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Binocular issue or...

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/20/2013 at 15:45
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Okay, I have a relatively new pair of Bushnell Legend 10x42 binocs.  When trying to ID soaring birds at distance against the bright sky, I am getting an optical anomaly. Below is a photo that I altered to illustrate what I am seeing.  I'm not sure whether this a chromatic aberration caused by faulty binocular optics, or if it is an effect of retinal after-image from gazing through the binoculars at a silhouetted object against the bright sky. Either way this effect is interfering with my ability to pick out fine details while birding.  Anyone else experiencing something similar, or have an opinion?  Thanks.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/20/2013 at 19:41
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That's classic chromatic aberration. That's not a problem with your eyesight, it's an optical flaw, present in a lot of different optics to varying degrees. It's caused by the different wavelengths of light not focusing on a common plane. It's most noticeable when viewing high contrast objects, such as dark objects silhouetted against a bright sky, or when viewing something with a very light colored surface meeting a very dark colored surface. The only solution is to buy optics with ED or extra low dispersion glass or complex optical designs that correct for this dispersion. Even then, C.A. is usually present to some degree, though it may be minimized. It's very difficult (and expensive) for optics designers to completely eliminate C.A.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/21/2013 at 03:38
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Also keep in mind that there are verying degrees of quality when it comes to ED glass.
The higher end binoculars have better  glass that does a better Job of reducing/eliminating CA than lower end binoculars.
 
Just because a binocular uses ED glass it still may not do a very good job at getting rid of CA.
 
With the high end binoculars you may have a little CA on the extreme edge of the FOV where it is not bothersome. The center of the view will be free of CA.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/21/2013 at 09:57
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Originally posted by timberbuck timberbuck wrote:

 Just because a binocular uses ED glass it still may not do a very good job at getting rid of CA.
 


Absolutely correct. The terms "ED," "HD," etc. are often nothing more than marketing terms, which may or may not mean the optic does a good job of controlling CA or other optical aberrations. Not all "ED/HD" optics are created equal, nor even close.

One thing's for certain, though... to get a view with little to no apparent CA, the optic will have to incorporate some kind of optical design incorporating complex "APO" (apochromatic) lens arrangements, "FL" (calcium fluoride crystal glass, lens elements doped with zirconium fluoride) lenses, or "ED" (extra low dispersion) lenses containing various rare earth elements, i.e., the optic will be more expensive, and usually quite a bit more expensive.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/26/2013 at 22:46
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Chromatic aberration it is...for sure.  However it is not an optical flaw as far as I know.  This subject comes up and goes round and round on every single optics discussion forum.  CA is present at all times in all binoculars.  It can be lessened in effect with ED or HD glass, some sort of glass with ultra low dispersion characteristics in the objective lens.  The problem is that with a single objective lens the three major colors, red, blue, and green focus at different distances behind the lens.  A single ED lens element included in a doublet objective will focus two of the colors at the same point and lessen the third band distance from the other two.  A triplet objective (usually with an air space) will tend to focus all three color bands at very close the same point.  However CA can't be eliminated, only controlled.  Some people have a terrible affinity for seeing this.  It also shows up in cheap cell phone photos too.  Some people get so involved in the optics hobby they take off and go looking for CA.  The problem is that once you find the damn stuff, well you're stuck with seeing it, but it looks like the CA gremlin found you.  Some people, including me (and I thank my lucky stars for the good fortune) don't see it.  Doesn't mean its not there, I just don't see it.  

As has been properly pointed out, not all ED glass is created equal, and frankly sometimes some sort of designation is used as a marketing ploy to grab a few sales.

Now the curvature of the field of the Bushnell Legend Ultra HD and the curvature present in my own vision don't get along real well.  Having said that, the Legend Ultra HD generally has a pretty good reputation for controlling CA.  The thing is there is another Bushnell Legend binocular which was replaced by the Legend Ultra HD.  This does not have ED glass.  You say a recent Legend, so everyone (me included ) is thinking Legend Ultra HD.

Now since you have it, you have only one hope with it.  That is to pray you have a defective unit and that this one can be fixed. Some collimation issues or prism misalignment may have occurred and be messing with the light path. Good luck with that as my experience with Bushnell service and warranty...well it sucks for lack of a better term.  But service repair is your option.  If Bushnell says there is nothing wrong with it, then you can sell it with a clear conscience.  OT site sponsor SWFA has a trade in program I have used with satisfaction, so if it is a non defective glass, you might try that route if you don't want to sell it yourself.  SWFA has the advantage of having a market for optics and for having staff that knows optics.

One of the less expensive ED binoculars that has a reputation of CA control rivaling the best is Zen Ray's ZEN ED 3 series binoculars.  I think you will need to get a different binocular when this all shakes out.


Edited by Klamath - November/26/2013 at 22:58
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/27/2013 at 17:51
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Go look through a pair of Vortex 8x42 Razors.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/28/2013 at 10:40
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Originally posted by Klamath Klamath wrote:

However it is not an optical flaw as far as I know.  This subject comes up and goes round and round on every single optics discussion forum.  CA is present at all times in all binoculars.

That depends on one's definition of "flaw." My definition of the term was not meant to imply the binocular in question is defective. It may very well be performing entirely to design specs. What I mean by "optical flaw" is that this is one negative trade-off of non-ED binos. It is an optical flaw in the sense that there's no such thing as a "perfect" optic with a complete absence of any negative optical traits. There are always design trade offs that have to be made to meet price, form factor, ergo, and/or specific image characteristics goals. The term "aberration" itself means flaw or deviation from theoretical perfection. You're correct that it's difficult, if not impossible to completely eliminate all CA in an image because of the fact that light passing through lenses separates into different color wavelengths that each focus on slightly different planes (dispersion). Nevertheless, some optics (usually more expensive "ED" types) obviously do a much better job of minimizing the degree of visible CA, and therefore are "less flawed." I know you know all this; just wanted to make sure the OP understands that the bino isn't necessarily defective and try to help him or her understand the concepts and terminology to look for when selecting binos that do a good job of controlling CA.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/28/2013 at 13:18
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I probably should have said CA is an optical characteristic.  One that affects some people more than others, and optical engineers need to work their design with this characteristic in mind as they envision their final product.  Like all characteristics, some we like better than others, and it affects people differently, pretty seriously in the case of the OP it would seem.  An optical instrument is a series of trade offs.  They have good points and bad points, sort of like our friends, we take the good with the bad, and if it works out the good outweigh the bad.

Anyway, have a Happy Holiday Season everyone. Big Smile
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