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This telephoto macro lens was the first lens I purchased for my DSLR. It was intended for insect macrophotography, and has not disappointed. At around f8, the lens is very sharp; it is softer wide open, but this is not a problem for macros as the depth of field at f2.8 is too shallow anyway. At normal macro apertures I have no complaints. F16 and smaller are best avoided owing to diffraction. The lens is optimised for macro use and is less successful as a telephoto, perhaps because contrast is rather low. It is also prone to flare - again, not usually a problem with macros.
Build quality is good, on a par with mid-range lenses from other manufacturers, and the lens is no larger than a decent standard zoom. The surface has a matt appearance, which shows up marks easily. The lens barrel extends to about double the normal length on zooming in: there are marks to show the magnification ratio. This lens is a true macro and gives 1:1 magnification. The front element does not rotate, and a plastic screw-on lens hood is provided, though I tend not to use it as the front element is quite deeply recessed, and the hood reduces the working distance.
For those unfamiliar with macro lenses, the magnification ratio is the ratio of the object size to the size of the image on the sensor. Magnification is determined by closest focusing distance; focal length determines working distance. So a 35mm lens may give 1:1 macro but the subject will be almost touching the front element. The 105mm Sigma has a working distance of about 10cm from the front of the lens (confusingly, some manufacturers give working distance from the sensor, without taking into account the length of the lens barrel) at 1:1. This is acceptable for hand-held outdoor macros as the lens does not cast a shadow on the subject. At lower magnifications you can photograph insects without getting so close as to scare them away. If your intended subject is very flighty, you might consider the larger, heavier, Sigma 150mm macro.
Switching between auto and manual focus requires you to (a) flick the switch on the lens barrel, (b) pull back the focussing collar and (c) switch the focussing method on the camera body. It is easy to forget one of these and when the camera on auto tries to focus the lens set on manual there is a grinding of gears, which can be doing no good. Autofocus is, quite frankly, terrible, so leave the lens on manual; it is usually easier to focus manually for macro work anyway. On autofocus, the lens tracks laboriously from one end of its long travel to the other. If using the lens as a telephoto, there is a focus limiter that cuts out the macro end of the travel. Autofocus is still not brilliant though.
In summary, an excellent dedicated macro lens; acceptable for occasional general use but certainly not a portrait lens.