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[OM] Re: focus and ? about lens filters

Subject: [OM] Re: focus and ? about lens filters
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 09 Jan 2008 07:21:29 -0500
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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