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. The Tamron 18-270mm PZD performed pretty well in real-world use on an EOS 650D, with its main practical disadvantage being slower autofocus compared to its peers.
The 18-270mm PZD, like many complex zooms, can be susceptible to flare when shooting into the light, although in our experience it generally behaved pretty well. Place the sun at the edge or corner of the frame and you'll get a flare pattern from internal reflections that gets progressively more intense and better-defined on stopping down, but it's not hugely intrusive, and overall contrast remains high.
|18mm, F22, sun at edge of frame||50% crop, top centre|
As with other superzooms you can also get problems from pointing the lens into the light such that the sun is just outside the frame, especially at longer focal lengths where the hood is least effective. But it's generally easy enough to spot through the viewfinder when this is likely to be a problem, and shield the front element with your hand.
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 large aperture. Like all superzooms, though, its relatively small maximum aperture means you're not going to get a huge amount of blurring most of the time - you'll get best results shooting wide open at telephoto.
The 18-270mm is much like other superzooms here - not especially pretty, but not terribly unattractive either. It provides slightly more attractive results with close-up shooting at the long end of the zoom, compared to more-distant backgrounds which can look slightly more fussy.
|250mm F6.3||Background detail, top left|
|46mm F5.0||Background detail, center left|
The 18-270mm, like most superzooms, shows significant lateral chromatic aberration at each end of the zoom range. This is visible as strong green/magenta fringing towards the edge of the frame at both ends of the zoom, that's most pronounced at 300mm. In the middle of the zoom range there's scarcely any CA visible at all.
Lateral CA can normally be removed pretty effectively in software if necessary, and we've illustrated this in the examples below using DxO Optics Pro 8. The 100% crops from the camera's JPEG output show obvious green/magenta fringing, which has cleaned up well in the corrected RAW conversions. Most current and recent Nikon DSLRs will automatically correct it in their JPEG output, too, but unfortunately Canon and Sony SLRs won't (their lens correction options only work with the companies' own lenses).
|F7.1, Canon EOS 650D, ISO 200||F8, Canon EOS 650D, ISO 200|
|100% crop, left (JPEG)||100% crop, lower right (JPEG)|
|RAW corrected with DxO Optics Pro 8||RAW corrected with DxO Optics Pro 8|
Like most superzooms, the Tamron 18-270mm PZD suffers strongly from distortion - barrel-type at wideangle, and pincushion through most of the rest of the zoom range. Note that because it's a third-party lens, the distortion correction functions built into modern SLRs won't work with it, as the required profiles are only available for the manufacturers' own lenses.
The Tamron's distortion is slightly complex in character, and this means that generic software lens correction routines won't always correct it completely. Only programs that use specific profiles for each individual lens (such as DxO Optics Pro or Photoshop's Lens Correction Module) will produce perfectly straight lines.
This is illustrated in the examples below - the 'corrected in PS' versions use Photoshop's generic distortion correction routines, and lines along the edges of the frame aren't perfectly straight (although they're a lot better than the originals). In contrast the 'corrected in DxO' versions use DxO Optics Pro 8's profiled distortion compensation, and produce a visibly better result.
18mm corrected in PS
18mm corrected with DxO
50mm corrected in PS
50mm corrected with DxO