Internet forums have been ablaze about the Sigma 19/2.8 lens ever since the first copies started popping up at the doors of eager NEX users. The virtual majority of the users seemed to be fairly satisfied with the imaging quality of the lens, although you could hear complaints once in a while as well. We recently compared Sigma 19mm f/2.8 EX DN to Sony E 16/2.8 only to discuver that Sigma is a stronger, in absolute terms, performer (of course that test needsto be taken with some grain of salt, since we're effectively were comparing lenses of different FL). But everything is obviously relative here - if we were to take Sony's 16mm lens as a benchmark, then Sigma seems to offer a much better quality, albeit at a longer FL. On the other hand, if we were to compare it to the Zeiss 24/1.8, than Sigma's distinction starts to fade away.
If we were to review the lens in isolation (meaning not using any other benchmark whatsover), we could say that performance characteristics of the lens are decent, but not remarkable. The patterns are very similar on both NEX-5n as well as NEX-7. Outstanding center resolution throughout the aperture range and average border performance. This is quite common in most wiide angle lenses (and this is particularly true on NEX-7, which puts significantly more pressure on the lens corners in general) - corners are softer than the center, particularly at wider apertures. Sigma follows this trend, but the absolute difference between center and borders is not as large as with the Sony's 16mm lens - that's why we can claim that Sigma is really an improvement over the otherwise mediocre Sony's UWA.
Of course, arguing about raw MTF50 chart is as useful to a practitioner as a cold shower to a cat. So instead let's take a look at the simplified SQF charts below (just don't forget that there's a reason SQF is called subjective). Here we can see that the lens does not offer much flexibility when it comes to prints, with all print sizes over 17in probably showing some visible quality problems around corners.
Before moving on to the next section, let's first take a look at the crops from the imaging target and then some real life tests. The crops show clear difference in image quality with both NEX-5n as well as NEX-7. Borders at f/2.8 are visibly softer in both cases, sort of confirming what Imatest seems to be complaining about.
Eyeballing the results from the real life tests, all done at the infinity with NEX-7 and uncorrected for artifacts, we can observe some minor softness around corners at f/2.8. As we stop down the lens, image quality around corners improves and by f/8 there is no major difference between the center and the corners. Center seems more or less uniform throughout the aperture range. Unfortunately, monitor is not necessarily the best medium for noticing subtle changes and at normal viewing distances some artifacts, even softness around corners, might not be very noticeable - you'd need to blow up the picture to higher magnification to be able to see subtle variation in quality as you stop down the lens from f/2.8. So the bottom line is that unless you are really obsessed at pixel-peeping, or plan to print in large format, you are probably gonna be reasonably happy with the overall quality of images produced by Sigma 19/2.8 DN.
Color & Rendering
Sigma 19mm f/2.8 EX DN is one of the dozens, if not hundreds, of lenses that don't really have anything unique about the way they render images. There is nothing wrong with that, meaning that users get fairly consistent and predictable performance, albeit without any memorable characteristics. Conventional, boring, predictable, consistent - pick your adjective here.
Like many UWA lenses out there, Sigma 19/2.8 EX DN produces reasonably well saturated colors,with the color gamut that seems to be shifted slightly towards blues. Not exactly cool blue a la Zeiss, but slightly warmer blue. Contrast levels streight out of camera are fairly good - global contrast varies from moderate at wider aperture to moderately high at smaller apertures. Tonal reproduction is average, with balanced range across shadows and mid-tones.
While you can obviously tweak the colors in post-processing and end up with pretty much any look you wish (there are some interesting color profiles out there on the web - some even claim color rendering styles matching Leica and Zeiss lenses), the old good NEX system itself is probably the best option for an average user - just let the built-in image processor optimze colors for you and for the most part you would not be disappointed in this department.
Like many UWAs, Sigma 19/2.8 also shows some CA.. Per Imatest, lateral CA around borders remains pretty constant on both NEX-5n and NEX-7, averaging ~1.5px across the aperture range on NEX-7 and ~1.2px on NEX-5n. This means that you would probay see some color fringing around borders at very high magnification, but the real life impact should not be a major issue.
DOF & Bokeh
A 19mm lens does not really offer much flexibility in controlling your DOF. With the MFD of 20cm and its max aperture of f/2.8, Sigma 19/2.8 offers DOF of ~1.14cm. Not too bad for the general purposes, but you still are not going to be able to blur the background significantly and achieve a decent level of isolation for subjects, as can be seen in the sample shots below. The OOF rendering of the lens is neutral, with OOF highlightsretaining even illumination and no major signs of longitudinal aberration.