On Sun, 17 May 1998 15:02:56 +0200, Ib Klewe wrote:

>Recently I read in a book that one should avoid using a teleconverter
>with a zoom lens. Why is that? I was considering buying a 2x converter
>for my tamron 28-200 mm

Most zooms are not sharp enough to suffer the losses of using
them with a 2X converter (there are a VERY few exceptions...),
and the resulting photos would not be very sharp. In addition,
the 2-stop loss combined with the slow maximum apertures of many
zooms makes the combination very difficult to use even if it were
sharp. A 28-200mm is of only marginal sharpness on its own and
would be quite soft (and slow) on a 2X converter. 2X converters
are generally most useful for small-stop macro work and for use
on the big fast teles (with which the very expensive 2X converters
are often quite sharp even with the lens wide open, and the
combination is fast enough to be useful).