Chromatic Aberration, CA, what, why, how? (3 Viewers)

[Thrupenney Bit]

Hi Gang,
As a newbie seeking advice, I keep seeing the comment, chromatic aberration, usually abrieviated as 'CA', frequently popping up when discussing binoculars and telescopes.
From reading reviews and comments, it seems to me it is an optical affect that outlines the target object being viewed - bird - with colours as a result of the optics used to view it.

Is it down to the optics used - all bins/scopes of a certain make or model?
i.e. does all makes of x cause it or have it, whereas make y model z is CA free?

Is it down to your personal eyesight or perception?
i.e. I view scope x as being fine but another person views the exact same scope as having CA, therefore it's their own personal optical makeup that has an effect on it?

Curious of Devon.
( possibly about to look at upgrading scope so needs to know! )

 

[Pinewood]

Hello thruppenybit,

Here is an article on chromatic aberration. The most egregious chromatic aberration appears as colour fringing. For binoculars, CA might appear as a sligtly less sharp line, say the edge of a dark building against a cloudy bright sky.

Yes some people are more sensitive than others to CA. When it comes to optics trying before buying is ideal, especially with the higher magnification of 'scopes. Zeis had a model, the 8x40 Victory I, which showed colour fringing under extreme conditions, like a black building against the sky. It was not noticeable otherwise, but it was certainly there. Some binocular reviewers like to increase the magnification of a binocular with a booster to check for CA. Their results are good to know but may not be of consequence in use.


Stay safe,
Arthur

 

[Thrupenney Bit]

Hello thruppenybit,

Here is an article on chromatic aberration. The most egregious chromatic aberration appears as colour fringing. For binoculars, CA might appear as a sligtly less sharp line, say the edge of a dark building against a cloudy bright sky.

Yes some people are more sensitive than others to CA. When it comes to optics trying before buying is ideal, especially with the higher magnification of 'scopes. Zeis had a model, the 8x40 Victory I, which showed colour fringing under extreme conditions, like a black building against the sky. It was not noticeable otherwise, but it was certainly there. Some binocular reviewers like to increase the magnification of a binocular with a booster to check for CA. Their results are good to know but may not be of consequence in use.


Stay safe,
Arthur

Thanks Arthur, that's very informative!
Have given it a quick read and will go back over it again slowly.

Having used top quality optical microscopes for extended periods of time when working it's something I'd never come across, though it is mentioned in the article. I've not noticed it when using my binoculars. Recently I've dug out my old scope to use in bird counts ( with x20W eyepiece ) and haven't noticed it there either, but I'm walking round a wetland, counting birds, being distracted by people out for a stroll chatting to me etc. so haven't really concentrated just on the image. Equally haven't noticed it there either!

I hope to be checking out some much newer scopes with higher magnification zoom eyepieces at the weekend, so need to at least have an idea of this famed 'CA' and make sure I can/can't see it if I'm thinking of purchasing something.
Thanks again.
Happy days!
Q

 

[tenex]

Here are a couple of good articles you might enjoy reading:
Optical Aberrations
The Seven Deadly Aberrations
What people are mostly talking about here is lateral CA. Besides being better controlled in some instruments than others, it can be more or less apparent according to conditions, bothers some more than others, and gets worse in the outer field than around the center.

 

[beacon hill]

Is it down to your personal eyesight or perception?
i.e. I view scope x as being fine but another person views the exact same scope as having CA, therefore it's their own personal optical makeup that has an effect on it?

Curious of Devon.
( possibly about to look at upgrading scope so needs to know! )

Great point. At our optics weekends many customers buying, whether first time or second / third pair upgrade
will not notice any CA until I point it out to them. Maybe it is there and visible, but they are not looking for it.

 

[Thrupenney Bit]

Morning.
Thanks both.
Will read the articles and inwardly digest.
All helps build my knowledge.
I will make a mental note to try and be aware if looking through a new to me scope and not just be bowled over by something new. Might be difficult though!!

