Given the excellent performance of Sigma’s earlier mirrorless lenses, and the good reputation of the 30mm f/1.4 for SLRs, I had pretty high expectations for this lens, despite the low price point. It mostly met those expectations, but there are also a few shortcomings.
The Sigma 30mm f/1.4 is somewhat of a mixed bag with regards to sharpness, as the performance is rather variable depending on focus distance. At close to moderate focus distances (say, from 2 feet to 15 feet), the 30mm f/1.4 produces very good sharpness from wide open across the image frame. It’s not blisteringly sharp at this aperture, but there is good resolution throughout, that will produce very detailed images. Stopping down to f/2.8 or beyond brings the resolution up to excellent levels, with great detail and a bit of pop.
Unfortunately, when focusing closer to infinity, the lens displays very strong field curvature, making it almost impossible to get sharp edges at a distance. When focused at infinity in the center, at say, f/5.6, the center will be sharp, while the edges will be focused much closer, at around 5-10 feet away. These edges are sharp, but when shooting for landscape work, you’ll often have edges in the distance that will be notably soft. You can mitigate this somewhat by stopping down to around f/11 or so, which compensates enough for the field curvature to provide decent resolution on the edges, but the overall sharpness of the image will drop somewhat due to diffraction. Still, for landscape work at a distance, f/11 or smaller is where you’ll want to be to counteract the field curvature.
As of this writing, the Sigma 30mm f/1.4 is the fastest lens specifically designed for APS-C E-mount, with autofocus, and as such, shooting at f/1.4 is going to be one of the key reasons to get this lens over something like the Sony 35mm f/1.8 or the Zeiss Touit 32mm f/1.8. In most situations, the bokeh is quite pleasing, producing relatively creamy backgrounds at closer focusing distances. However, the further you focus out, and the higher contrast the background, the more the bokeh suffers. The lens has notable bokeh fringing (which I’ll discuss further in a bit), which can show up as green accents in the background, or around specular highlights. It can also get a bit nervous in some situations. However, the overall impression is positive.
Color, Contrast and Chromatic Aberration
The Sigma 30mm f/1.4 has a lower contrast profile than the f/2.8 Art primes Sigma released a few years back, but for this type of lens, which may often be used for portraiture, the lower contrast at wider apertures will work to its advantage. At smaller apertures, the contrast picks up a bit, yielding nice pleasing images with a bit of pop. Color tends a bit to the warm side, but not by much. Color response is natural, with decent saturation that reacts well to postprocessing, but lacks native richness to the files.
One downside to the lens is chromatic aberration. The Sigma 30mm f/1.4 shows both visible lateral chromatic aberration towards the edges, as well as the magenta and green fringing of spherochromatism in out of focus areas. While both of these can usually be corrected with modern postprocessing tools, it’s definitely a weak point of the lens.
Distortion, Flare and Vignetting
The Sigma 30mm f/1.4 also cuts a few corners when it comes to these other optical aberrations. The lens suffers from rather high native barrel distortion, with straight lines showing a noticeable bow towards the edges of the frame. When shooting JPEG or when using most RAW converters, the built-in distortion profile will correct this before you see it (or if you turn on the profile in Lightroom). I didn’t notice too much of a hit at the edges from the distortion correction, but there is a small impact.
The lens is a bit of a mixed bag with regards to flare performance. At wider apertures (f/4 and wider), the lens loses a bit of contrast with a bright source like the sun in the frame, but doesn’t show ghosting too badly. When stopping down, however, the lens produces nice sunstars from the 9 bladed aperture, but unfortunately also shows very visible ghosting, as can be seen in the shot above.
Like most fast prime lenses, the Sigma 30mm f/1.4 shows visible light fall off at wide apertures, which disappears at smaller apertures. This behavior doesn’t bother me in a lens of this type.