For the same model of roof binoculars which one is less prone for CA 8x32 or 8x42? (1 Viewer)

[binolover]

For the same model of roof binoculars which one is less prone for chromatic aberrations 8x32 or 8x42?
I ask because some of people I asked said that the full size binoculars of the same magnification will handle everything better.
Other people argue that when reducing chromatic aberration the smaller the front lens the better will reduce them.

What is your opinion? Will the full size or the mid size 8x32 of the same model roof ED binocular will be less prone to CA?

 

[Dennis Mau]

Increasing aperture almost always increases CA, especially in lower quality binoculars. Smaller aperture binoculars will have less CA, but it also depends to a large extent on the design of the binocular and the quality of the glass used. In your example, the 8x32 will have less CA than the 8x42, assuming they both have the same F-ratios. Increasing the aperture of a binocular exacerbates the different wavelengths of light focusing at slightly different places on the image plane. The CA in a binocular depends on both the aperture size and the focal length. A binocular with a fast F-ratio and a big aperture will have more CA than a binocular with a slow F-ratio and a small aperture. That is why the older achromat telescopes were so long and thin to reduce CA before they developed better apochromatic glass. If the aperture is increased, the false color is increased. If the focal ratio is increased, the false color is decreased.

https://www.cloudynights.com/topic/929135-the-effects-of-increasing-aperture-on-ca-and-or-sphero-chromatism/

"You’ve probably seen these, but a picture is worth a thousand words. They illustrate the situation with a typical Fraunhofer achromat. In the first diagram, imagine increasing the diameter or decreasing the focal length. In either case, the longitudinal difference between where red and blue converge increases. The table numerically illustrates the relationship. By changing the glass and going with an ED objective, the same principles hold true, but with the different optical densities effecting the angle of refraction (Snell’s law), that shortens the distance between where red and blue converge. The difference between the two focal points is still there, just less, so you can effectively increase diameter and decrease focal length to a much better result. There will still be limits. Hence, going to a triplet design at F6, etc."

 

[binolover]

Increasing aperture almost always increases CA because it magnifies aberrations mor

Thank you for the answer.

Some people said that the binoculars with a very large aperture need ED glass. And not only roof. They said that even porro that pass of 50mm diameter should have ED glass to compensate for the large aperture.

 

[Conndomat]

You should also consider the length of the binoculars!

There may be some models where an 8x32 is better for CA, but this is usually not the case.

Andreas

 

[binolover]

You should also consider the length of the binoculars!

Thank you.
Se you mean the length of the binoculars might compensate and reduce CA?
So a 8x42 have larger diameter lenses but are also longer than 8x32. So the extra length compensate for the extra diameter lenses and might have the same amount of CA?

 

[Tringa45]

There are two types of CA, longitudinal CA which is caused by the objective lenses and lateral CA at the field edge, which is present in almost all eyepieces.
The larger the apparent field of view, the more likely you will see lateral CA.
Longitudinal CA is to some extent dependent on the focal ratio of the objectives but most binoculars are around f/3,5 anyway, so correction of CA is a question of the design and the choice of optical glass sorts and has nothing to do with the size of the objectives.

John

 

[tenex]

Increasing aperture almost always increases CA because it magnifies aberrations more, especially in lower quality binoculars. Smaller aperture binoculars will have less CA, but it also depends to a large extent on the design of the binocular and the quality of the glass used. In your example, the 8x32 will have less CA than the 8x42.

Rubbish, simply false as a generalization. What "magnifies" aberrations is magnification, but manufacturers aren't idiots and typically devote extra attention to the higher powered models, just as to those with larger objectives. I don't think I've ever seen a particular model in a given line stand out as having much more obvious (lateral) CA, and certainly not as a regular function of objective size.

 

[Tringa45]

Rubbish, simply false as a generalization. What "magnifies" aberrations is magnification, but manufacturers aren't idiots and typically devote extra attention to the higher powered models, just as to those with larger objectives. I don't think I've ever seen a particular model in a given line stand out as having much more obvious (lateral) CA, and certainly not as a regular function of objective size.

I temporarily disabled my ignore list and immediately regretted it.
Why does he feel compelled to post when he understands nothing?

John

 

[binolover]

Longitudinal CA is to some extent dependent on the focal ratio of the objectives but most binoculars are around f/3,5 anyway, so correction of CA is a question of the design and the choice of optical glass sorts and has nothing to do with the size of the objectives.

