"ED" vs "HD" glass

[ScoutMan]

Is there a difference? Is there any advantage of one over the other?

[oncebittern]

Is there a difference? Is there any advantage of one over the other?

ED is Extra-low Dispersion, whereas HD is High Definition.

Further details of exactly what characteristics each has would probably be best found from the relevent manufacturer's websites.

[henry link]

Neither of these marketing terms describes an actual glass type. The family of "ED" glass types consists of a number of different glasses with Abbe numbers between about 80 and 95. All have the potential to reduce longitudinal CA compared to a conventional crown and flint achromat, but only if they are properly matched with other types. The greatest potential for CA reduction comes from the types with the highest Abbe numbers. The binocular manufacturers seldom reveal what "ED" glass type they use and never reveal the matching types.

[Kevin Purcell]

Is there a difference? Is there any advantage of one over the other?

Along with a host of other two and three letter trademarks they're all marketing terms indicating (as Henry explains) the use of low-dispersion (high Abbe number) glass usually a fluorophosphate glass.

Search the forum for a couple of other technical threads.

[FrankD]

I love Henry's comments. I always feel like I learn a bit more about optical design after reading them.

You too Kevin.

Thank you both for sharing your knowledge on the subject at hand.

I guess there isn't any way to determine the Abbe number for the various extra low dispersion glass currently on the market?

[Kevin Purcell]

I guess there isn't any way to determine the Abbe number for the various extra low dispersion glass currently on the market?

Well there is the Refractive index database if you know the manufacturer and type of glass

http://refractiveindex.info

But the difficult bit is getting the bin maker to tell you whose glass they're using and which glass type they're using. See my other post buried on the $320 bin thread (FK and PK versus LaK).

http://www.birdforum.net/showpost.php?p=1510126&postcount=19

But really unless you know the glass type and the design and have seen the outputs from the simulations (and can ask the design "why did you do that?") you might not be much wiser.

Looking in the "small end" often tells you more.

[FrankD]

Good info Kevin. Thanks.

I then have to assume that just because the glass has a higher Abbe number then it does not necessarily mean that it will perform "better" unless all else is equal?

[Kevin Purcell]

Good info Kevin. Thanks.

I then have to assume that just because the glass has a higher Abbe number then it does not necessarily mean that it will perform "better" unless all else is equal?

It just affect the design ... the high Abbe number means lower dispersion (lower LCA) but that comes (as you can see from the diagram) a lower refractive index so it's more difficult to make stronger lenses (another reason why you won't see them in an eyepiece).

ED glass only works when the optical design is optimized for it: it enables lower CA designs.

But that should be true of any "sane" optical designer. Otherwise you'll know when you look through the bin.