|
Page 2 of 2
Distortion
The Sigma produces an absolutely negligible degree of barrel distortion on DX cameras.
The chart above has a real-world size of about 120x80cm.
Vignetting
The Sigma lens enjoys a sweet spot advantage when used on APS-C DSLRs so the light falloff isn't quite as pronounced as within its native format. At f/1.4 there's still some vignetting which may be visible in critical situations. Stopping down to f/2 resolves this issue from a field perspective.
MTF (resolution)
Full format prime lenses tend to have an easy play on DX DSLRs in terms of resolution. However, the Sigma struggles somewhat at least at max. aperture. The center quality, and that's certainly where it counts most in this lens class, is already excellent. The border and corner quality is just fair here though. Stopping down to f/2 boosts the borders to good levels and also the corners recover quite a bit as well. The image quality is extremely high between f/2.8 and f/8. The peak performance is reached around f/5.6.
The field curvature is marginal and the overall centering quality of the tested sample was very good. In addition, the amount of focus shift when stopping down was very low.
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 for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations
There is one issue that has prevented better corner/border results: the resolution figures suffer quite a bit due to rather pronounced coma at large apertures - very sharp contrast transitions towards the center, soft contrast transitions towards the outer image field. The shot below, a crop of our MTF test chart, illustrates the issue. As to be expected, it is less pronounced than on FX, though.
Chromatic Aberrations (CAs)
Lateral chromatic aberrations (color shadows at harsh contrast transitions) are generally very low with an average pixel width of 0.5px to 0.7px at the image borders. Note that CAs can easily be corrected in software or by the camera itself (if you shoot JPEGs and own a current Nikon DSLR).
Bokeh
The quality of the bokeh (out-of-focus blur) is a primary aspect for such a large aperture lens. The Sigma is capable of delivering very pleasing results here. Out-of-focus highlights have a very smooth inner zone and they remain circular till f/2.8. At f/4 you can spot some traces of the aperture shape. The "cat's eye" highlight shape in the image corner is largely avoided on DX DSLRs although there's a slight tendency towards an ellipsoid shape at max. aperture (due to mechanical vignetting). The foreground blur is buttery and the background is also very smooth albeit a tad more nervous.
Bokeh Fringing
Bokeh fringing is a general problem in this lens class. As you can notice below the halos have different colors - magenta (red + blue) in front of the focus point and green beyond. The Sigma struggles somewhat more than usual here. The color fringing is very pronounced at f/1.4 and f/2. The effect remains visible at f/4 and there are also traces left at f/5.6.
These shots also show the almost complete lack of focus shift when stopping down, as mentioned in the MTF section.
|
Move the mouse cursor over the f-stop marks below to observe the respective bokeh fringing
|
| f/1.4 |
f/2 |
f/2.8 |
f/4 |
f/5.6 |
|
|
Sample Images
You can find some sample images taken with the Nikon D3x in our Nikon FX review of the lens.
VerdictThe Sigma AF 85mm f/1.4 EX HSM DG delivers a convincing performance on DX DSLRs. The lens is impressively sharp in the image center - even at f/1.4 - but it's comparatively softer towards the borders/corners here. However, the Sigma is nothing short of excellent in the range between f/2.8 and f/8.
Lateral CAs are minimal and not field-relevant. Same goes for image distortions. The vignetting is only a slight problem at f/1.4 and not an issue anymore from f/2 onwards. The quality of the bokeh is generally excellent except in very tricky situations. The amount of bokeh fringing is on the high side,so you may spot some colored out-of-focus halos in critical situations.
The build quality of the lens is up to pro standards. The metal lens body is rubber-coated thus providing a good grip. The tightly assembled focus ring turns smoothly but it's a bit tricky to find the exact focus due to the very short focus path. On the other hand the same short focus is also responsible for fast auto-focusing so there's a certain trade-off here. We had no significant issues with focus accuracy on the tested sample. The Sigma AF 85mm f/1.4 EX DG HSM is quite expensive in absolute terms, especially for a third-party lens. However, compared to Nikon's latest offering it costs much less and as such is a highly interesting alternative.
| Optical Quality (Technical): |
 to  |
|
|
| Field Quality: |  (Shallow Depth-of-Field Photography) |
| Mechanical Quality: |  |
| Price/Performance: | |
| | |
| | What does this mean ? |
|