Thank you, John.
For different models of binoculars I expected to involve a totally now design with a totally different type of optical glass.
But for the same model made in 2 sizes 8x32 and 8x42 I don’t expect to use two different type of optical glass.

 

[Tringa45]

Thank you, John.
For different models of binoculars I expected to involve a totally now design with a totally different type of optical glass.
But for the same model made in 2 sizes 8x32 and 8x42 I don’t expect to use two different type of optical glass.

If they were scaled designs then the 8x32 woud have eye lenses 3/4 the size of the 8x42 and only 3/4 the eye relief, but that is seldom the case.
The 8x32 and 8x42 Zeiss SF for instance are radically different designs.
Schott, btw, offer about 120 different sorts of optical glass with varying refractive indices and Abbe nos. and any one binocular would contain multiple sorts.

John

 

[Dennis Mau]

There are two types of CA, longitudinal CA which is caused by the objective lenses and lateral CA at the field edge, which is present in almost all eyepieces.
The larger the apparent field of view, the more likely you will see lateral CA.
Longitudinal CA is to some extent dependent on the focal ratio of the objectives but most binoculars are around f/3,5 anyway, so correction of CA is a question of the design and the choice of optical glass sorts and has nothing to do with the size of the objectives.

John

Not all binoculars have an F-ratio of around 3.5. An f-ratio of 3.5 is unusually low for binoculars. They can vary depending on the size of the binocular from 3.2 to as much 5.0 or more in larger binoculars, and it affects the amount of CA they have, with the faster F-ratios having more CA and the slower F-ratios having less CA.

https://www.cloudynights.com/topic/227145-binocular-f-ratios/

https://www.cloudynights.com/topic/13186-focal-length-in-binoculars/#entry157926

World's Best 8X42: The Zeiss 8X56FL

The image provided by the best current binoculars looks so good that it’s easy to fall into the assumption that binoculars are now so close to optical perfection that further improvement wouldn’t accomplish anything as it would be invisible to the eye. Every now and then I disabuse myself of...

www.birdforum.net

 

[Tringa45]

Not all binoculars have an F-ratio of around 3.5. They can vary depending on the size of the binocular from 3.2 to as much 5.0 or more in larger binoculars, and it affects the amount of CA they have, with the faster F-ratios having more CA and the slower F-ratios having less CA. If you put me on ignore, how are you going to learn anything?

I stated "most binoculars are around f/3,5" and you then repeated my own argument.
Learn something from you? An inflated ego perhaps.

John
PS:- The extensive edit in post #2 is still valid but completely out of date. Slow focal ratio achromatic doublets are hardly made anymore and with modern optical glass it is posible to make a 100 mm f/8 apochromatic doublet.
A Kowa 88 is f/5,7 with a triplet objective and doublet(?) focussing lens. I challenge anyone to see longitudinal CA there.
PPS:- Editing your own naive posts is an insidious tactic but they remain if quoted, as in tenex' post #7.

 

[Maljunulo]

I stated "most binoculars are around f/3,5" and you then repeated my own argument.
Learn something from you? An inflated ego perhaps.

John
PPS:- Editing your own naive posts is an insidious tactic but they remain if quoted, as in tenex' post #7.

That’s only when he doesn’t delete them.

 

[Dennis Mau]

I stated "most binoculars are around f/3,5" and you then repeated my own argument.
Learn something from you? An inflated ego perhaps.

John
PS:- The extensive edit in post #2 is still valid but completely out of date. Slow focal ratio achromatic doublets are hardly made anymore and with modern optical glass it is posible to make a 100 mm f/8 apochromatic doublet.
A Kowa 88 is f/5,7 with a triplet objective and doublet(?) focussing lens. I challenge anyone to see longitudinal CA there.
PPS:- Editing your own naive posts is an insidious tactic but they remain if quoted, as in tenex' post #7.

No, most binoculars do not have an f-ratio of 3.5; a typical f-ratio for binoculars is closer to f/4 to f/6, with only higher-end models reaching a lower f-ratio like f/3.5, but this is not the standard for most binoculars on the market. A binocular with an F-ratio of 3.5 would be an ultra-wide, very fast binocular.

"f/4 to f/5 seems to be where nearly all binoculars are in f-ratios. Why doesn't anyone make an f/3 or an f/8 binocular? Or even folded optics binoculars, what happens when you get away from f/5? Let me guess: price goes up?"

https://www.cloudynights.com/topic/261219-binocular-fratios/

 

[tenex]

Why does he feel compelled to post when he understands nothing?

