Sigma 70mm f/2.8 EX DG Macro - £370/$500
Put this lens on a Nikon D3100, Nikon D5100 or any previous Nikon bodies of this class and your first reaction will probably be that the autofocus is broken. That's because the lens doesn't have a built-in motor so, for autofocus, relies on the screw drive that's only featured in high-end bodies such as the Nikon D7000, Nikon D300s and Nikon D700.
While this can be frustrating for general photography, it isn't a major downside for macro shooting, where you'd normally use manual focusing anyway. On the upside, the focus ring itself is very smooth and has a long travel that enables precise adjustments.
The focus limit switch is handy too, limiting the range either side of about 50cm. But again, this is only really useful on bodies that can drive autofocus. Without internal focusing the lens also doubles in length at its shortest focus distance.
Sharpness lags behind that of the similarly priced Tamron 90mm f/2.8 SP AF Di Macro when you're shooting at the largest aperture of f/2.8. There's less of a difference between f/8 and f/16, though. Contrast is a little less inspiring from this Sigma lens, too.
There's a lack of sharpness when shooting at the maximum aperture, but things improve at f/5.6 and beyond.
Sharpness at f/2.8: 1791
Sharpness at f/8: 2320
Sharpness at f/16: 2002
Chromatic aberration is controlled very well. There's no real evidence of colour fringing within test images.
Fringing at f/2.8: 0.17
Fringing at f/8: 0.12
Fringing at f/16: 0.13
It's not the best in the group, but with only slight pin-cushion distortion it's not too far behind the leaders.
Image quality verdict
This lens works quite well as a macro optic, but the lack of sharpness and contrast at large apertures is less than ideal for close-up shots.