Telescope Reviews: ED glass in binoculars

This discussion continued from a recent post of mine to the refractors forum regarding ED glass

ED means that the glass has Extra-low Dispersion. That by itself has no meaning as far as color correction and no bearing on the final color correction of a doublet, triplet or multi-element lens. Dispersion does not govern the color correction of a lens. The term "ED" does not imply any particular level of color correction. There are extra-low dispersion glasses that produce bad color correction, and there are high dispersion glasses that produce excellent color corrections.

The term "reduced secondary spectrum" is used to describe a lens that has better than achromat correction, but it will still have residual color. If a lens color error is not specified as apochromatic, you can pretty much bet on the lens still being a simple achromat. In many cases the term ED is nothing more than a marketing ploy.

From the notes of a world renowned lens designer
“The term ED means Extra-low Dispersion. A lot of glass has extra-low dispersion (generally any glass having a Vd value > 70). Not all glasses having this property will produce measurably better color correction than a standard crown-flint combination achromat. Measurably better would be better than 1 part in 2000 color error over the C-F spectrum (red to blue-green). Therefore, you can easily claim to have a (Semi-ED) lens if you use the common design FK5 crown and SF1 flint, yet have exactly the same color error as any normal achromat.

It is not ED that produces better color correction. It is the fact that some glasses, notably the ones using fluorite elements in their construction, have a property known as abnormal dispersion. This property allows two dissimilar materials (such as crown and flint) to have opposite and equal color errors which cancel when they are combined. Most (not all) of these glasses with abnormal dispersion also are Extra-low Dispersion glasses, but the two properties should not be confused”.
Roland Christen

Color correction of two different achromat scopes can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.

Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens.

From this, you can see how easy it may be to market a small, not so fast, ED scope to the viewing public and have it declared at least by potentially half the viewers that is is nearly color free, and yet it is still an achromat.

ED lenses can be doublets or triplets. Even in a well made ED doublet with well-matched ED glass, a longer F ratio will show better color correction. For instance an ED doublet 80mm f/7 will have less color than an ED 80mm f/5. Generally, no ED doublet with lower Abbe# ED glass will match the performance of higher Abbe# glass.

edz

OK, so how does this relate to ED glass in Binoculars?

Setting aside what ED really means to the end user, take a moment to think about where ED glass is employed in the design and what may be the overall result. In an optical system, color error is far more dependant on the correction of the objective lens than the eyepiece. The ratio of contribution to (longitudinal) color error can be found by comparing the focal length of the objective versus the focal length of the eyepiece. Some may recognize this as magnification. So, at 10x the objective contributes 10x the color error as the eyepiece. There are many eyepieces labeled ED, however, their total contribution to color correction is quite small. In a 20x binocular ED color correction contribution from an eyepiece is limited to correcting 5% of the longitudinal CA error.

So, having two binoculars of apparently the same quality, why is it one binocular can look so much better than another? Refer to the portion above about bias. Color correction of two different achromat scopes (two different binoculars) can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.

Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens. Relate that to a 50mm or 40mm binocular and the color error is already very small due to aperture.

Furthermore, it is not uncommon at all for a binocular to have internal vignette that reduces aperture to something even smaller than the nominal stated based on objective size. Well, at the same time aperture is reduced (benefical towards reducing false color), this increases the focal ratio (also benefical towards reducing false color), reducing some aberrations, including color error. For two equal sized (nominal) binoculars, but one having greater vignette than the other, a portion of the false color is slightly suppressed by the greater internal vignette.

Many designs especially binocular designs, are faster than desirable for optimum ED color correction and their color performance suffers as a result. In the almost uncountable scope discussions that we have, it has been said data consistently shows better designs incorporate higher Abbe# ED glass in an objective of focal ratio significantly more than twice the diameter in inches. For instance, as relates to a binocular, the WO22x70 Apo (apprx f/5.8 to f/6) is specified as APO and as using FPL-51 ED glass (a lower Abbe# ED glass) in a doublet configuration. This binocular still shows some false color. It remains questionable that it produces what would be considered an apochromatic image.

There are many testimonies about scopes using FPL-51 ED glass that still show moderate to significant false color. Triplets using ED glass of various types have consistently proven to have great visual color correction. The Tak Astronomer 22x60 4 element Flourite objective shows no noticable visual color error.

So here we have not only many reasons why it may be quite easy to market ED in a scope and show good results, even though the results might not be entirely a result of a better combination of ED glass, but also we have a number of issues that are specifically binocular related that would tend to lessen even moreso the degree of performance improvement resulting from ED use in a binocular.

Don’t be so overwhelmed by what you read on some websites about the outstanding performance of ED binoculars. There are just as many, if not more, fine non-ED binoculars. How often have you seen reviews of ED binoculars in which the author isolated various tests and reported contribution to overall performance from such aspects such as focal ratio, aperture size, vignette, etc. Could it be that the ED marketed models performance is not entirely due to the inclusion of some ED in the design? I leave that to you to decide.

edz

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