Whether flat or meniscus I found it interesting that some large lenses
do indeed have a protection plate. But it's also apparent that flat
plates don't always go well with digital sensors. Hoya makes that point
on the coatings for their digital specific filters. I took that as
hogwash but maybe not. I say maybe not since, if the problem could be
fully resolved with coatings then I think Canon would have taken that
route rather than redesign to a meniscus lens.
Chuck Norcutt
Marc Lawrence wrote:
> 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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