The 18-270mm produced very impressive test results towards the middle of the zoom range (around 35-100mm), but was less convincing at wideangle and towards telephoto. It stands up well in comparison to both the Nikon 18-200mm F3.5-5.6G ED VR and the Canon EF-S 18-200mm F3.5-5.6 IS; it's softer than the Nikon at wideangle and the Canon at telephoto, but beats both in that mid-range. Our test sample exhibited a slight decentering defect, which most obviously gave a soft lower-left corner at 18mm and F3.5-5.6; we would expect a better-aligned sample to show measurably higher sharpness at wideangle.
|Sharpness||Sharpness is very good indeed from 35-70mm, especially at optimum apertures around F5.6-F11 (indeed at 35mm F8 it's excellent right across the frame). The lens is less good at wideangle, although quite acceptable at apertures of F8-F11. Likewise sharpness falls again towards the telephoto end; at 270mm the lens appears to be sharpest wide open, but this likely reflects a slight focus shift due to spherical aberration.|
|Chromatic Aberration||Chromatic aberration is remarkably well controlled considering the huge zoom range. At wideangle we see red/cyan fringing which is relatively low for this class of lens, but between 35 and 100mm CA is essentially negligible. However at 200mm, green/magenta CA appears again, and at 270mm it becomes distinctly problematic.|
|Falloff||We consider falloff to start becoming a potential problem when the corner illumination falls to more than 1 stop below the center. The 18-270mm performs very well in this regard; there's about one stop of falloff wide open at 18mm, which essentially disappears by F4.5; but apart from that, nothing which is likely to be photographically relevant at all.|
|Distortion||As is common with superzooms, distortion is quite visible across much of the range. Barrel distortion is rather pronounced at wideangle (+3%), and at longer focal lengths we see pincushion distortion which peaks at 50mm and -2%. (In comparison, the Canon 18-200mm shows more extreme barrel distortion at wideangle, and the Nikon higher pincushion distortion in the mid-range.)|
|Maximum magnification is 0.32x, achieved at 270mm and a closest focus distance of about 42cm (significantly closer than Tamron's stated 49cm), giving a usefully long working distance of 18cm from the subject to the front of the lens.
Image quality is OK; the image is very soft at F6.3, with significant halation due to spherical aberration, but the center sharpens up well at F8. The corners of the F8 chart shot are soft due to curvature of field, but sharpen up by F16. The image also shows mild barrel distortion and some chromatic aberration.
The spherical aberration also manifests as a significant focus shift on stopping down, demonstrated later in the review.
|Macro - 74 x 49 mm coverage
Distortion: Mild barrel
Corner softness: Poor
Focal length: 270mm (405 mm equiv)
FX (Full Frame) Coverage
Both the Nikon and Canon mount versions of this lens will mount on full-frame DSLRs; on Nikon cameras (D3, D3X, D700) DX crop mode will be automatically selected (and the camera will therefore shoot at reduced resolution). The lens's image circle doesn't cover the 35mm full frame format properly at any focal length, giving severe vignetting; this is very obvious and well-defined at 18mm, but gets increasing diffuse at longer focal lengths. This is illustrated in the samples below.
The bottom line is that if you shoot using a full frame camera, a lens with appropriate coverage really does make far more sense.
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 turned out to be perfectly competent in normal use, with few serious deficiencies.
Given the fact that the 18-270mm has no fewer than 18 elements in its construction, control of flare is impressive. Even when shooting into the low winter sun, the lens simply refused to show much in the way of flare problems at all. With the sun in the corner of the frame at wideangle, just the faintest of flare patterns becomes visible on stopping down to F16 or smaller; in strong backlight at full telephoto (and therefore with lots of light impinging on the front element) there's scarcely any loss of contrast visible. Not bad at all.
|18mm F18, sun in corner of frame||270mm F7.1, strong backlight|
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. This lens can allow you to achieve quite substantially blurred backgrounds, especially at longer focal lengths and wide apertures.
Bokeh is generally reasonably neutral, neither adding to nor detracting from the image as a whole. For closeup shots, backgrounds are smoothly blurred and perfectly attractive; however at shorter focal lengths and with more distant backgrounds, bokeh can start to become a bit less smooth and more 'busy'. To be fair though, few superzooms are going to do any better.
|270mm F6.3||50% crop|
|85mm F5.3||50% crop|
Superzooms are always going to suffer from chromatic aberration to some degree, but Tamron has managed to produce a lens which is overall no more afflicted than others in its class, despite the extended zoom range. Red/cyan ringing is visible at wideangle of course, but its no worse than the other superzooms we've tested, and in the middle of the zoom range it disappears completely. The biggest problem with CA is reserved for the extreme telephoto end, and significant green/magenta fringing can be visible on many shots. However it's also important to appreciate that the visibility of fringing is highly dependant upon image content; if you shoot typical telephoto subjects such as wildlife for which the subject tends to be fairly central, with the corners beyond the depth of field, then fringing will be much less obvious (see the samples gallery for more examples).
