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Achro, Semiapo, ED, Fluorite, etc.

Started by RolandC, 11/15/2011 08:14AM
Posted 11/15/2011 08:14AM | Edited 11/15/2011 08:18AM Opening Post
So many choices, result is that sometimes confusion reigns. So what are these different types?

The attached image shows a diagram that places all the available glasses from the major manufacturers according to their dispersions. It is well known (and has been known since Newton's time) that the longitudinal chromatic aberration (or secondary color) of any two or more glasses depends on their partial dispersions.

If two glasses have the same partial dispersion, then the resultant color error will be zero. However, to make a practical lens that has some realistic focal ratio, the dispersion Vd of the two glasses must be far apart, ideally 2:1 or more.

A quick look at the diagram shows that almost all glasses with few exceptions, lie close to each other on the Abbe Normal line. Therefore most of these glasses will produce the same color error of 1 part in 1800, give or take. The standard for achromats is the venerable BK7 - F4 (or F2) combination which produces almost exactly that. So, for a lens of 1000mm FL, the blue and red will be approximately 1/2 mm longer than the green. The violet will be even further out, and that can be calculated using the partial dispersion from red to violet, versus this chart which shows partials from red to blue.

I did a quick combine of some favorite glasses that can produce a semi-apo, ED doublet apo, Fluorite doublet and highly corrected triplet lens, along with their respective color error. Certainly others exist with slightly different corrections, but these are the glasses that you can actually get in reasonable quality levels.

Note one interesting glass that was touted some years ago as a "semi-ED" glass, which is FK5. "FK" is the Schott designation for Flour-crown, and this type of glass has some fluorite in it, but in this case not much at all. In any case, as you can see, there really are no mates far enough separated in dispersion (Vd) that would produce any sort of meaningful color correction, any better that the plain vanilla BK7-F4 variety.

So, there you are. There is no free lunch.


Attached Image:

RolandC's attachment for post 54455
Posted 11/15/2011 12:38PM #1
VERY interesting, as usual, thanks for posting grin


Gilles COHEN

Gilles Cohen
Posted 11/15/2011 01:47PM #2
Nothing like a good picture to say a thousand words! Thanks for taking the time.
Posted 11/15/2011 04:34PM #3
Once you know the glasses, you can put them on the chart and color error is known.

Posted 11/16/2011 12:42PM #4
Thanks. This really shows that it IS the mating element, not the ED element that determine the color "error" for a doublet.

For example combining FPL53 and KF6 shows no real advantage over the FPL51/FK6 combo as the slopes of those two lines are almost identical. So I'd go for the cheaper glass for a given quality.

The PSK5/N-KzFSN11 combo is really interesting. Any practical problems with either glass?

Thanks again



"Don't sweat the petty things and don't pet the sweaty things" 8O
Posted 11/16/2011 06:46PM #5

You almost had me until I saw that Young Frankenstein reference...

Attached Image:

mogayn's attachment for post 142448