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Sigma 85mm f/1.4 DG DN ART - Review / Test Report - Analysis |
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Lens Reviews -
Sony Alpha (Full Format)
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Page 2 of 3
Distortions
When looking behind the scenes, we can spot a high degree of pincushion distortion of more than 3%. This is unusually high for such a prime lens, but then end-users won't really notice this, thanks to auto-correction.
Vignetting
The vignetting characteristic is typical for a lens in this class. RAW images show a light falloff of ~2.2EV (f-stops) at f/1.4. Stopping down results in a gradual decrease in vignetting. At f/5.6, the light falloff remains still somewhat noticeable at around 1EV. Auto-correction helps, although it doesn't reduce the issue to zero. At f/1.4, there's still a falloff of ~1EV and slightly less than that from f/2 onward. Auto-correcting vignetting isn't lossless, so it's sensible not to overdo it.
MTF (resolution) at 42 megapixels
The Sigma 85mm f/1.4 DG DN ART is sharp, plenty sharp. The center quality is already excellent at f/1.4, and the outer image field is easily on a very good level. Stopping down increases the quality slightly until reaching an outstanding peak at f/4. Diffraction is the limiting factor beyond. You won't really notice this at f/5.6, but f/11 and beyond should be avoided unless you require the depth-of-field.
The field curvature is very low. The tested sample had a good centering quality.
Please note that the MTF results are not directly comparable across the different systems!
Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure of sharpness.
If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations
Chromatic Aberrations (CAs)
Lateral CAs are very low and nothing to worry about.
Bokeh
You won't really invest in such a lens if you didn't intend to use it for shallow depth-of-field photography, so now that we know that the lens is sharp - let's see how it renders out-of-focus areas.
Out-of-focus highlights are nicely rendered near the image center, at least. As you can see below, the inner zone of the discs is very smooth, with just a hint of outlining. Stopping down to f/2 maintains the circular shape, and a more edgy aperture shape is just barely visible at f/2.8. The 11-rounded aperture blades are doing a nice job here.
The Sigma lens isn't immune to the deterioration of the highlight discs towards the borders/corners. Unfortunately, they deteriorate "very early" - at f/1.4, the discs are only circular near the center. The corner highlights are "cat eyes". This is a mechanical vignetting effect. Stopping down broadens the "perfect" zone, although the quality of the corner discs is certainly not ideal, even at f/4.
The quality of the general blur in the focus transition zone is good in the image background, although less so in the image foreground, which shows double halos at contrasty edges.
Bokeh Fringing / LoCA
Boheh fringing/LoCA is an axial color fringing effect with purplish halos in front of the focus point and greenish beyond.
The Sigma lens doesn't feature an APO design (like most lenses), so there's some pronounced color fringing present at f/1.4. It's reduced at f/2 and mostly gone from f/4. This can't be easily corrected via post-processing.
If you scroll through the images, you may notice that the focus extends toward the rear when stopping down (without changing the focus point) - so there's a bit of a focus shift going on (RSA = residual spherical aberrations). However, the "0" at f/1.4 remains in focus at smaller aperture settings, so it's not much of a deal in real life.
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