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[OM] Re: focus and ? about lens filters

Subject: [OM] Re: focus and ? about lens filters
From: Marc Lawrence <montsnmags@xxxxxxxxx>
Date: Wed, 09 Jan 2008 14:53:10 +1000
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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