Tamron SP 15-30mm f/2.8 Di VC USD G2 - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

Ultra-wide zoom lenses are rarely champions in terms of image distortions and this also applies to the Tamron SP 15-30mm f/2.8 Di VC USD G2. The issue is, as usual, most pronounced at the 15mm with a heavy barrel distortion of ~3.4%. However, the barrel distortion eases rapidly when zooming out and it's not really annoying anymore at 18mm. Neutral distortions are reached around the 20mm mark and there are mild pincushion-type distortion towards "longer" focal lengths.

Vignetting

Vignetting is, of course, present in all images but, surely also thanks to the immense front element, it's not quite as bad as one might think. Again, the issue is most pronounced at 15mm with a clearly visible light falloff of 2.3 EV (f-stops) at f/2.8. However, if you stop down to f/5.6 the vignetting is actually pretty good by full-format standards. The vignetting isn't quite as pronounced at other focal lengths but unless needed, you should stop down to f/4 at least.

MTF (resolution at 50 megapixel)

High-resolution image sensors tend to be brutal to lenses that feature an extreme design but the Tamron does actually a pretty good job. The center is generally excellent straight from f/2.8 and the near center is also sharp. The outer image field is soft at the extreme ends but usable in the middle range (at f/2.8). Stopping down to f/4 solves most of the softness issues at the borders/corners and there's a general boost in the image center. The sweet spot is reached around the f/5.6 mark. The center remains impressively sharp and the borders/corners are on a good to very good level here - across the range. Visible diffraction effects kick in at f/11 but the setting is still fine for mainstream usage.

The field curvature is slight at 15mm but negligible at other settings. The centering quality of the tested sample was good.

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

Chromatic Aberrations (CAs)

Lateral CAs (colors shadows at the image borders) are quite well-controlled but you can spot them during pixel peeping. The average CA pixel width remains under 2px at the image borders in the middle part of the zoom range. Interestingly they are slightly better at the extreme ends which is a little unusual.

Bokeh

Modern ultra-wide lenses are heavily relying on aspherical elements which tends to have negative side effects on out-of-focus highlights. Below, you may spot that the highlights aren't perfectly clean but they are certainly better than in most lenses in this class.

The general rendition of out-of-focus areas is actually pretty good as you can see in the sample image below. Many ultra-wide lenses suffer from smearing or rather ugly halo effects but, while not perfect, Tamron did a good job here.

Flare

The first generation of this lens was quite prone to flare which is probably why Tamron felt the need to improve upon this by applying its new "AX" coating. Still, when pressed you can produce greenish ghostings as shown below albeit the image contrast usually remains on a high level. You may also spot that the lens renders the sun with a starlight effect which is a desirable effect in such scenes.