In amateur astronomy, one can now buy a 100 mm diameter objective employing "ED" (extremely-low dispersion) glass as one of the doublet's lenses, plus the optical tube and focusser, for about 800 EUR (ordinary achromatic doublet costing 1/3 of that). It offers massive improvement in chromatic aberration.
Such a massive lens has enough material to make hundreds of microscope objective lens elements, so ED glass could be used even in cheapest objectives. Why are such glasses not more common in microscopy?
I assume these exotic glasses are only employed in "Fluor"-class objectives (they often don't use real calcium fluoride anymore, do they?).
ED low-dispersion glasses
Re: ED low-dispersion glasses
Nikon, among others, made/makes extensive use of ED glass in their higher end objectives for both compound and stereo microscopes - as well as in their better telephoto lenses for cameras. The ED objectives for stereo scopes such as the SMZ-U are fairly massive and use near telescope and binocular amounts of this glass.
The cost of ED glass elements is probably driven not just by the elements added to reduce dispersion, but increased costs to maintain precision. Plus a bit of charging what the market will bear. I do know that fluorite glasses were harder to grind and polish into good lenses and that some of the ED additives that were tried tended to introduce strain (making the glass unsuitable for polarization or DIC - prime markets for higher end objectives). And given the performance expected of higher end objectives, there is likely greater care and cost in everything from lens grinding, to coatings, and the parfocal and parcentered assembly.
The cost of ED glass elements is probably driven not just by the elements added to reduce dispersion, but increased costs to maintain precision. Plus a bit of charging what the market will bear. I do know that fluorite glasses were harder to grind and polish into good lenses and that some of the ED additives that were tried tended to introduce strain (making the glass unsuitable for polarization or DIC - prime markets for higher end objectives). And given the performance expected of higher end objectives, there is likely greater care and cost in everything from lens grinding, to coatings, and the parfocal and parcentered assembly.
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apochronaut
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Re: ED low-dispersion glasses
As soon as ED glass became available, microscope companies that produced any better colour corrected objectives at all , started including it in their designs. Traditionally fluorite was the mineral of choice for fluorites and apochromats but it became supplanted in many cases by ED glass types. Even superior achromats such as " advanced achromats" at AO had a little in them. That is what allowed AO to continue with 34mm parfocal barrels for so long, yet put planapochromats in them.
You can roughly determine when they arrived in a company's optical designs by knowing when they started using the same eyepieces for both achromats and apochromats.
You can roughly determine when they arrived in a company's optical designs by knowing when they started using the same eyepieces for both achromats and apochromats.
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apochronaut
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Re: ED low-dispersion glasses
A further note to add to PeteM's observations on the technical precision required in a quality microscope objective, is the number of lens elements that need to be critically aligned and optically coordinated in a microscope objective.
Even a simple higher N.A. achromat from 1960 containing one element of LD glass can have 7 individual elements in 3 groups. Today a planachro can have 12 elements with 3 or more kinds of glass and a planapo 20 or more elements with 5 or more kinds of glass. Packing all that into a tube between 34 and 95mm long and making it work to as tight a specification as a microscope objective works at is quie remarkable. Some of the lenses are 1/2mm in diameter and focal length.
Even a simple higher N.A. achromat from 1960 containing one element of LD glass can have 7 individual elements in 3 groups. Today a planachro can have 12 elements with 3 or more kinds of glass and a planapo 20 or more elements with 5 or more kinds of glass. Packing all that into a tube between 34 and 95mm long and making it work to as tight a specification as a microscope objective works at is quie remarkable. Some of the lenses are 1/2mm in diameter and focal length.