On Tue, 14 Jan 2003 18:46:07 +0000, David Littlewood <david@nospam.demon.co.uk> wrote:

>In article <o9j82vkd98assht0ggnm4ehvdlacugqfvj@4ax.com>, Howie Axelrod

><highwind1@attbi.com> writes

>>I took the Nikon School course this weekend and the instructor

>>indicated that in the case of long lenses, or zoom lenses (I'm not

>>sure which he indicated), the lens filter must be removed to get

>>correct focus at infinity.

>>

>>Anyone ever heard of this?

>>

>>Is it for Long lens, Zoom telephoto, or both?

>>

>>Any reference material (wen\b) on this would be appreciated.

>A filter at the front would make no difference.

>

>A rear filter (as often used on long teles and zooms) will affect the

>focussing distance (and probably also the correction of aberrations).

>Thus, such a lens must always have a plain glass "filter" in place even

>if no filtration is required.

>

>A glass element will make the film appear closer to the lens than it

>really is. Without that glass element, it is possible that the lens

>cannot be brought to infinity focus (closest to the film).

Since most teles that have the removable rear filters

also are "ED"-type, and have "beyond-infinity" focus

range (to accommodate the FL changes with temperature),

removing the rear filters generally still permits focus

on very distant subjects. Also, I have checked wide-open

performance with/without front and/or rear filters

installed, without being able to observe any differences

except focus shift with the rear filter removed...

BTW, some short FL lenses use rear filters (Nikkor 16 and

15mm lenses), and I think these do require the filter to

be in place for correct operation (but I don't remember

if this is really true...;-).