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Tamron 17-70mm f/2.8 Di III-A VC RXD - Review / Test Report - Analysis |
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Lens Reviews -
Sony Alpha/NEX (APS-C)
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Distortion
Image auto-correction is the de-facto standard today. Activated, the distortions are negligible.
When looking a bit behind the scene, the Tamron lens produces a fairly strong barrel distortion at 17mm switching to medium distortions from the middle range up to 70mm. In the grand scheme of things, this is still moderate compared to some other standard zoom lenses that we tested.
Vignetting
In standard shooting mode (auto-correction activated), the vignetting is moderately high at f/2.8 and not overly relevant anymore from f/5.6 onward.
The RAW vignetting reveals that there's quite a bit of correction necessary, especially at the extreme ends of the zoom range. While a light falloff of more than 2 EV (f-stops) wouldn't be all the unusual if it was a full format lens, it's off our charts in the APS-C scope. Keep in mind that vignetting correction comes at the cost of increased image noise.
MTF
While you may not be all that impressed so far, this may change with respect to the resolution characteristic of the Tamron 17-70mm f/2.8 Di III-A VC RXD. It produces a very homogenous, high image quality across the zoom range and the relevant aperture range. The center quality is generally excellent between f/2.8 and f/5.6, and the outer image field is still very good, with just a little drop in quality at 70mm. Diffraction has an impact from f/8 but f/11 remains perfectly usable.
The field curvature is quite low. The centering quality of the tested sample was Ok.
Please note that the MTF results are not directly comparable across the different systems!
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 of sharpness.
If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations
Chromatic Aberrations (CAs)
Olateral CAs are usually auto-corrected, so you don't have to worry in real life. When looking into the RAW results, the CAs are moderate, although they increase somewhat in the far corners. The average CA pixel width varies between 1-1.5px at the image borders.
Bokeh
If you followed the social media coverage about this lens, you might have noticed critical comments about the quality of the bokeh. We can confirm this to some degree, but you have to keep some context really - we are talking about a standard zoom lens here. There is no modern, standard zoom lens with a (near-)perfect bokeh out there. These lenses tend to have multiple aspherical elements, and it would be highly unusual if this didn't have a negative impact on the out-of-focus rendering.
In the case of the Tamron lens, this is most obvious with out-of-focus highlights. The inner zone of the discs is very busy, and the disc edges are slightly outlined.
The circular shape of the highlights deteriorates somewhat towards the image borders. However, the effect is comparatively moderate on this lens, with a very broad zone with circular highlights. As usual, stopping down restores the corner discs to their circular shape.
While the highlight discs may not be ideal, the rendering in the focus transition zones is actually pretty good.
The background blur is smooth and symmetrical, and even the foreground blur is pretty good.
Bokeh Fringing / LoCA
Boheh fringing (also called LoCA) is an axial color fringing effect with purplish halos in front of the focus point and greenish beyond.
The Tamron lens isn't an APO design so it exhibits some fringing at f/2.8. This is reduced at f/4 and traces remain at f/5.6.
Sun Stars (Experimental)
Below is s sequence of 100% cropped images from 17mm f/2.8 all the way up to f/16 - illustrating the sun star behavior. Sun stars are an aperture effect that shows up if a bright light source is part of the scene (usually in night shots) - here, illustrated using an LED. The effect is barely present at f/2.8 - simply because the aperture is essentially circular. The first star rays appear at f/5.6, and they get increasingly pronounced the more you stop down. The rays are nicely pointy, as you may notice below.
Flare
Flare artifacts are a fact of life for all lenses. It's just a matter of how many are produced in scenes with strong light sources. Overall, Tamron did a relatively good job here. Below is a rather extreme scene shot taken into the sun (and yes, normally, you wouldn't take this image, it's for illustration only). You can notice some colored blobs and stripes in the circled area below. These rainbow colors are not ideal, whereas the low loss in overall contrast is good given the extreme light conditions.
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