This is a massive range to cover with one lens successfully. It is L glass &
as such a great lens for what it is. However, read around the various sites
for reviews & it's difficult to get away from the consensus that the best
zoom compromise (at the moment) is the fabulous 70-200 f2.8L IS.
Reviews of this lens are seldom short of 'superb'. It's not a prime so
before any 'purist' leaps in saying this or that prime are better - zooms
are a compromise to gain flexibility of in shot cropping & portability and
therefore it's trying to get the 'least compromise' versus max performance
for your money.
That taken, the 70-200 has a full f2.8 through it's focal range and thus a
wonderful ability to throw any background beautifully out of focus. However,
coupled with the IS an f2.8 when fully extended at 200 is giving 2 full
stops more light (quicker shutter speed, less camera shake) which coupled
with the IS gyros really makes it possible to grab shots hand-held that
would previously have required a tripod.
Also, the f2.8 is vital once you start looking at the issue of
teleconverters. They will only allow full-autofocus functionality if the
COMBINED f factor does not exceed f5.6. Therefore, if you start with a lens
(even of L quality) that is 5.6 at one end of it's focal length then as soon
as you attach a 1.4x converter you lose autofocus at the long end of it's
zoom range [pointless].
Whereas, f2.8 throughout the focal range will allow one to attach not just
the 1.4x but (if necessary) the 2x teleconverter & still have full autofocus
throughout the total range of the lens.
"Joe" <com> wrote in message