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This is a large prime lens and currently costs £3000 to £4000 depending on where you get it, so reading this you will probably have sussed out it's spec on the sigma website already. Being a sigma lens it is available for sigma, canon and nikon lenses. I use a nikon D300 so on this I base my review, although it will be equally applicable to other mounts.
Most people who get this lens will either do so for photographing wildlife or sports. And for these it is ideal. 500mm gives you plenty of reach, letting you photograph the action from the sidelines with ease, or get close to wildlife without disturbing it.
The maximum aperture of f/4.5 is nice and bright. Wide open this lens performs well, not as well as say the Nikon f/2.8 primes, but these lenses are far more expensive and bigger too. There is almost no distortion, and no noticable chromatic aberration either. Maximum sharpness is reached at about f/7.1 and it certainly is really crisp at this point. Contrast is good, although not quite as exceptional as on the nikon 70-200 f/2.8 or nikon 300mm f/2.8 this is the only reason I have docked one mark off the rating. Having said that it is very very good, and you can easily tweak the contrast slightly in photoshop.
Autofocus using the HSM provides great performance which is also silent! It can even keep up with fast flying birds most of the time if you are good enough to keep on the bird. According to Sigma this lens won't autofocus with a 1.4x converter, which certainly is true if you use a sigma teleconverter. However, with other converters you can AF! On a canon teleconverter you need to tape over a couple of the electrical contact pins, and with a nikon converter you need to grind out a small metal tab and bob's your uncle! Both techniques can be found on the web, and result in reasonably good autofocus performance and a 700mm lens! You can also use a 2x converter, which I havn't tried, but I suspect even using the above tricks you would still be limited to manual focus.
On the lens itself there is a rotating filter holder, a pinch catch allows you to open this chamber to use the supplied polariser (not that I ever have) as I don't want to loose any light. There is also an AF/MF switch, and then a focus range switch too. This limits the focus to between 4m and 8m, 8m to infinity or the full 4m to infinity. Being a HSM lens you can also tweak the focus when it is set to AF, and the focusing ring is really nice and smooth too.
Being a big 500mm lens it is quite surprising that it isn't actually all that huge! It is clearly smaller and lighter that the 500mm f/4 lenses from both canon and nikon. Yes it weighs just over 3kg, so you don't really want to try and hand hold it, but stick it on a monopod or tripod lens and it is not ungainly. The tripod mount has a number of holes so that you can get it to balance nicely, and this also doubles up as a carry handle too. The tripod collar cannot be removed, but it can be rotated after loosening a little knob, this makes it easy to change from landscape to portrait, and means you can move the mount to the side when using a beanbag/car window etc.
The lens hood that is provided is not amazingly long, which could prove annoying in some instances. But it is solid, and has a positive bayonet connection and then a knob to lock it in position. This is far better than the silly clicky button on Nikon lenses which have a habit of falling off when you are stood at the edge of a cliff or boat to be lost forever!
Having got mine second hand I don't have the supplied case, but I can't say I'd ever use it anyway. This lens fits in my Tamrac expedition 7x rucksack with the body attached and hood stowed away reversed on the lens. All of which fits as hand luggage on the majority of air travel providers.
The finish of the lens is a hard wearing and smart textured black. Using realtree no-mar tape you can easily camouflage the hood and front end of this lens, or there are neoprene jackets available.
Having invested rather heavily in this lens I am pretty sure it is going to last me many years, and take me into a whole new league of wildlife photographing opportunities.