In article
>In B&W's range of "filters" there's also some lenses designed to reduce the
>minimum focal distance with from 25 to 75 percent.
>Could be handy when you're using a zoom that won't focus closer than one
>meter. Do these lenses work very well at all? I suppose they will reduce
>the maximum focal distance as well, meaning you cant focus to infinity.
>How about influence on optical quality?
Unless the close-up lens used is weak ("+1"), and therefore not very useful,
there will almost certainly be noticeable image quality degradation unless
a small f-stop is used. More expensive than the single element close-up
lenses are the 2-element "achromats", which can be very good by middle
lens stops (and even better at small f-stops). Use of either type will
prevent infinity focus. An achromat (made by Nikon, Canon, Leitz, Minolta,
and Sigma, as I recall....) is a good way to go for making good quality close-up photos with many zooms.
Hope This Helps