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. The front is flat but the back side has an extremely
shallow sinusoidal-like curve. Commonly used in Celestron and Meade
catadioptric telescopes for the correction of coma in fast, spherical
mirrors. I don't know of any cases where a Schmidt plate is used in
conjunction with all-refracting optics but I don't see any reason why it
couldn't be. That's my 2 cents.
Chuck Norcutt
Marc Lawrence wrote:
> Andrew Fildes wrote:
>> I seem to remember reading somewhere that a couple of the super
>> dooper Canon lenses like the legendary 200/1.8 actually had a window
>> pane front element - in effect a built-in protection 'filter' (that
>> filtered nothing) in the assumption that many users would go in harms
>> way and it would be a lot cheaper to replace than some beautifully
>> curved piece of rare glass.
>> Of course, I could be wrong as usual....
>
> The Canon Camera Museum seems to confirm that...I think:
>
> http://tinyurl.com/2gtcua
>
> Cheers,
> Marc
> Noosa Heads, Oz
>
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