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...