9 out of 10 points and recommendedVery practical lens, fast, small, solid image qualitySigma quality control, minimum focusing distance, barrell distortion, field curvature, some CA
Sigma 30/1.4 is a lens for photographers, not for pixel peepers expecting super sharp corners wide open.reviewed February 21st, 2008 (purchased for $500)
My copy of the lens is the second one I got - the first one had decentering problem. The current copy has served me now for a year or more, and has served me well, once I learned the lens and that took some time.
Let's begin with the contruction of the lens. I like the included lens hood, the basic solid contruction of the lens, focus distance window (though would like to have 3m or 4m marked between 2m and infinity and a IR focus spot), metal mount and focus ring. What is not good, in my copy of the lens, is the ugly sound it makes when focusing when tilted about 20 or more degrees up or down. When the lens is level, focusing is silent.
About focusing - this lens can be problematic, especially if your camera has as bad AF-system as for example Canon 350d (Rebel XT) has. I do actually have this camera and this copy of the lens focuses quite a bit better than what I would expect, pretty much every picture is in focus. Lucky me, I didn't even have to send the lens+camera for calibration.
I'll begin with field curvature. This lens has plenty of it - the edges are focused closer than the center, quite a bit closer, so if you plan to take group pictures, you may need to stop down a bit more than you'd like to. This also contributes somewhat to the test results you'll find in the net - shooting a test chart at close distance will produce worse results than shooting real life objects. The field curvature does provide an advantage too: focus and recompose works really well with this lens!
Sharpness is stunning in the center area (maybe 85% of the image) even wide open. Contrast is good wide open. At f2 contrast turns excellent and sharpness improves slightly in the center (not much room there for improvement, at least using 8M pixel camera). Stopping down more doesn't really do much to center sharpness or contrast, at least not on a 350d, except beyond f11 when sharpness starts going down, though remaining perfectly usable even at the minimum aperture of f16.
Edge and corner sharpness has always been a matter of controversy with the Sigma 30. Pixel peepers complain how the corners never get as sharp as the center, while actual photographers seem to enjoy shooting with the lens regardless. What is the truth? I will tell you :) First of all, there is the issue of field curvature I mentioned earlier. Second, I simply do not understand what is so important in the corners of the image when one is shooting wide open? Is one shooting landscapes at f1.4? The areas where interest lies in the images are sharp with this lens wide open. The corners at f1.4? Yes, they stink, and are absolutely useless, but really, who cares?
Anyhow, corners do get better at smaller apertures. Now, there is one more thing about the edges and corners - at close to medium distances they are far better than near the horizon for example. In my opinion, this is not a big deal, as when I shoot landscape, the corners either have some foreground object, or clouds/sky, in either case the lens performs well at approperiate apertures - something like f9 to f10 seems to be the optimum for landscape shooting. The only minor problem is. that even with small apertures, the extreme left and right edge do remain rather average at long distance, perfectly all right, but not great like the center area.
Barrell distortion is rather heavy for a prime, though it seems to vary vis-à-vis focusing distance - the closer you focus, the more barrell distortion you should expect. In my landscape shots I've not seen any distortions ever, not once, but occasionally shooting something much closer does induce quite a bit of this flaw. Pixel peepers may find shooting brick walls to be another rewarding source of complaint about this lens.
Flare is very well under control, especially considering that it is a f1.4 lens, even in contra light situations. Sun in the image should not be a problem.
Chromatic aberrations are something you need to take care in post processing as you will lose some sharpness otherwise. Both red/cyan and blue/yellow may need fixing in post processing.
Bokeh - usually smooth and creamy, even at medium apertures. However, some kind of backgrounds, like patch of leaves against the sky, may cause non-pleasing triangular highlights. Usually not a problem, but some might find it irritating - I am sure there are plenty of examples of this in the net for you to see.
Vignetting, according to all the test sites, is heavy, though I've not found this to be the case in real life phography. There is some vignetting at the wider apertures, but nothing really worry about.
In summary, Sigma 30/1.4 is a rewarding lens, a great tool for a photographer who is interested in a lens that functions well in practical circumstances, in the field. It has it's own character which should be understood - understading the basics of photography does the trick: when shooting landscapes, you use a small aperture, when shooting people, a larger ones. Pixel peepers will find the fluffy clouds in the corners to be hopelessly soft when they shoot landscapes at f1.4, but no photographer will notice this and this lens is built for the real photographers.