O.K. everybody, get out your pack mules. Sigma has introduced what they
describe as 'the world's first Ultra-Telephoto Zoom lens' - the Sigma
APO 200-500/2.8 EX DG. And yes, that f2.8 is a constant f2.8. As for the
size of this lens, let's just say it would look right at home hanging
off the wing of a 747. It is huge.

Producing a lens of this sort was no mean feat. According to Sigma, the
new lens incorporates 4 Special Low Dispersion glass elements as a means
of maintaining high levels of optical performance and minimal levels of
optical aberrations throughout the zoom range. For filtering, there's a
filter-draw towards the rear of the lens that also allows for rotating
Polarizers.

Size and specs aside, the imaging possibilities made possible with this
lens are enormous; just make sure you have a serious tripod with you
because you might hurt yourself lifting this rig to your eyes.

Included with each lens is an optically-matched 2x teleconverter that
converts this baby into a 400-1000/5.6 lens. Keep in mind if you plan on
using this lens on an APS-sized DSLR (1.5x FOV) it is effectively a
300-750/2.8, or a 600-1500/5.6 with the 2x converter.

Alas, this amazing lens is somewhat of a teaser in its own right as
pricing and availability

http://tinyurl.com/38vflm