 

[Thrupenney Bit]

Morning.
Thanks both.
Will read the articles and inwardly digest.
All helps build my knowledge.
I will make a mental note to try and be aware if looking through a new to me scope and not just be bowled over by something new. Might be difficult though!!

Now I've woken up a bit, I've gone through both articles mentioned by tenex.
Definitely need to re-read several times to get to grips with it.
I get the impression that a lot of the fundamental research was carried out very early by the then pioneers, von Siedal and Petzval mentioned in the first article.
As an aside, when move house a few years back and I tried to resolve exactly what the brass cylindrical object, around 9" long and 3" diameter was. I'd played with it since childhood, moving the winder up and down and watching the lens inside move but had no idea exactly what it was.
I discovered it was a very early Petzval lens, manufactured in London 1800's and an early example from the factory/workshop where made. It was an early portrait lens for the camera of the time.
I had no idea Petzval was such a pioneer in optics.

 

[TheBlackGrouse]

Like Pinewood said: ''Yes some people are more sensitive than others to CA''. Personally I don't care about CA at all, but a flat image and the rolling ball effect can drive me crazy. It is very well possible that your eyes and brain simply don't notice CA, or they ignore it. Maybe you just don't care like me, in that case you can concentrate on other characteristics that can be dealbrakers like field of view, eye relief, edge sharpness etc.

 

[Thrupenney Bit]

Like Pinewood said: ''Yes some people are more sensitive than others to CA''. Personally I don't care about CA at all, but a flat image and the rolling ball effect can drive me crazy. It is very well possible that your eyes and brain simply don't notice CA, or they ignore it. Maybe you just don't care like me, in that case you can concentrate on other characteristics that can be dealbrakers like field of view, eye relief, edge sharpness etc.

Thanks for your comment.
It's an intriguing topic! I have no idea if I'm aware of it or not, but something I hope to keep in mind when looking down some modern optics at the weekend.

 

[Patudo]

Are you really sure you want to go down that rabbit hole? Once you teach yourself to see CA you'll never be able to unsee it, and you could end up spending significant dosh in the quest to find optics that minimize it.

Seriously, sometimes ignorance is bliss.

 

[Thrupenney Bit]

Are you really sure you want to go down that rabbit hole? Once you teach yourself to see CA you'll never be able to unsee it, and you could end up spending significant dosh in the quest to find optics that minimize it.

Seriously, sometimes ignorance is bliss.

Oh fair comment.
I must admit to not seeing it yet, I just wanted to try and understand what it was.
As said, despite using top notch microscopes in my working life I've not been aware, nor using my bins and older scope.
I hopefully will be at a field day this weekend, and want to look at other scopes and just be aware of what things to look out for... or not!!!

 

[CMB]

Oh fair comment.
I must admit to not seeing it yet, I just wanted to try and understand what it was.
As said, despite using top notch microscopes in my working life I've not been aware, nor using my bins and older scope.
I hopefully will be at a field day this weekend, and want to look at other scopes and just be aware of what things to look out for... or not!!!

I am one of the unfortunate folks who are sensitive to CA. It is not something I look for, and it is not something I trained myself to see. When I am scanning with optics, and hit a high contrast area, it simply is there. Red on one side of an object, blue on the other. There's green in there too sometimes.

How thick/wide the red and blue outline are, and how visible it is in the center of the field of view (if at all) depends on how well corrected the optics are.

Some people don't see it at all. My spouse does not see CA. And I have several birding friends who do not see it. I can set a scope up so that CA is very visible to me, and they can't see it.

If you look at some high constrast objects with a variety of different brands of optics, and don't see it, then be very happy and move on. You will have a much wider range of optics to select from in terms of price and brands.

 

[Thrupenney Bit]

I am one of the unfortunate folks who are sensitive to CA. It is not something I look for, and it is not something I trained myself to see. When I am scanning with optics, and hit a high contrast area, it simply is there. Red on one side of an object, blue on the other. There's green in there too sometimes.

