The Sigma 500mm f4.5 EX HSM Telephoto Lens
This is a great telephoto lens for the bird photographer and one that I have used extensively up until quite recently. This lens produces outstanding detail and is not excessively heavy or large, in fact it's downright light and compact compared to almost all similar alternatives.

The optical quality from this telephoto prime lens is superb, chromatic abberation has always been fully under control by the glassware, even on the Nikon D2x (a camera that finds any weakness in the glass in front).  It's ability to resolve detail is amazing and don't listen to anyone who says this isn't a sharp lens... tarring a lens as soft because of their own inability to use long telephoto lenses correctly is their problem. With good support this lens will produce razor sharp images for professional needs.

The auto focus speed of the HSM focus motors in this lens is very fast indeed... matching Nikon's AF-S and only fractionally slower than the latest Canon USM system. Don't consider you are settling for a substandard AF speed by purchasing a Sigma HSM lens.

One aspect that can put people off from owning an f4.5 telephoto lens versus an f4 is the one of adding a 1.4x teleconverter and losing AF capability... this is only true of a few cameras and teleconverters, notably Canon and there are ways around this. The problem with Canon cameras is that the non-pro bodied cameras will refuse to AF when they are told via electroncic means that there is less than f5.6 worth of light in front of it. A 1.4x Canon teleconverter (adding 1 f stop of light loss) between camera and an f4.5 lens will transmit to the camera that f5.6 has been exceeded. In reality that camera is still more than capable of AF at beyond this f5.6 mark, though not quite at the top AF performance of f5.6 or below... and Canon want AF to work at top performance or not at all, so that their AF reputation is second to none. The AF speed when using the 500mm f4.5 and a 1.4x teleconverter is fast and reliable, whether Canon or Nikon cameras are being used.
Sigma 500mm f4.5 EX HSM
The Sigma 500mm is sitting between a 600mm f4 and the 80-400mm lenses, coffee mug for scale
To get around this problem of the Canon teleconverter telling exactly what's in front of it, you need to use a 'dumb' teleconverter that doesn't tell everything to the camera. The Kenko Pro 300 1.4x is one such teleconverter, and an excellent one at that. An alternative is to tape over the three pins on the lefthand side of an EOS fit teleconverter, this will also allow AF operation.

Basically, this lens will AF quickly with almost any camera (not D2x for some odd reason) when used with a Kenko Pro 300 1.4x teleconverter. Nikon users can rejoice because by slightly modifiying the peerless TC-14E teleconverter ( Read Here ), you can AF very quickly at  700mm with no real impact on the results versus the bare lens.

I used to hear this talk about the Sigma lenses not being built to withstand the rigours of hard work that a professional may put it to... again, my findings are the opposite. After nearly 2 years of hard work the lens was presented to it's new owner as mint. The body finish is robust, even though I'd prefer a flat finish to the slighlty 'furry' finish (which to it's credit never feels cold!).

The tripod foot is an excellent design with very comfortable finger grips that suits my style of walking around with tripod in one hand and camera + lens in the other. The tripod collar and securing bolt allows very smooth rotation of the body.

The basic specifications are:
Available in Canon Nikon Minolta Pentax and Sigma mounts.
Lens Construction 12 Elements in 9 Groups
Angle of View 5 degree
Number of Diaphragm Blades 9pcs
Minimum Aperture F32
Minimum Focusing Distance 400cm(157.4”) 
Maximum Magnification 1:7.7
Filter Size 46mm (drop-in type)
Dimensions: Diameter 123mm (4.8”)
Length 350mm (13.7”)
Weight 3150gr/111.1oz
Here's a shot with a Fuji S2 pro with the Sigma 500mm f4.5 EX HSM. Believe it or not, this was taken at ISO800 but I think it demonstrates what this lens can achieve.
This image opens up fairly large.
Birding Top 500 Counter