Of course if you shoot JPEGs with Nikon bodies such as the D300 and D90 which feature automatic CA compensation, the effects of lateral CA will be significantly reduced anyway.
|F13, Nikon D300||F9, Nikon D90|
|100% crop, top left corner||100% crop, top right corner|
|100% crop, Nikon D300 JPEG||100% crop, Nikon D90 JPEG|
Tamron has seen fit to include the word 'Macro' in the 18-270mm's full title (although it can be easy to miss amongst all the other letters), so we might reasonably expect close-up performance to be something above the average. But while the closest focus distance is indeed pretty impressive, and the magnification therefore towards the top of the class (about equal to the Canon 18-200mm), optical performance is somewhat problematic due to the effects of spherical aberration. This manifests in two ways, namely extremely soft results wide open, coupled with a substantial focus shift on stopping down.
The example below illustrates these effects using a three-dimensional test subject. The camera was a Nikon D300, and the lens was set to 270mm and minimum focus. A macro rail was then used to focus on the fourth match head from the front, using magnified live view at F8 (judging precise focus at F6.3 was near-impossible due to the blurring effect of halation). The picture was then shot at apertures from F6.3 through to F16.
|270mm F6.3||270mm F8||270mm F11||270mm F16|
When shot at F6.3, the point of sharpest focus is on the third match head, and the entire image is heavily softened due to intense halation. The image sharpens considerably at F8, with optimum focus moving towards the fourth match head as expected. On stopping down further, the region of sharpest focus shifts progressively further away from the camera (naturally depth of field also increases). More controlled macro rail tests indicated a focus shift of about 6mm between F6.3 and F8. The upshot of this is that when shooting macro, it's advisable to focus right at the front of the region you want to be sharp, and to shoot at apertures of F8 or smaller where possible.
The 18-270mm features Tamron's new vibration reduction system (first seen on the 28-300mm F3.5-6.3 Di VC full-frame superzoom), which claims to allow handholding at shutter speeds 4 stops lower than usual before blur from camera shake becomes apparent. The mechanism is near-silent in use, with just some high-pitched whirring noises when it activates and deactivates, presumably from the VC group moving in and out of the 'at rest' position.
We've generally found the stabilisation units in SLR lenses to be pretty effective in real-world use, and to quantify this, we subjected the 18-270mm to our studio image stabilisation test, using the wideangle and telephoto settings plus one mid-range focal length (50mm). With its effective focal length range of 27-405mm, we'd normally expect to be able to get good results handheld at 1/50 sec at wideangle, and 1/500 sec at telephoto without image stabilisation. The subject distance for these tests was approximately 2.5m for 18mm and 50mm focal lengths, and 10m for 270mm.
We take 10 shots at each shutter speed and visually rate them for sharpness. Shots considered 'sharp' have no visible blur at the pixel level, and are therefore suitable for viewing or printing at the largest sizes, whereas files with 'mild blur' are only slightly soft, and perfectly usable for all but the most critical applications.
|18mm VC OFF||50mm VC OFF||270mm VC OFF|
|18mm VC ON||50mm VC ON||270mm VC ON|
Tamron has only been making optically stabilisated SLR lenses for little over a year, but you wouldn't know it from these tests; their vibration control technology is a close match to both Canon's IS and Nikon's VR in our tests. Whilst it doesn't quite seem to deliver the 4 stops claimed by Tamron, it's not too far off, especially at wideangle (unfortunately it appears to be a bit less effective at telephoto, where you need it the most).
At 18mm, we got bettter results a 1/3 sec with VC on than at 1/25 sec with it off; at 50mm, similar results at 1/6 sec with VC on vs 1/50 sec with VC off; and at 270mm, similar results at 1/30 sec with VC on compared to 1/250 sec with it switched off. The take-home message is that activating the VC system allows you to get usable shots (sharp or at worst slightly blurred) at shutter speeds at least three stops slower than when it is turned off. Even at slower speeds, there's a good chance of getting 'keepers' hand held if you're prepared to take multiple shots. This therefore greatly increases the lens's versatility in low-light conditions (or indeed any other situation where a slow shutter speed would be desirable).
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