|
Sigma 18-50mm f/2.8 DN DC Contemporary - Review / Test Report - Analysis |
|
Lens Reviews -
Fujifilm X
|
|
Page 2 of 3
Distortion
Image auto-correction is the de-facto standard today. If activated, you don't have to worry about distortions, as shown below.
This changes quite a bit when looking at the RAW characteristic. A barrel distortion of ~5.2% at 18mm may not be the worst that we've seen, but it's nothing to be proud of due to the amount of correction necessary at this setting. This changes to medium barrel distortion at 24mm before switching to pincushion style at longer focal lengths. Overall, it's quite obvious that low native image distortions were not a primary design objective.
Vignetting
In standard shooting mode (auto-correction activated), the vignetting is well-controlled. The light falloff mostly varies around 0.6 EV (f-stops) at f/2.8 and a bit less than that at smaller aperture settings.
As you may have guessed already, the situation is drastically worse again in the original RAW files. As shown below, the light falloff is way off our usual APS-C format vignetting scale at f/2.8. This is more resembling to what we tend to see from full-format lenses (on full-format cameras). Stopping down to f/4 helps a bit, but the issue is only somewhat tamed at f/5.6 and decent at f/8.
MTF (resolution at 26 megapixels)
The Sigma 18-50mm f/2.8 DN DC Contemporary shows a comparatively even performance across the zoom range. At 18mm, the center quality is great straight at f/2.8, and the outer image field still reaches good to very good levels. The sweet spot is reached at f/5.6 with slightly elevated performance over the f/2.8 results. Diffraction sets in at f/8, and f/11 should be avoided. The characteristic remains broadly intact at 24mm with a marginally higher corner quality. There's a slight decrease in the broader center quality at 35mm @ f/2.8 and somewhat more so at 50mm. However, the quality is still perfectly fine by most standards. Stopping down a little boosts the center quality somewhat if needed.
The field curvature is low. The centering quality of the tested sample was Ok.
Please note that the above applies to 26 megapixels. We've seen a more pronounced decrease in corner performance at 18mm when used on 40 megapixels.
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)
Auto-correction will take care of lateral CAs. When looking into RAW files, the CAs are moderate at f/2.8 but increase gradually the more you stop down - peaking at an average pixel width of 2px at the borders at the wide-end at f/11. This is quite high already, but f/11 shouldn't be a mainstream setting on APS-C anyway.
Bokeh
An f/2.8 APS-C standard zoom lens may not be the last word in terms of shallow depth-of-field capabilities, but you can achieve good object isolation at shorter focus distances. So let's have a look at the bokeh quality.
Standard zoom lenses tend to struggle with out-of-focus highlights due to the extensive use of aspherical elements in their design. However, the Sigma lens delivers a comparatively decent quality in this respect. The (center) highlight discs are nicely circular at f/2.8 and f/4 before the more edgy aperture shape sets in at f/5.6. The inner zone of the discs is just slightly nervous, and the edges show a bit of outlining. Stopping down emphasizes this somewhat.
When looking at the complete image frame, the circular shape of the highlight discs is maintained up to the midfield. Beyond, they deteriorate due to mechanical vignetting, as you can see below. The discs are very much cut in the very corners. If you look closely, you may also spot that the edge rendering of the discs gets somewhat busier beyond the center. Stopping down to f/5.6 recovers most of the circular shape.
The general blur is very pleasing in the focus transition zone of the foreground (shown to the right below). The background blur rendering (to the left) is a tad more nervous but still pretty good for a standard zoom lens.
Bokeh Fringing / LoCA
Boheh fringing (also called LoCA) is an axial color fringing effect with purplish halos in front of the focus point and greenish beyond.
The Sigma lens exhibits visible fringing at f/2.8 with traces remaining at f/4. The issue is gone at f/5.6.
Sun Stars (experimental)
Below is s sequence of images from f/2.8 all the way up to f/16 - illustrating the Sunstar behavior (using an LED light). Sunstars are an aperture effect that shows up if a bright light source is part of the scene (usually in night shots). At f/2.8 the effect is very underdeveloped simply because the aperture is basically circular. Stopping down to f/4 and f/5.6 doesn't do much. Traces of a fan-like star start to develop at f/8, but it's only really visible at f/11, with best results at f/16. Overall, the effect sets in quite "late" on this lens.
|