How thick/wide the red and blue outline are, and how visible it is in the center of the field of view (if at all) depends on how well corrected the optics are.

Some people don't see it at all. My spouse does not see CA. And I have several birding friends who do not see it. I can set a scope up so that CA is very visible to me, and they can't see it.

If you look at some high constrast objects with a variety of different brands of optics, and don't see it, then be very happy and move on. You will have a much wider range of optics to select from in terms of price and brands.

Thanks for your thoughts.
I'll keep my fingers well and truly crossed when having a look on Sunday!

 

[Hermann]

One thing to bear in mind is that with a scope at higher magnifications it's not just the colour fringing that affects the view. It's also a loss of contrast across the whole field of view - the image looks a bit "mushy" for want of a better word.

For instance, I once compared the old Leica Televid 77 to the Apo-Televid 77: At 20-30x there was some difference, but the image in the Televid was still pretty good, just not as vivid and "clear" as in the Apo. At higher magnifications (especially at 40x and above) the difference was becoming more and more obvious the higher the magnfication was. I've had similar results when comparing a Nikon II to a Nikon ED II or a Kowa TSN-1 with a TSN-3.

In other words: I'm not really susceptible to CA. I only "see" it when it gets really bad in binoculars. But with scopes, especially at high magnifications, things are quite different.

Hermann

 

[Thrupenney Bit]

One thing to bear in mind is that with a scope at higher magnifications it's not just the colour fringing that affects the view. It's also a loss of contrast across the whole field of view - the image looks a bit "mushy" for want of a better word.

For instance, I once compared the old Leica Televid 77 to the Apo-Televid 77: At 20-30x there was some difference, but the image in the Televid was still pretty good, just not as vivid and "clear" as in the Apo. At higher magnifications (especially at 40x and above) the difference was becoming more and more obvious the higher the magnfication was. I've had similar results when comparing a Nikon II to a Nikon ED II or a Kowa TSN-1 with a TSN-3.

In other words: I'm not really susceptible to CA. I only "see" it when it gets really bad in binoculars. But with scopes, especially at high magnifications, things are quite different.

Hermann

Thank you Hermann.
My scope has a x20w eyepiece and so far I've not been aware of it but if I'm looking through higher magnification eyepieces it might become apparent.
I will bear it in mind when sampling the scopes.
Thank you for that thought!

 

[TheBlackGrouse]

One thing to bear in mind is that with a scope at higher magnifications it's not just the colour fringing that affects the view. It's also a loss of contrast across the whole field of view - the image looks a bit "mushy" for want of a better word.

Hermann

Correct, my text here was mainly about binoculars where it's just the colour fringing, with high magnification scopes it's a different story.

 

[Thrupenney Bit]

As I'm considering updating my scope, and if I do will probably experience zoom eyepieces for the first time, this is something I need to be aware of.
That's the reason I'm picking everyone's brains here, and finding peoples comments very helpful.

 

[fazalmajid]

CA is caused by glass not having a constant refraction index as a function of wavelength, which means different colors are focused in different places. Generally the way to correct it to use glass of different characteristics, specially ED or fluoride glass. Look at stark bright/dark transitions at the edge of the frame and if you can see rainbow or purple fringes there.

 

[Thrupenney Bit]

CA is caused by glass not having a constant refraction index as a function of wavelength, which means different colors are focused in different places. Generally the way to correct it to use glass of different characteristics, specially ED or fluoride glass. Look at stark bright/dark transitions at the edge of the frame and if you can see rainbow or purple fringes there.

Thank you.
I'm starting to understand it is because different wavelengths behave differently and that ED glass fluorite etc can make a difference and possibly improve things.

Hopefully tomorrow I'll be able to sample different scopes and yes, will be taking all this advice with me in my head and checking out fringes of objects.
Thanks again.

 

[fazalmajid]

Also have a look at point light sources in the periphery. There are other types of image-degrading aberrations like coma, where points grow "hair" (coma means hair in Greek), astigmatism, spherical aberration and so on.