Originally Posted by Pete
My eye glasses have plastic lenses and they work very well -- except when I scratch them or scuff them. As I understand it, the Schneider EL will have glass front and back with the plastic in-between. I'm sure it will have slightly less detail and contrast than their all-glass lenses, but the question is how much less. In my opinion, there's a big performance gap between a Panamorph and a standard ISCO/Schneider. I'm hoping the Schneider EL will slot in between these two...not as soft looking as a Panamorph, not as highly resolved as an all-glass ISCO/Schneider. In the absence of comparison, the Panamorph looks pretty good. I'm anticipating that the EL will look better, but we shall see.
If you know me by now, you'd know there's a very complicated explanation for why equating plastic eyeglass lenses with plastic projection lenses isn't valid. But I'll spare you all that.
Plastic eyeglass lenses are truly woeful for projection. I tried it for kicks (cost a few bucks) several years ago. Eyeglass lenses' curvature is measured in 1/4 diopters, with +/- 0.09 diopter tolerances (by australian standards anyway), which can amount up to about a 2% tolerance on a glass curvature... utterly unacceptable for projection purposes. On a 250mm nominal focal length lens you could have bits of that were 244mm focal length and other bits that were 256mm focal length, all mixed up together on the same surface: hardly the stuff of legendary optical performance. A flat spot on a glass lens of just 0.1% difference to the rest of its surface makes that lens a reject, as you can never focus it evenly.
For projection lenses you're talking 0.25% radius tolerance all over a large lens, as a starting point
, and consistent over the entire surface, better, if your manufacturer can manage it. You really would
need glasses - the kind you mail-order off the back of Superman comic books - to view a picture projected through eyeglass-quality plastic lenses. That, and the human eye has an ability to adapt to slight inconsistencies in eyeglass focal lengths and still deliver a reasonable image on the retina. There's no "adaptive" process in projection. You have to get it right first and all other times (which is why most use, and always have used, glass).
I'm not quick to naysay the new Schneider lens, because I don't have a clue under what conditions it was made. Plastic eyeglass lenses I have
experience with. They're fun, but useless for even half-serious viewing.
The "10%" light loss figure could have been misunderstood, or quoted out of context. I do know that some plastics are clearer
than glass, and are used in preference to it in some cases. They are certainly cheaper - by the thousand - but the setting up of dies and so on costs a literal fortune. So, unless Schneider are intending to make these lenses by the thousand, or at least many hundreds, I can't personally see the point. But as I said, of course I'm not at all privy to their business plan.
When you're using plastics in doublets (as I assume any decent projection lens would have to do) the problems increase, as matching indices of refraction across a large production run is often batch-dependent. Even slight variations, sometimes within a batch, can make errors more glaring at the margins.
It does seem like a retrograde step - going back to plastic - but if anyone can do it and garner OK results, I guess a big firm like Schneider can. However, glass is still the preferred option, for mine, but I would
say that, wouldn't I?
P.S. Japan Dave... you have zero sense of humour mate.