Chuck Norcutt wrote:
> I could hypothesize that the front element is actually a Schmidt
> corrector plate to correct the coma of such a fast optic. The
> characteristic of a Schmidt corrector is that it looks nearly flat to
> the eye but isn't.
Although this page:
http://tinyurl.com/2dqxe6
('Photography in Malaysia' page)
says
"Lens construction:- 12 Elements in 10 Groups with three UD glass
elements (at 2, 3 and 5 - protective glass as first element)"
I can't see where they reference this information from. Perhaps it's
from Canon's Lens Work, or perhaps it's just an oft-repeated assumption.
It is stated for the FD equivalent lens too. This is stated by another
in relation to other EF lenses as well (ie. 500/f4), but in
non-definitive terms like, for instance, the words "I believe...".
I just found a link at Canon that states that:
"Most super-telephoto lenses have a protective glass at the front. When
this glass has a flat surface, the light that reflects off the image
sensor reflects back from the protective glass, occasionally resulting
in spot-shaped ghost. To eliminate this phenomenon, all of Canon's
large-aperture IS super-telephoto lenses adopt a meniscus lens shaped
protective glass."
http://www.canon.com/camera-museum/tech/report/200308/report.html#t9
A diagram is included.
All of which in my mind doesn't leave a definitive answer.
Cheers,
Marc
Noosa Heads, Oz
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