high-contrast object. It may be purplish or yellow/green and if it is evident it definitely makes the image "not perfectly clear".marcus said:
If the "fringe" can be seen everywhere in the image and changes from purple to green during focusing, it is so called "longitudinal CA" and derives from the objective. It means that different colors are not focused at the same plane.
More often the fringes are mainly visible at the edges of the field of view and when the eye is not perfectly aligned with the eyepiece. In this case the fringing comes from the eyepiece and is called "transverse CA" "lateral color" or "chromatic difference of magnification". This means that all colors are nicely focused at the same plane but they form images which don't overlap perfectly.
Examples of purple color fringing are shown in this digital camera review http://www.dpreview.com/reviews/sonydscf828/page16.asp .
Ilkka
AlanFrench
Well-known member
iporali said:
Longitudinal chromatic aberration is a variation of focus on-axis with color. Lenses are generally designed so that they bring blue and red to a common focus, and the region around green focuses close to the lens. At the low powers used in binoculars, longitudinal CA is not an issue.
The color fringing seen along the edges of high contrast objects is lateral color - a variation of magnification with wavelength, and the culpert is generally the eyepiece. (A lens has to handle a view angle of 7 degrees or so, while an eyepiece has to handle a view angle of 50 to 70 degrees or so.)
The other day I was looking at a brightly lit white flower against a dark green forest background. You could nicely see the lateral color, and how it increased as you moved the flower closer to the edge of the field.
Clear skies, Alan
Sorry for hijacking this thread, just a brief question, I promise.
I was wondering if you could explain, quite how the addition of a fluoride-glass lens in the objective of the Zeiss FLs has been enough to remove the lateral color of the eyepiece. I have understood that the eyepiece design in the FL is very similar to the previous Victory (which did suffer from lateral color). I have no clue.
Ilkka
Alan,AlanFrench said:
I was wondering if you could explain, quite how the addition of a fluoride-glass lens in the objective of the Zeiss FLs has been enough to remove the lateral color of the eyepiece. I have understood that the eyepiece design in the FL is very similar to the previous Victory (which did suffer from lateral color). I have no clue.
Ilkka
AlanFrench
Well-known member
Illka,
How do you know that the Zeiss FLs have no lateral color? In spite of a few reports to the contrary, I be very surprised - pleasantly surprised - to find they show no lateral color. I have not used an eyepiece that did not show some near the field stop.
At any rate, the objective can have lateral color too, although the eyepiece usually has more and that is the main issue. You've got me curious. When I get a chance, I'll design a 40mm f/5 achromat and a 40mm f/5 doublet using fluorite. We'll see if the lateral color of the lens improves with the use of fluorite.
Clear skies, Alan
How do you know that the Zeiss FLs have no lateral color? In spite of a few reports to the contrary, I be very surprised - pleasantly surprised - to find they show no lateral color. I have not used an eyepiece that did not show some near the field stop.
At any rate, the objective can have lateral color too, although the eyepiece usually has more and that is the main issue. You've got me curious. When I get a chance, I'll design a 40mm f/5 achromat and a 40mm f/5 doublet using fluorite. We'll see if the lateral color of the lens improves with the use of fluorite.
Clear skies, Alan
Curtis Croulet
Well-known member
The thread on "Zeiss Victory FL" has several messages addressing chromatic aberration ("CA") in this product, which I assume is really lateral color. I'm not very sensitive to CA in binoculars, and I haven't seen it yet in my new 8x42 FL. However, several reviewers do report CA in the FL, but considerably less of it than in other high-end models.
henry link
Well-known member
I would have agreed with Alan about longitudinal CA not being a problem at binocular magnification until I started using the 8X42 FL. It's a case of noticing how much of a problem something is only after you have seen it's absence. The color fringe from longitudinal CA in other 8X bins isn't that wide, but I now think it has a noticeable effect on image quality by lowering contrast and reducing sharpness.
There is lateral color in the FL. It's really not much different in that regard from other good binoculars with the same AFOV. The surprise is how much "cleaner" the center of the field appears in the 8X42 FL without the slight longitudinal CA visible in other 8X bins.
Alan, FWIW you'll need to design a very fast triplet objective (my guess is around f/3.3-f/3.5) using what Zeiss calls "fluoride glass".
