The term ED might imply that it is fully corrected for color error at all visible wavelengths. However ED does not mean anything about correction at all. ED glass CAN be used to correct color error, but does not necessarily do so on its own.
Full color correction comes from a combination of ED glass and crown glass, either in doublet or triplet configuration. If ED glass is combined with Flint glass or Crown-Flint or other similar types, you will have more or less color error. Only crown glass mates can achieve true apo performance over a wide wavelength range.
Why do manufaturers use ED glass with flint or crown-flint and not the ideal crown glasses? After all, a crown glass is actually purer, clearer, less prone to striae and less costly than any crown-flint or flint glass. They do this to achieve fast focal ratios without messy aspherization, or to keep the cost of fabrication down because flint glass makes for easy manufacturing and looser tolerances. The tolerance level will need to be as much as 10 times tighter when you use a crown mate with ED glass, but the color correction achieveable this way can be as much as 10 times better.
Either glass can be combined with suitable mate to produce a highly corrected lens. It's just a matter of focal ratio achievable.
For a zero color lens, FPL51 would need to be about F15 in a doublet, F8-ish for a triplet in a 6" size. FPL53 would need to be about F12 for a doublet, F6-ish for a zero color lens. FPL52 lies somewhere in between. Fluorite would be similar to FPL53 in min achieveable focal ratio.
If zero color is not required, then doublets of various amount of correction can be made with any of the above. I would say that for the same color error, FPL53 at F7 would be similar to FPL51 at F9 in a doublet. So, you see it is not necessarily that one has less color, rather for any specific amount of color, one type of ED would need to be longer than another type.
The manufacturer makes the choice of correction based on cost/performance. Allowing more color error usually lowers the cost, but at some point this flattens out. Trying to achieve zero color with the cheaper ED glass can actually increase the price dramatically because the mating element becomes expensive, and the fabrication difficulty becomes enormous. It becomes a matter of putting all these variables together to achieve a certain result in the cost/performance spectrum.
Therefore manufacturers will try various approaches to see how it is received by the audience. Sometimes it is a simple matter of dropping the aperture down a few notches and keeping the focal length the same that can produce a winning formula. For instance, an 85mm F6.4 might be too difficult to make with a certain formula, but by dropping the size down to 72mm F7.5, it becomes cost effective (best of all for the manufacturer, most people won't perceive a difference in aperture).
Winners can be found in all categories from the least cost/performance to the highest cost and highest performance models. |
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