I have owned and used the APO Macro version of this lens in NIKON mount for around a year before selling both the lens and my Nikon outfit. I now use Canon DSLR's but please, don't hold this against me! I also have a friend who has the cheaper non APO version, so it is interesting for me to be able to compare the two directly.

Finish and fit is to a good level and materials used are of good quality. I don't know if it is simply a variation between samples but my own APO version appeared to have somewhat 'smoother' zooming than my friend's non APO one. As usual for Sigma, a decent lens hood (barrel type) and a soft case are included in the package. The lens sports a 58mm filter thread, so filters are relatively inexpensive to purchase.

Sigma market this lens as an APOchromatic lens which I suspect, is slightly exaggerating the reality. In fact, they do use two SLD (Special Low Dispersion) elements in the design which comprises 14 elements in all, in 10 groups and, I have no doubt it is this, that 'lifts' the performance of the lens above that of it's less well endowed sibling - the non APO version. It also benefits from improved lens coatings, enhancing it's resistance to flare and ghosting which is more of a problem with digital cameras and their highly reflective sensors. Hence the DG (Digital) designation.

Rather unusually for a lens in this price bracket, it also features a nine blade diaphragm mechanism which provides greatly improved bokeh or OOF (Out Of Focus) highlights than might usually be expected. Closest 'normal' focusing distance is 1.5 metres which provides a maximum magnification ratio of 1:4.1 at 300mm. However, a manual 'macro' switch (limited to being used between 200 to 300mm focal lengths), provides a much improved reproduction ratio of 1:2 or half life-size. Very impressive and extremely useful but NOT true 'macro' which, is generally considered to be 1:1 or actual 'life-size'. This is still impressive however , as the lens does not boast the sophistication of floating elements to 'aid' close focusing.

Now to the lens's performance. Firstly, distortion is held down to low levels throughout the zoom range. Initially (at 70mm), there is slight barrel distortion, but certainly not enough to cause concern. This evens out to become pretty well neutral at around 135mm then changes to pincushion distortion between 200 to 300mm. Again, this is not serious and in fact, such a performance is to be commended.

Resolution or 'sharpness' is a mixed bag. On one hand, with the smaller APS-C digital SLR's sharpness is really quite impressive, while on FF (full-frame) cameras, it is nowhere so good.

This is maintained throughout the zoom range to around 200mm focal length but, at 300mm, performance at maximum aperture f/5.6 is reduced to only GOOD++ in the centre and 'just' GOOD at the edges. Corners, it has to be said are 'just about' verging on SOFT. Stopping down to f/8 improves things considerably when central resolution becomes VERY GOOD with edges reaching VERY GOOD- Corners attain an acceptable GOOD+ rating. Stopping down to f/11 brings a further slight improvement to VERY GOOD both in the centre of the field and at the edges. Only the extreme corners show a slight falling off to become just GOOD.

The disappointment comes when used on full-frame digital cameras where performance is really quite poor. Even at 70mm, at f/4, the resolution can best be described as 'only' GOOD, with edges and the corners in particular, being SOFT. Stopping down to f/5.6 does improve things slightly, when GOOD+ is achieved in the centre. However, edges and corners hardly seem to improve at all. f/8 turns in the best performance (on full-frame) that this lens can achieve, where at the centre of the frame, it just about squeezes a VERY GOOD- rating. Even the edges improve very slightly to just about GOOD+ which I suppose, is not too bad.

In my opinion, this is a pretty good lens for use on APS-C sensor digital SLR cameras but I could not possibly recommend it for use on full-frame digital. Results (on APS-C), can be very good indeed, particularly at this price point. Although the lens is not perhaps the easiest on the planet to use - long extension when focusing close, rotating front element, noisy AF, no full-time manual focusing and manual switching for the 1:2 macro mode - it is however, still a good choice for anyone who might use these longer focal lengths only occasionally and is therefore less willing to invest large sums of hard-earned cash on a more sophisticated model. For these potential purchasers and APS-C digital photographers in particular, definitely recommended.