There is lateral color in the FL. It's really not much different in that regard from other good binoculars with the same AFOV. The surprise is how much "cleaner" the center of the field appears in the 8X42 FL without the slight longitudinal CA visible in other 8X bins.
Alan, FWIW you'll need to design a very fast triplet objective (my guess is around f/3.3-f/3.5) using what Zeiss calls "fluoride glass".
Last edited:
AlanFrench said:
Alan,
All I know is that the Zeiss FLs are said to have less lateral color than the Nikon SEs, which have significantly less than any roof-prism binoculars that I am aware of. If that really is the case, the "CA-problem" is IMO practically solved. I wouldn't have guessed that an fl-objective can make such a big difference. Of course I believe that traces of lateral color can always be seen - even with the FLs.
Wow, I really look forward to hearing about your results - thanks! I too have been under the impression that most of the lateral color come from the eyepiece, but to my knowledge Zeiss didn't significantly change the eyepieces in the FLs.
Thanks once more Alan, and Henry too!
Ilkka
AlanFrench
Well-known member
henry link said:
Henry,
A doublet should at least answer the question of whether with fluorite or fluor-crown glass (which I assume is the translation to the odd "fluoride glass" - I love it when the ad hype kicks in) reduces lateral color as compared with a doublet with normal glasses. A triplet simply allows a bit more control of aberrations (more degrees of freedom).
Ideally, one should look at the entire system - including prisms and eyepieces, but I do not have the software to go that far. I can do a triplet, though. It may be a little while before I get where I want to be with the doublet comparison.
Keep in mind that longitudinal chromatic aberration is an on-axis aberration, so I don't think it is reasonable to consider it the cause of a reduction in sharpness across the field. I still suspect the eyepieces are the biggest factor in sharpness at the edge of the field (field curvature, astigmatism, lateral color).
Clear skies, Alan
Alan: As you know I do not agree that binoculars do not exhibit longitudinal CA. (I would be interested to know why you think Zeiss employ triplet objectives with a low dispersion element. You seem to suggest that it is a marketing stunt and yet many people including myself see reduced CA.)
As you well know longitudinal CA from an objective increases with objective size and decreases with F ratio (focal length divided by objective diameter). Binocular objectives have a fast F ratio (~F4 or less) which must give rise to a trace of long. CA. Long. CA is consistent with seeing different coloured fringes either side of an object.
Compact binoculars such as the Zeiss 8x20 roof prism show almost no CA to my eyes. This is consistent with the small objectives and the long optical assemblies that suggest objectives with large F ratios.
It seems to me that roof prism binoculars usually show significantly more CA than porro prism ones, and that the CA is consistent with lateral CA in that it consists of a purple halo. I was very disturbed by one well respected 8x32 roof prism binocular. The image seemed to be constructed from a 'true' image, superimposed on a concentric but slightly larger purple image. This was most disturbing when panning the image and I cannot understand how others manage.
It strikes me as odd that a roof prism binocular usually (but not always) shows more CA than a porro prism one. What's more the CA seems to my eyes to be mostly lateral CA in that it consists of a purple halo originating from a slightly magnified secondary image. Curiously the Zeiss 8x30 BGAT showed very little CA of any kind showing that CA can be reduced to levels that I consider acceptable. I have heard that the Zeiss 8x30 BGAT achieved focus by moving the prisms rather than by moving an internal lens that sits between the objective and the prism. Maybe this is not relevant!
As you well know longitudinal CA from an objective increases with objective size and decreases with F ratio (focal length divided by objective diameter). Binocular objectives have a fast F ratio (~F4 or less) which must give rise to a trace of long. CA. Long. CA is consistent with seeing different coloured fringes either side of an object.
Compact binoculars such as the Zeiss 8x20 roof prism show almost no CA to my eyes. This is consistent with the small objectives and the long optical assemblies that suggest objectives with large F ratios.
It seems to me that roof prism binoculars usually show significantly more CA than porro prism ones, and that the CA is consistent with lateral CA in that it consists of a purple halo. I was very disturbed by one well respected 8x32 roof prism binocular. The image seemed to be constructed from a 'true' image, superimposed on a concentric but slightly larger purple image. This was most disturbing when panning the image and I cannot understand how others manage.
