Quote:

---

the Zeiss FL series completely eliminates this effect over the central half of the field of view

---

I wouldn't expect that Zeiss FL is one of those we need to be trying to determine if the ED formula is really responsible for performance or not. I'd more than likely give them some credit that they've got it. BTW, how much are those worth? Would they perhaps be at the pinnacle of the highest end of design/cost? Would those be a good example to argue against the points I made above? I think not.

Since there are both very expensive and not so expensive glasses that can be used to satisfy the combinations of ED lenses that can be designed, have you thought about why it cost so much for a really effective ED apochromat lens, or for that matter even an effective ED achromat? Although not the only reason, adding an ED element increases spherical abberation and coma. So the design configuration works towards reducing CA, but now there are other problems that must be overcome. This requires additional lens design and working and therefore higher cost. Or perhaps it is simply considered by some the compromise that must be overlooked in order to deliver a cost effective product to the public. I'm sure there are examples of both on the market.

Every wavelength of light has a different focal length. Since a lens presents rays across not a flat plane at the focal point, but a curve, if it were corrected to have CA minimized at the center, it will still present the focal point of the same wavelength rays at a different position to the eyepiece as you move further off-axis. An eyepiece cannot focus across all the points of a curve at the same time, so while focused on any given point, some other point must be out of focus. The presence of lateral color is showing the amount of off-axis curve that cannot be accomodated by the eyepiece, focal plane junction.

I'll assume it is for this reason, often the CA correction in a doublet is corrected at the position 70% off axis. This at least helps minimize this effect. Perhaps a lens design that moves the CA correction away from the 70% off-axis position inward is one that shows considerably greater lateral color. But this is now considerably off-topic to the discussion of ED glass. This is lens design.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21

Post Extras