Sigma 19 mm f/2.8 EX DN
The Sigma 19 mm f/2.8 EX DN is a physically quite small, lightweight (140 g) and affordable lens for Micro 4/3 cameras. It provides a reasonably fast AF and manual focus via a by-wire focus ring, which is the only control present on this lens. On Micro 4/3, a 19 mm lens is a medium wideangle, roughly like a 38 mm on full frame. This lens focuses down to 20 cm, lacks environmental seals, and is primarily a consumer product. The optical formula, however, is relatively complex at 8 elements (of which 3 aspherical) in 6 groups.
This lens was made in two versions sharing the same optics. The earlier version, introduced in 2012, has a barrel with external parts entirely made of plastic (except the rear mount, which is metal) and a finely ribbed focus ring. This is the version reviewed on this page. The subsequent version is externally clad with a thin metal sleeve on the barrel and focus ring, and the latter is entirely smooth. While the earlier version is opaque black, the later version is shiny black or shiny chrome-like metal. I don't really like the newer look of this lens series, but don't care much for looks when choosing my camera equipment.
Focusing is carried out by a linear motor. Aperture is also electronically controlled by the camera. Turning the lens upside down causes an apparently loose and heavy internal part to change position, with an accompanying rattling sound and perceptible vibration. This moving part is not the image stabilization assembly, because this lens has none. This may feel disconcerting, and has caused worried posts by owners of this lens on multiple bulletin boards, but apparently all specimens of this lens display the same behavior, as well as the 30 mm lens from the same series and possibly also the 60 mm, which I don't own .
The lens is supplied with a proprietary lens shade that mounts on a proprietary bayonet around the front of the barrel and can be reversed in place for storage. This round lens shade is quite wide and short. A petal or rectangular lens shade would have been more appropriate for this lens.
Image quality is reasonable, and the image periphery improves if the lens is stopped down to f/5.6 or f/8. There is a little fall-off in the corners and a little barrel distortion and transversal chromatic aberration, but in most cases I find I can live with them unless I need the best possible quality. DXOMark rates this lens as roughly average in image quality, with the newer version marginally better than the original one.
This is an inconspicuous lens that can find multiple uses when weight and size are critical, for example on a flight without checked-in luggage, in street photography, and where a larger and more expensive lens would attract undesired attention.
This lens has been discussed among UV photographers because it transmits usable amounts of UV down to around 370-380 nm and is one of few Micro 4/3 lenses that are usable in UV imaging and provide AF. UV transmission is limited to the longest UV wavelengths, and therefore the lens produces largely monochromatic UV images. Coupled with in-body image stabilization, this lens can be used hand-held in architectural and landscape UV imaging and provides a sufficiently evident wideangle perspective rendering.
A 30.5 mm UV- or IR-pass filter is large enough to avoid vignetting with this lens, and can be mounted on the 46 mm filter mount of the lens via step-down rings. In fact, the step-down rings can be permanently left mounted on the lens and help a little to prevent lens flare.
The short focal length of this lens can cause false-color shifts between center and periphery of the image when used with dielectric UV-pass filters like the Baader U, especially in images of vegetation recorded in full sunlight. Converting the images to monochrome and masking the center of the image in post-processing to reduce its brightness is usually enough to correct this problem. This lens can also be used for NIR imaging.
UV images with this lens are not as sharp as in the visible range, but still acceptable. In particular, the loss of resolution in peripheral areas, possibly due to spheric aberration and/or chromatic aberration, is more evident in UV than in VIS.
An additional page discusses studio tests of UV imaging with this lens.
The Sigma 19 mm f/2.8 EX DN is a small and affordable medium-wideangle lens for Micro 4/3. Image quality and price/performance ratio are reasonable. This lens is also usable in UV imaging and one of the few AF wideangles usable for this purpose on Micro 4/3.