It strikes me as odd that a roof prism binocular usually (but not always) shows more CA than a porro prism one. What's more the CA seems to my eyes to be mostly lateral CA in that it consists of a purple halo originating from a slightly magnified secondary image. Curiously the Zeiss 8x30 BGAT showed very little CA of any kind showing that CA can be reduced to levels that I consider acceptable. I have heard that the Zeiss 8x30 BGAT achieved focus by moving the prisms rather than by moving an internal lens that sits between the objective and the prism. Maybe this is not relevant!
Hermann
Well-known member
Leif said:
The Zeiss 8x30 BGAT focusses by moving the objective lenses, just like the old 10x40 BGAT did. Zeiss is certainly no stranger to making binoculars with reduced CA. In the 10x50 porros of the early 1960's Zeiss used high index glass to reduce CA. I don't have the literature at hand at the moment, but I seem to remember Zeiss claimed to have halved CA compared to conventional binoculars in the promotional literature at the time.
Hermann
henry link
Well-known member
Leif,Leif said:
I have a Zeiss brochure from 1992 which has a cut-away view of the 8X30B Dialyt. It shows a moving objective with no protective element in front. Perhaps one was added later to increase water resistance.
BTW the objective doesn't appear to be anything unusual, a cemented doublet. The 10X50 Hermann mentioned had, I beleive, an air spaced doublet.
Henry
Last edited:
AlanFrench
Well-known member
Leif said:
Henry,
My opinion that longitudinal CA is not an issue with low power optical systems, such as binoculars, based largely on discussions with folks who know far, far more about optics than I do.
I had two telescopes here for a couple of months several years ago - a TV Pronto and a UO 80 (an 80mm f/6.4 or so achromat). Used at low powers the images, on axis and for much of the field, were sharp and color free. At high powers subjects showed color fringing, and the views were far inferior to those through a lens using an ED element.
The folks I have talked to about optics design and make diffraction limited systems, so they are not binocular experts. It would be nice to find someone who actually designs binoculars willing to share optical knowledge with us inquisitive folks. (There do not seem to be any "experts" on birding optics with much optical knowledge.) I have always been curious about how the prism's color effects are taken into account. One optical designer once told me the big old battleship binoculars were designed as a unit, and that the lenses by themselves would not work as well as a standard telescope achromat.
Clear skies, Alan
I have recently purchased the Leica 10x42 BNs and have noticed some CA in certain situations (high contrast + long distance) - it appears as a slight yellow halo to one side of the object being viewed.
It is a very strange phenomena, however, because in nearly identical situations later in the day, I saw no CA at all (I was even looking for it!!).
The angle of light seems to be a crucial factor in CA, and perhaps variation in our own eyes?
Cheers,
Poephila
PS
These glasses are otherwise sensational!
It is a very strange phenomena, however, because in nearly identical situations later in the day, I saw no CA at all (I was even looking for it!!).
The angle of light seems to be a crucial factor in CA, and perhaps variation in our own eyes?
Cheers,
Poephila
PS
These glasses are otherwise sensational!
Last edited:
henry link
Well-known member
AlanFrench said:
Alan,
I would agree about longitudinal CA being unimportant if binoculars had f/6.4 objectives but they are usually f/4 or less. The Zeiss 8X42 FL is the only bin I have tested that could have it's magnification boosted 5X and still show very little longitudinal CA. Other bins look truly terrible when their magnification is boosted that much. At binocular magnification the CA is visible, just not very obtrusive. I think once you see a binocular image without longitudinal CA you'll realize you have been seeing it all along, but ignoring it.
I have one experience that seems to confirm that binoculars are designed as a unit, including the prisms. I removed the objective (about f/3.6) and the eyepiece from a Nikon 8X30 E and placed them in a hollow tube thinking the image would be superior without the porro prism. In fact it was far worse, extremely soft with lots more chromatic aberration than with the prism in the light path.
There is certainly quite a difference between the small community of APO astronomical telescope designers who join in internet debates about their products and talk to customers on the phone, and the nameless, faceless people who design binoculars for the big optics firms. It would be very interesting if the binocular designers, whoever they are, would occasionally tell us something directly about their ideas instead of having everything coming to us dumbed down by the advertising department.
Henry
Last edited:
