Testing: Longitudinal Chromatic Aberration and focus shift
Lenses with focal ratios of f2.8 or larger are often prone to longitudinal color aberrations (loCA, a.k.a. “axial color” or “bokeh CA”). These show up as magenta coloration in the foreground and greenish hues in the background and are not easily corrected in post-processing. The new Tamron is very good in this respect: there is almost no loCA to be seen at f2.8.
But there is a little focus shift when stopping down from f2.8 to f4.0: See how the 70mm mark on the left gets less sharp when the lens is stopped down to f4.0.
For comparison have a look at the Sony FE 16-35mm f2.8 GM here.
Sharpness and contrast
Let’s have a look at the theoretical performance of the new Tamron 17-28mm f2.8 Di III first and compare it to the performance of the Sony FE 16-35mm f2.8 GM:
These MTF charts show the computed lens-performance of lenses wide open without influence of diffraction. Higher values are better (more contrast) and the closer the solid and the dotted lines are together the less astigmatism (= resolution depends on the orientation of the test-pattern) the lens has. The x-axis displays the distance from the optical axis (=center of the sensor) in mm.
From the charts the new Tamron 17-28mm f2.8 Di III should be pretty good within the APS-C image-circle. But towards the full-frame corner the lens develops some strong astigmatism on the short end. On the long end astigmatism is well controlled but sharpness of the FF-corner still shows a clear drop compared to the APS-C-corner. But let’s see how this theoretical performance translates into real life results in the sharpness test based on Siemens-stars at 4 mm (center), 13 mm (APS-C-corner), and 20 mm (FF-corner) off axis. Processing was done in Lightroom 8/CRAW 11 from RAW to Adobe Color profile with the built-in lens profile for shading and CA compensation applied. Distortion compensation is currently not recognized by Adobe’s RAW converter. Noise-reduction is set to 0, sharpening to 50/0.5/36/10, with no extra tone, color, or saturation adjustment. White-balance was adjusted to a neutral white and I did some exposure compensation to make the brightness of all crops match. So you will not see light fall-off in the corners.
The following are all 100% crops!
First up is an overview of the wide-open performance at different focal lengths. You can jump to the detailed results at different apertures and comparisons with the Sony FE 16-35mm f2.8 GM by clicking on the crops of the respective focal length.
Within the APS-C image-circle the new Tamron delivers good to very good sharpness throughout the zoom-range. But the FF-corner looks pretty soft in comparison especially at 17mm (with strong astigmatism) and 28mm.
If you want to see all the details and comparisons with the Sony FE 16-35mm f2.8 GM, read on. Or you can fast-forward to the performance at long distances.
The following 100% crops for each focal length show the new Tamron 17-28mm f2.8 Di III from f2.8 down to f11 compared to the Sony FE 16-35mm f2.8 GM at f2.8.
Performance at 17mm:
The Sony is sharper wide open than the Tamron in the center and the FF-corner. At f4.0 performance of the Tamron within the APS-C image circle becomes really crisp while the FF-corner needs further stopping down to get better.
Performance at 20mm:
F4.0 makes the center look excellent, while the APS-C-corner profits from stopping still further down to f5.6. This also makes the FF-corner pretty good. At f2.8 the Sony is as sharp as the Tamron at f4.0.
Performance at 24mm:
Compared to the Sony the Tamron’s center is now right up there but the APS-C-corner and FF-corner is still a bit behind at f2.8.
Performance at 28mm:
F4.0 yields a very nice performance in the APS-C image-circle but for optimal performance of the FF-corner stop further down to f5.6. Both the Sony and the Tamron look pretty comparable within the APS-C image-circle but the Sony still has the lead in the FF-corner.
Performance at long distances
The Siemens-star test-targets are shot at a distance of 45x focal length (i.e. at around 1.2m for 28mm focal length). But performance of lenses also depends on the shooting distance. Therefore I present another series of test-shots of a city around 1 km away. Processing was done in Lightroom 8/CRAW 11 from RAW to Adobe Color profile with the lens-profile automatically applied (except for distortion compensation). Noise-reduction is set to 0, sharpening to 50/0.5/36/10, with no extra tone, color, or saturation adjustment. I used autofocus at the largest aperture and did not change focus for other apertures. All shots were made at ISO 100 and image stabilization switched off.
First up is an overview of the wide-open performance at different focal lengths. You can jump to the detailed results at different apertures and comparisons with the Sony 16-35mm f2.8 GM by clicking on the crops of the respective focal length. As usual I have selected the diagonal that provided the better corner results as almost any lens is a bit decentered.
In this long-distance test the new Tamron 17-28mm f2.8 Di III shows good to very good sharpness. Even the FF-corner does look very usable at f2.8 in this test only softening a bit at 28mm focal length.
If you want to see all the details and comparisons with the Sony FE 16-35mm f2.8 GM, read on. Or fast-forward to the next chapter on vignetting.
The main image shows the complete scene wide open to give you an impression of the angle of view and to judge vignetting. Following the main image are 100% crops for each focal length from the new Tamron 17-28mm f2.8 Di III down to f11 compared to the Sony FE 16-35mm f2.8 GM at f2.8. Both lenses were shot on the same day within minutes of each other. You can access the full resolution shots up to f11 via the links beneath the main image or clicking on the respective crops. Please respect our copyright and only use those images for personal use.
Results at 17mm:
The Tamron and the Sony look very similar in this comparison.
Results at 20mm:
Again both lenses perform very similar.
Results at 24mm:
The Tamron produces the crisper APS-C-corner and FF-corner at 24mm than the Sony.
Results at 28mm:
At 28mm the Tamron again beats the Sony at the APS-C and FF-corner.
In this test the new Tamron 17-28mm f2.8 Di III looks very similar to the Sony at 17mm and 20mm focal length and beats the Sony at 24mm and 28mm. A very respectable performance at long(er) distances.
Vignetting and distortions
Vignetting and distortions are lens aberrations that can be easily corrected by software. Lightroom and Photoshop have offered lens profiles for some time which could be applied as an option. With most mirrorless systems though, Adobe applies lens profiles automatically whether you want them or not. But on Sony Alpha mirrorless cameras you can switch lens compensation off separately for vignetting, colour aberrations, and distortions.
To make it easier to see light fall-off in the corners of a full-frame sensor I’ve arranged a series of three shots each with the new Tamron at different apertures. All images were developed to the same brightness in the center. The first line shows vignetting at 17mm focal length, the second at 28mm both without shading compensation applied:
Following is the same but with shading compensation from the lens profile applied:
The sample images above show that the lens produces some heavy vignetting wide open which is not completely eliminated even with lens profile applied.
Distortions are of the barrel type at 17mm with a strong mustachio effect and turn to pin-cushion pretty fast. At the time of writing the Adobe RAW-converter did not recognize the distortion compensation parameters. It simply rendered the RAW-files at full original distortions. At the end of the samples page there are images at different focal length that show the distortions of uncorrected RAWs developed in Lightroom versus corrected JPGs straight out of camera with lens profile applied. Hopefully Adobe/Tamron can fix this bug quickly.
Rendering of point-light sources at night-shots
Night-shots pose a different challenge for lenses as the contrast is even higher than under bright sun and point-light sources can reveal some weaknesses such as coma, haloing and colour-aberrations that do not show up as prominently in other test-shots. The 100% crops below the main image show the effect of coma in the FF-corner at the short end of the new Tamron and the Sony FE 16-35mm f2.8 GM at various apertures:
On the short end the new Tamron 17-28mm f2.8 Di III has very little coma even at f2.8 just like the Sony FE 16-35mm f2.8 GM.
Coma at the long end:
Same at the long end: both lenses show very little coma.
Rendering of out-of-focus point-light sources
This test is for the rendering of point-light sources in an out-of-focus background. The circle of confusion that is produced by this test is pretty indicative of Bokeh performance (in the background) and light fall-off. Ideally the out-of-focus image of the point-light is evenly lit and perfectly circular, with no “onion-rings”, and without coloration. Large aperture lenses normally produce an effect known as “cat’s eye” the further away from the optical axis the point-light is projected. This is due to optical vignetting in the lens barrel when light enters the lens from an angle.
All images were shot at the longest focal length and largest aperture. The new Tamron is first followed by the Sony FE 16-35mm f2.8 GM.
Shot at 28mm, f2.8 the new Tamron should produce Bokeh balls that are 20% smaller in diameter than from the Sony FE 16-35mm f2.8 GM at 35mm, f2.8. This difference is not fully reflected in the test-shots above as focus/magnification was not set exactly comparably. Comparing the other qualities the 17-28mm f2.8 Di III shows the brighter outline and stronger onion rings than the Sony GM. The cat’s eye effect is relatively mild on both lenses and Bokeh balls remain pretty circular when stopped down to f8.0.
Now let’s see how this analysis of out-of-focus point-light sources translates into Bokeh-performance shooting a book-shelf at 28mm.
As was to be expected Bokeh is not very pronounced with a lens of such short focal length and a focal ratio of only f2.8. Foreground looks a bit smoother than the more important background which suffers from outlining producing double contours. This makes hard contrast edges look nervous and not very soft. To get a more pronounced Bokeh from this lens you need to get much closer to your subject. See some of the flower shots on the next page. The Sony FE 16-35mm f2.8 GM profits in this comparison a little from its longer focal length. But other than that the Sony’s Bokeh does not look much better than from the Tamron.
As the new Tamron can achieve quite a good magnification at closer distances it is interesting to see how the lens performs in close-up shooting. When trying the lens at its longest focal length of 28mm it soon became clear that the close-up shots were pretty soft. See the first crops shot at 28mm f2.8. Shooting at 17mm focal length produced much crisper results as can be seen in the following crops. They were shot at 1:4.6 magnification where the area of sharp focus is just 110 x 166mm. The crops shown below are from 0mm, 10mm, and 18mm off the center of the sensor respectively:
The new Tamron produces pretty usable results in the APS-C image-circle at 17mm focal length even at f2.8. Still better when stopped down to f5.6 or f8.0. You just need to remember to use the shortest focal length when shooting close-ups.
Flare, ghosting, and sun-stars
Catching a strong light-source shining directly into the lens is always a risky business: it could produce strange colourful ghost-images or reduce contrast considerably through flare and glare. The appearance of flare and ghosting depends on factors like the aperture and the angle of the light hitting the lens. So to judge the proclivity of the new Tamron 17-28mm f2.8 Di III for these artefacts I went through a series of well calculated shots against a strong light source to provoke glare and ghosting.
In the following images the little bright square inset in the upper left shows the respective area with an exposure compensation of +3 EV to make it easier to see which levels of black the lens renders at that point:
The new Tamron 17-28mm f2.8 Di III is pretty clear of flare and glare artefacts except when the light-source is at or near the corner. Outside these artefacts the new lens renders a deep black, so there’s little veiling glare.
The images also show that sun-stars of the Tamron 17-28mm f2.8 Di III are well defined at the wide end of the lens while they lose some of their definition at 28mm focal length.
Next check out my sample images!Check prices on the Tamron 17-28mm f2.8 Di at B&H, Adorama or WEX. Alternatively get yourself a copy of my In Camera book or treat me to a coffee! Thanks!