Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests.
Many fast lenses have problems with flare when pointed at bright light sources - the sheer amount of glass makes suppression of internal reflections relatively difficult. With this in mind, the Tamron performs well when the sun is within the frame, giving barely any ghosting from internal reflections, or general loss of contrast from veiling flare. This is shown in the first example below.
This impressive resistance to flare persists when shooting into the light at the telephoto end of the zoom. In the second example, the sun just is outside the frame, above and to the left of the tree, and yet overall contrast is retained very well. Overall this is a pretty impressive performance.
|Canon EOS 6D, 24mm, F22||Canon EOS 6D, 70mm, F11|
The studio tests suggest the 24-70mm has relatively low lateral chromatic aberration, and this is confirmed in real-world use. As usual, CA is at its worst at wideangle, but even then it's barely worth worrying about. On Canon SLRs (which can't correct CA from third-party lenses in their JPEG processing) you'll see a little red/cyan fringing around high-contrast edges in the extreme corners of the frame; Nikon SLRs, in contrast, will compensate for this automatically. Any competent modern Raw converter will correct CA pretty easily too.
In the examples below we're looking at colour fringing in the corners of the frame at 24mm. Here we're comparing an out-of-camera JPEG from the Canon EOS 6D with the corresponding Raw file converted using Adobe Camera Raw, with CA correction enabled. There's a little fringing in the JPEG, which is removed completely from the corrected Raw conversion.
Lateral Chromatic Aberration - Canon EOS 6D, 24mm F8
|Camera JPEG||Raw converted in ACR with CA correction|
|Camera JPEG, 100% crop||100% crop: fringing removed|
The tests show that the 24-70mm F2.8 exhibits about 2 stops vignetting wide open at the each end of its range, but somewhat less in the middle, which is fairly typical for its class. At both 24mm and 70mm it gives quite abrupt vignetting in the extreme corners, which can be visually quite intrusive in some some situations. Naturally this can be corrected in post-processing using software such as Adobe Lightroom or DxO Optics Pro. But while most SLRs can correct vignetting when used with the manufacturer's own lenses, they won't with third party optics like the Tamron.
The rollover below shows images shot at F2.8 using the Canon EOS 6D, looking at RAW files converted in Adobe Camera Raw with and without vignetting correction enabled. At 24mm the uncorrected vignetting doesn't look too bad in isolation, although here it's accentuated when flicking back and forward with the corrected version. However at 70mm the vignetting is quite intrusive, and we suspect many users would prefer to tone it down a little in post-processing in this kind of shot. To be fair though, this isn't really any worse than the Canon EF 24-70mm f/2.8L II USM.
|Canon EOS 6D, 24mm F2.8, uncorrected||24mm F2.8, corrected|
|Canon EOS 6D, 70mm F2.8, uncorrected||70mm F2.8, corrected|
The Tamron shows fairly strong barrel distortion at wideangle, which can be a problem with certain types of shot. This distortion is notably higher than its Canon and Nikon counterparts, and while it's also pretty strong compared to the Sigma and Sony 24-70mm F2.8s, they both exhibit complex 'moustache' distortion. The rollover below shows an instance where the Tamron's barrel distortion has resulted in a decidedly odd-looking picture out-of-camera, and the corrected version looks much better.
Canon EOS 6D, 24mm
|Distortion uncorrected||Distortion corrected in ACR|
Background blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and/or a large aperture. The 24-70mm's fast maximum aperture means it can provide nicely-blurred backgrounds at the telephoto end.
The degree of blur isn't all that matters, of course, but also its aesthetic quality (which is what the word 'bokeh' refers to). This changes with focal length, aperture, focus distance and background distance, so isn't possible to pin down in simple terms. But after shooting hundreds of real world shots with the lens, we'd say the Tamron generally acquits itself well, delivering attractively-blurred backgrounds most of the time. A couple of examples at different focus distances are shown below.
|EOS 650D, 70mm F2.8, close-up||EOS 6D, 70mm F2.8|
|Background detail||Background detail|
In the first close-up shot, the Tamron does particularly well - the background is beautifully blurred, and the transition from the in-focus rose bud to the out-of-focus leaves smooth and attractive. These qualities tend to persist on stopping down, too. In the second example the distant background is a bit more 'busy', but it's far from unpleasant if you view the image as a whole.