This page is part of a set describing tests of small-format lenses in UV photography. The main page describing these tests is available here.
Sigma 19 mm f/2.8 EX DN for Micro 4/3
Unlike the preceding lenses, this is a native Micro 4/3 lens supporting autofocus and aperture control by the camera. My second-hand specimen is the earlier version of this lens with plastic barrel and comfortable, fluted focusing ring, not the metal-clad, completely smooth opprobrium of the later model. Optically, the two models are supposed to be identical. The filter mount is 46 mm and the minimum focus 20 cm. The optical schema is complex, with 8 elements (of which 3 aspherical) in 6 groups. Manual focusing is by wire and there is no distance scale. Aperture must be controlled by the camera, and there is no aperture ring. A versions of this lens for Sony E-mount cameras is also available. A lens shade is included with this lens, but cannot be used once a step-up filter adapter is mounted at the front of the lens. The lens shade shown in the above figure is sufficiently wide to prevent vignetting on Micro 4/3. The lens shade and two 52 mm filters can be stacked on top of a 46 mm to 52 mm step-up ring without vignetting.
Something large and heavy rattles inside this lens when it is shaken. This is normal for this series of Sigma lenses, and does not indicate that the lens is broken. I believe that an internal autofocus group is "loose" inside the barrel when the power is off. There is no in-lens image stabilization, which is another thing that can cause internal parts to rattle around.
NUV attenuation is over 3 stops with respect to the reference lens. This is at the limit of usefulness. However, this is counterbalanced by a very good image resolution and contrast with the Baader U.
In many situations, like hand-held photography, it is good to have AF in spite of the low NUV transmission (assuming that there is enough continuous UV available to autofocus with sufficient precision). Among the tested lenses, it is also among the ones with the lowest focal length that do not introduce a significant vignetting, or a significant loss of contrast in NUV images.
NUV image quality is quite good (dust particles visible in the 1:1 crop are on the subject, not on the sensor). This lens also allows by far the highest subject magnification (without extension tubes) among the lenses tested in this set. The minimum focusing distance is only 20 cm. For this reason, I am showing also a reduced version of the whole image, as an example of the good NUV contrast of this lens. Together with AF, these characteristics make it one of the most interesting NUV lenses among those discussed in this set, in spite of a borderline NUV transmission. A marked cutoff in NUV transmission well within the band passed by the Baader U filter results in a violet general tone, indicative of a prevalence of longer NUV wavelengths.