my mention of Canon's IS was too brief and introduced some
confusion, so I'll try to clarify everything.
A f4 IS lens IS f4 and therefore will NOT give you what
a f2 lens has:
- the bright and snappy viewfinder image
- the action stopping capacity
- the higher flash GN
- the shallower DOF
- the ability to maintain an acceptable f-stop with a tc
"IS gives you two fstops" ONLY means that you can handhold
a IS lens at two stops slower than an equivalent non-IS lens.
for some applications, this is just not relevant, but for
others, esp. travel photography and action photography, this
means A LOT.
The 28-135/3.5-5.6 has at least the handholding capacity
of a 28-135/2-2.8, which would be an enormous lens.
However, it costs two times less than a 28-70/2.8 and
weights two times less too. Moreover at 2.8 the DOF is
quite limited, which is not necessarily the effect that
you want.
The 28-70/2.8 is sharper, but this is NOT because it is a
faster lens. It would be probably easier to build a sharp
28-135/3.5-5.6 (and certainly a 28-70/3.5-4.5). As a side
comment, it is regrettable for those of us working on a
tripod or limited by weight that manufacturers have only a
fast/sharp line and a slow/less-sharp line.
Q.-Tuan Luong, Computer scientist at SRI, +1(650)859-5138
check out large format landscapes at
http://www.ai.sri.com/~luong/landscapes/lf/