Actually it's a case of a little knowledge being a dangerous thing. There's usually something to what Denco says... but it's generally incomplete if not entirely irrelevant, as he apparently can't tell. Or it comes as often from worthless sources as respectable ones, as again he can't tell. And he'll try to cover this up when pointed out, so if I respond I always quote the material in question to preserve it. Or he'll change the subject, so now we're arguing about what a typical focal ratio is instead of the thread topic, what models have more CA. Some knowledge goes in, bits of it come back out, but nothing much happens in between. And others should learn from this! But sadly we already have.

 

[jafritten]

Actually it's a case of a little knowledge being a dangerous thing. There's usually something to what Denco says... but it's generally incomplete if not entirely irrelevant, as he apparently can't tell.

Ernst Leitz and Carl Zeiss would have referred to this phenomenon as gefährliches Halbwissen. A very useful term.

 

[Maljunulo]

Dunning and Kruger must laugh their heads off as they read his posts.

 

[Dennis Mau]

It would also depend upon the focal length of the 8x32 and 8x42 binocular you were comparing. If they both had the same focal length, the F ratio in the 8x32 would be greater due to the smaller aperture, but if the 8x42 had a longer focal length it could have CA control similar to the 8x32. F ratio is a big factor in how much CA an optic has, because a slower or higher F-ratio controls CA better.

F Ratio= Focal Length/ Aperture

 

[binolover]

It would also depend upon the focal length of the 8x32 and 8x42 binocular you were comparing. If they both had the same focal length, the F ratio in the 8x32 would be greater due to the smaller aperture, but if the 8x42 had a longer focal length it could have CA control similar to the 8x32

Could we use an aperture like in photo cameras in front of each binoculars’ lens to make the portion of the objective in use smaller or larger depending when we need more or less large front lenses?
You can see in the attached photo how it looks such aperture.
I have seen on an astronomy website a binocular modding technique where they cut circles and the put them in front of the large lenses to reduce the aperture.

So when is too much light we will close the aperture to make the front lens smaller. Front lens smaller means less chromatic aberrations because the focal lens will remain unchanged but the F ration would change to control better chromatic aberrations.

When in low light conditions and when we don’t need chromatic aberrations control because there are not too many colors anyway we would open large the aperture to take the avantage of the light gathering of a very large front lens.

Should such system work well?
Why the current binoculars don’t have adjustable aperture?

 

[Dennis Mau]

Could we use an aperture like in photo cameras in front of each binoculars’ lens to make the portion of the objective in use smaller or larger depending when we need more or less large front lenses?
You can see in the attached photo how it looks such aperture.
I have seen on an astronomy website a binocular modding technique where they cut circles and the put them in front of the large lenses to reduce the aperture.

So when is too much light we will close the aperture to make the front lens smaller. Front lens smaller means less chromatic aberrations because the focal lens will remain unchanged but the F ration would change to control better chromatic aberrations.

When in low light conditions and when we don’t need chromatic aberrations control because there are not too many colors anyway we would open large the aperture to take the avantage of the light gathering of a very large front lens.

Should such system work well?
Why the current binoculars don’t have adjustable aperture?

Exactly, and many astronomers stop down their binocular objectives for exactly that reason. That is a good idea, and it would be interesting to see how it would work. That is exactly Henry did when he stopped down his Zeiss FL 8x56 to reduce CA. You are using the center part of the objective instead of the edge, where there are fewer aberrations when you stop down an objective lens.

World's Best 8X42: The Zeiss 8X56FL

The image provided by the best current binoculars looks so good that it’s easy to fall into the assumption that binoculars are now so close to optical perfection that further improvement wouldn’t accomplish anything as it would be invisible to the eye. Every now and then I disabuse myself of...

www.birdforum.net

Stopping down a binocular

I’ve got a pair of Zeiss conquest HD 15x56 that I like quite a bit but, in daytime viewing, I see too much CA. There’s been some discussion on here about using the objective covers to stop down binoculars, to reduce aberrations like CA. I have the objective covers for this binocular and I’d like...

www.birdforum.net

https://www.cloudynights.com/topic/68956-clear-aperture-and-stopping-down/

https://www.cloudynights.com/topic/532659-stop-down-test-for-binoculars/