Tamron SP 24-70mm f/2.8 Di VC USD G2 - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

The Tamron SP 24-70mm f/2.8 Di VC USD G2 produces a varying degree of distortions throughout the zoom range. At 24mm there is a clearly visible barrel distortion of 3.5% which is a little higher than average for a lens in this class. Distortions are negligible around the 35mm mark but increase again towards the long end of the range. At 70mm you have to live with medium pincushion distortion.

Vignetting

In terms of light-fall off the Tamron has its weak spot at 24mm (as usual in this class). At 2.5 EV (f-stops) it is very substantial at fully-open aperture. Stopping down to f/4 makes this more manageable but even at f/8, the fall-off remains around the 1 EV (f-stop) mark which is a little high in our book. When zooming out, the initial vignetting is lower, of course, but remains beyond what we've seen on the Canon EF 24-70mm f/2.8 USM L II. However, it is not relevant anymore from f/5.6 onward in the 35-70mm range.

MTF (resolution) at 50 megapixels

It may surprise you, but despite the moderate range high-speed standard zoom lenses are incredibly difficult to design - and to manufacture. Thus it doesn't come as a surprise that the Tamron shows a mixed performance when throwing a high megapixel sensor at it. As you may have guessed, this is most obvious at f/2.8. At 24mm the center quality is generally very good and remains on a high level in the broader center zone. However, the image corners are very soft. Stopping down to f/4 solves the issue by boosting the corners to good levels at least. The peak performance is reached between f/5.6 and f/8 with a very good performance across the image field. This characteristic remains similar at other focal lengths with the same corner weakness at large apertures although the border zone is also increasingly affected. Even so the lens remains very sharp around the f/8 mark at all settings. Diffraction has, as usual, a more pronounced effect from f/11 onward.

The centering quality of the tested sample was Okay albeit not perfect. Getting that golden sample with perfect centering can be daunting and the inclusion of an image stabilizer doesn't make things easier in this respect (...).The amount of field curvature is comparatively low.

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

MTF (resolution) at 21 megapixels

Why are the MTF numbers sometimes "better" on 21 megapixels compared to 50 megapixels ? There are two reasons for this. Lateral CAs are lower in terms of pixel widths at 21mp simply because the pixel density is also lower. Extreme CAs that may exist at 50mp are therefore less affecting the measurements at 21mp. Generally, we are also using a certain degree of sharpening during the image conversion (just like in real life images) and because the 21mp results are "sharper" on pixel level they are relatively more receptive to (mild base-) sharpening.

Many users are still using cameras with a more moderate pixel count so let's have a look how the lens performs with the eased requirements at 21 megapixels - also for comparing the results to older (EOS 5D II) reviews.

Unsurprisingly, the performance is better on a per-pixel level. However, the corner quality is still not hot at f/2.8 albeit the borders are at least acceptable. Stopping down to f/4 solves most of the corner issues and the sharpness is pretty impressive at medium aperture settings.

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 chromatic aberrations (color shadows at the image borders) are low at the image borders although they are amplified quite a bit in the extreme corners (not shown below). The CAs peak at the extreme ends at f/2.8 with an average pixel width of around 1.5px. That's just visible albeit not really disturbing. Stopping doesn't really make a difference at 24mm albeit there's some improvement at 70mm. The CA are not as an issue around the 50mm setting.

PS: The CA figures were taken at 50 megapixels.

Bokeh

The bokeh is the aesthetic quality of the blur produced in the out-of-focus areas of an image. The Tamron SP 24-70mm f/2.8 Di VC USD G2 is certainly not as capable in producing a very shallow depth of field as fast prime lenses in its range but it is certainly usable for portraits etc.

The lens features several aspherical elements and unless they are manufactured to the very highest standards, such elements have a negative impact on the rendition of out-of-focus highlights. As you can see below, the highlights discs show an onion-like substructure as well as heavy outlining. On the positive side, the discs are circular till at least f/5.6 albeit they are, as usual, deteriorating in shape towards the image borders. Please note that the Canon EF 24-70mm f/2.8 USM L II isn't any better either.

When looking at the general quality of the blur in the focus transition zone, it is actually quite smooth both in the image background and the foreground is just a tad busier.