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I have owned this lens (in Canon mount) for some time and use it frequently. Being a DG designated lens, it is suitable for use on both FF (full-frame) and cropped sensor APS-C digital SLR cameras. I use my example on Canon EOS 5D and EOS 400D digital SLR cameras where on the latter, it effectively becomes a 112mm focal length f/2.8 Macro lens with larger than 1:1 'life-size' reproduction ratio potential. Very useful!
The 'EX' designation which, in SIGMA terminology, relates to 'EXCELLANCE' and their 'professional' quality range of lenses which generally, exhibit a somewhat higher specification, build quality, finish and design feature-set than their 'DC' range of lenses. It therefore comes in the familiar smooth matt black 'crinkle finish which is quite attractive and so far, apparently hard wearing. Although this unique finish does seem almost to 'attract' dust or at least, allow it to stick to the surface more easily than 'shinier' finishes, it is very easy to wipe over and keep clean.
The construction of the lens consists of 10 elements in 9 groups and includes 3 SLD (Special Low Dispersion) elements. The diaphragm has 9 blades ensuring very pleasing, smooth and 'buttery' bokeh or OOF (Out-Of-Focus) highlights. Very useful on a macro lens where almost everything but the subject itself 'will' be out of focus.
Like all Sigma EX lenses, this example also comes complete with soft case, front and rear caps and a lens hood (metal, screw on, barrel type) - which rather unusually has a filter thread cut into the end of it, permitting the attachment of common 77mm size filters. The lens itself has a smaller filter thread of only 62mm. Common still but less so than 77mm size.
Although an EX lens, unusually and perhaps, a little disappointingly, it doesn't come with either IF (Internal Focusing) or Sigma's superb HSM (Hypersonic Motor) AF (Auto Focusing), which would normally provide almost silent and very fast auto-focusing. The lens therefore seems somewhat unsophisticated, noisy, slow and perhaps even a little 'harsh' when auto-focusing. Whenever working at more normal distances, it might prove beneficial to use the focus limiter switch to help speed things up a over the more limited range. It isn't such a problem however, when working in macro mode where it is often more practical to focus manually in any case.
The lens does extend considerably as you focus more closely, although thankfully, the barrel doesn't rotate during focusing making it easy to use polarising filters. The extended lens barrel has reproduction ratios marked upon it which is helpful, although of course, it must be remembered, these don't hold true when using the lens on a cropped-sensor APS-C digital SLR camera.
The fixed focal length 'prime lens' design is common with macro lenses where 'zooming' is much less of an advantage and where ultimate resolution is required being more easily achieved with this configuration. The 70mm focal length is a good choice in this case for it is 'just' long enough to be workable on FF (full-frame) DSLR's, providing of course, you are not working with 'flighty' subjects, such as butterflies etc, which will almost certainly be disturbed by the necessary close working distances imposed. It does have a slight advantage of marginally greater DOF (Depth-Of-Field) on a full-frame cameras where longer focal length lenses can leave you struggling for that little bit 'extra'.
When used on APS-C cameras however, the lens effectively becomes an even more useful 110mm, 112mm or 119mm f/2.8 Macro, depending on which camera make you use, due to the crop factor produced by the smaller sensors used in these designs. This provides that little extra working distance between camera and subject and also, provides slightly 'shallower' DOF (Depth-Of-Field) which can be useful for 'isolating' your subject against a blurred background.
Performance it has to be said, it excellent. Vignetting 'is' noticeable at maximum aperture when used on FF (full-frame) cameras but, not so much as to present any problems. On APS-C cropped sensor cameras, it is negligible, due to the fact that such cameras utilise only the central 'sweet spot' of the lens's image circle. Even with full-frame, stopping down just 1 f/stop to f/4 virtually eliminates vignetting altogether.
CA's (Chromatic Aberrations) or 'colour fringing' are also quite well controlled on this lens and will only ever be noticeable when shooting very high contrast or backlit subjects at f/2.8. Very easily corrected in software anyway.
Resolution or 'sharpness' is surely what all macro lenses are about and this lens certainly fulfils it's promise. It is really very sharp indeed, particularly for close-up and macro work. Even at maximum aperture f/2.8, the centre of the field is EXCELLENT- with the edges being 'just' behind at VERY GOOD++ and extreme corners at VERY GOOD+ which is really quite impressive. Stopping down just 1 f/stop to f/4 produces truly EXCELLENT results across almost the entire field with the exception perhaps of the extreme corners which attain EXCELLENT- However, stopping down further to f/5.6 brings even these extreme corners into EXCELLENT territory.
Performance remains very high until f/11. However, as to be expected, diffraction begins to effect sharpness at f/16 at which stage performance attained is VERY GOOD in the centre of the field, VERY GOOD- at the edges and GOOD+ in the corners. At f/22, although still really quite impressive and certainly very useable, performance does drop in real terms to just GOOD+ in the centre, GOOD at the edges and GOOD- in the corners. A really very impressive performance equalling that of the renowned Canon EF 100mm f/2.8 Macro USM lens in my opinion although of course, without the benefits of the IF (Internal Focusing) and USM (Ultrasonic Motor), for a lens which doesn't alter it's size when focusing and ultra fast, near silent AF. It should be remembered however, the Canon lens is somewhat more expensive, doesn't include a lens hood or the rotating tripod collar which I would consider to be more 'necessary' with the Canon lens.