On Sat, 17 Aug 2002 21:35:53 GMT, Rudy Garcia wrote:

>In article <3d604ee2.3383401@newsstand.cit.cornell.edu>,
> d_ruether@hotmail.com (Neuman - Ruether) wrote:
>
>> On Sat, 17 Aug 2002 01:26:43 GMT, Rudy Garcia
>> wrote:
>>
>> >In article ,
>> > stephen@bokonon.stevedunn.ca (Stephen M. Dunn) wrote:
>> >
>> >> In article <3D595613.658506B6@videotron.ca> Alan Browne
>> >> writes:
>> >> $Stick with the filter. It is just clear glass with (or without) a
>> >> coating.
>> >> $Pros can afford to have a front element replaced (or the whole lens), but
>> >> I
>> >> $can't, can you?
>> >>
>> >> Actually, that brings up another point. Some pro lenses have a
>> >> flat piece of clear glass as the first element for exactly this
>> >> reason - protection. Canon usually does it for lenses where the
>> >> first refractive element is either ultra-low dispersion glass,
>> >> which is relatively expensive to replace, or fluorite, which is
>> >> also very expensive and also much softer (and therefore more
>> >> easily damaged) than optical glass. I wouldn't be surprised
>> >> if Canon weren't the only lens maker to do this on some lenses.
>> >>
>> >> For that matter, the big lenses that don't take front-mounted
>> >> filters (because they'd be way too big - e.g. a 300/2.8, where the
>> >> diameter of a front-mounted filter would be well over 100mm) usually
>> >> take drop-in filters towards the back. If you're not using a filter,
>> >> guess what you're supposed to put in its place? Yup - a flat
>> >> piece of clear glass.
>> >>
>> >> I haven't heard anyone complaining that either of these flat
>> >> pieces of clear glass ruin the optics of these lenses :-)
>>
>> >I believe those lenses have the flat filter (front or back) computed
>> >into their optical formula. That is, they are designed to be there for
>> >optimum optical performance.
>>
>> For optimum focus - removing the filter does not appear to
>> change anything but focus in the image...
>>
>> >There is one situation where a protective filter may affect the image if
>> >you are critical. That is in macro. The issue is not flare, but the
>> >undesired refraction from the "flat" filter.
>> >
>> >In macro work the subject is sooo close to the front of the lens that
>> >most of the subject rays being imaged are striking the filter at an
>> >angle that is way off the normal and are therefore refracted. This
>> >results in a displacement of the light rays as they go through the
>> >filter.
>> >
>> >Depending on the symmetry of the shot, the amount of displacement will
>> >vary significantly. You may not notice this if you are doing macro with
>> >a lens that is not really designed for this task as it will have enough
>> >distortion by itself as to hide the undesirable refraction from the
>> >filter. The thicker the filter glass, the more refraction you get from
>> >it.
>>
>> Um...
>> The same should be true for super-wides, but
>> removing/replacing a large/thick Nikkor filter
>> from an 18mm or 17-35m does not appear to change
>> the image at all...
>> David Ruether
>> d_ruether@hotmail.com
>> http://www.David-Ruether-Photography.com
>> Hey, check out www.visitithaca.com too...!
>
>I've seen it with a 50mm at 0.7 X there was distortion on an off
>center of symmetry subject with the filter on. Redid the shot later
>without the filter and the distortion was gone.
>
>It wasn't noticeable on every shot. Just ones where there was the right
>amount of asymmetry of the subject relative to the optical axis of
>symmetry, then the refraction produced by the filter was noticeable.
>Again, this was in macro work and in a critical application where
>correct rendition with minimal distortion was needed.
>
>If I was shooting a bug instead, I probably would not have noticed it.

I appear to stand (rarely...;-) corrected...;-)
Thanks.