Longitudinal Chromatic Aberration and focus shift
With lenses offering an aperture of f2.8 or larger I test for longitudinal CA (loCA, a.k.a. “axial color” or “bokeh CA”). The new Tamron shows a clear magenta coloration in the foreground (left) and somewhat weaker greenish hues in the background (right). This also shows up in real-life shots. By f5.6 the effect is gone. The test also revealed a bit of focus shift.
Sharpness and contrast
Let’s have a look at the theoretical performance of the new lens first compared to the Sigma 50mm f1.4 “Art”:
These charts show the lens-performance at the largest aperture. Higher values are better and the closer the dotted and the continuous lines of each color 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. I’ll show you the real-life performance at 4 mm (center), 13 mm (DX-corner), and 20 mm (FX-corner) on a D810.
From the charts, the new Tamron should perform better than the Sigma. The Tamron shows a very flat and graceful decline of contrast and resolution towards the corners of the FF/FX sensor while the Sigma drops significantly from 10mm onwards. Remember though that the Tamron is shown here at f1.8 which is 2/3 of a stop darker than the Sigma at f1.4. But let’s see how this theoretical performance translates into real life results in the sharpness test based on Siemens-stars.
What follows are near-center results (first column) followed by APS-C/DX-corner results and FF/FX-corner results on a D810. The D810 results from the DX-corner should be a very good approximation for performance on a 16MP DX sensor (like the D7000), because the pixel-pitch of both sensors are the same. But differences in the AA-filter and micro-lens-design of a D810 and a D7000 might yield different end-results.
Processing was done in Lightroom 5.7.1 from RAW at Camera Standard settings. Noise-reduction is set to 0, sharpening to 35/0.5/36/10, 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 match. CA-removal is ON.
The following are all 100% crops!
These 100% crops directly from a 36MP D810 sensor show that this lens delivers very good performance across a full-frame sensor right from the start. To get the highest resolution in the center you need to over-shoot a bit (back-focus) which led to the magenta color-shift in the f1.8, f2.0 and f2.8 crops. All crops for each aperture are from the same image as there was no need to optimize focus for the different crops: proof that the lens has no field-curvature to speak of. DX-corner and FX-corner show a little less contrast wide open but coma seems well under control in the FX-corner. At f2.8 and from f4 onwards the lens is very sharp and contrasty across the whole frame until diffraction sets in at f11. Distortions are very low.
Following is the performance of the Tamron 45/1.8 VC at f2.0 (1st row) directly compared to the Zeiss 55/1.4 Otus (2nd row), Sigma 50/1.4 Art (3rd row) and the Nikon 50/1.4G (last row) at f2.0. The three competitors were shot on a D800 with a slightly softening AA-filter and got a sharpening of 70/0.5/36/10 to make them comparable.
This comparison is proof that the Tamron is indeed a great performer. It looks almost as good as the Zeiss Otus were it not for the coloration from longitudinal CAs. And both lenses share an almost complete lack of field curvature at this shooting distance. Compared to the Sigma the Tamron is on a par in the center and beats it in the DX- and FX-corner. The Nikon only claims fourth place in this comparison suffering from lower overall contrast and strong coma.
Performance at large distances
The Siemens-star test-targets are shot at a distance of 40x focal length (i.e. at 1.8m for 45mm focal length). But performance of lenses also depends on the shooting distance. Therefore I present another series of test-shots of a landscape dubbed the “Unremarkables” where you can measure distances in km, not meter. Processing was done in Lightroom 5.7.1 from RAW at Adobe Standard settings. Noise-reduction is set to 0, sharpening to 35/0.5/36/10, with no extra tone, or saturation-adjustment. There’s no tinkering with vignette-control so you see it here as it is produced by the lens. Focus was acquired at the largest aperture manually and not changed for other apertures.
You can click on each image to access the large original. Please respect our copyright and only use those images for personal use.
The main image shows the complete scene at f1.8 to give you an impression of the angle of view and to judge vignetting. This is followed by one row of 100% crops at different apertures each from near the middle, the DX/APS-C-corner, and the FX/FF-corner. You can access the respective shots up to f16 via the links beneath the main image.
Following is a comparison at f2.0 with the Sigma 50/1.4 “Art” (2nd row) and the Nikon 50/1.4G (3rd row). Both shots from the Sigma and the Nikon were made last year in summer on a D800 and profited from clearer atmospheric conditions. For the comparison I adapted white-balance and exposure in post-processing but did not touch contrast. The images with the Tamron on the D810 (which has no AA-filter) were sharpened at 35/0.5/36/10 while the shots from the Sigma and Nikon on the D800 (with AA-filter) were sharpened at 70/0.5/36/10. So although not exactly comparable the crops should give you a good impression of how those lenses compare.
Performance of the Tamron wide open in this long-distance shot is again very impressive in the FF/FX-corner which profits from the very low coma of this lens. But unlike in the comparison of the Siemens star test-shots the Zeiss Otus is now clearly ahead of the Tamron. Compared to the Sigma “Art” and the Nikon the Tamron looks a bit softer in the center than the very sharp Sigma but better than the Nikon. In the DX-corner the Tamron, Sigma, and Nikon look alike at f2.0. And in the FX-corner the new Tamron performs clearly better than the Sigma or the Nikon and is only bested by the Zeiss Otus. Stop the Tamron down to f4.0 and its performance becomes very good across the full-frame sensor.
All-in-all the new Tamron 45/1.8 VC is a very good performer near and far with a slight weakness at the border of the APS-C/DX image circle at long-distance shots. Field curvature is negligible and distortions are moderate. Only vignetting is an issue.
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 color-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 FX corner of this lens at various apertures. From f4.0 the effect is negligible:
As demonstrated in the other tests above the lens has indeed a very good correction for coma. This is responsible for the excellent resolution in the FF/FX-corner.
Unfortunately some CA show up none-the-less in the form of blue/magenta halos around point-lights at f1.8. This effect is visibly reduced already at f2.0 and gone at f2.8 as the following crops from the center show:
I think this effect is due to longitudinal CA and can be reduced by pulling focus a bit more towards the camera. But that would compromise optimal sharpness. Following is another 100% crop from a typical situation where this “magenta ghosting” raises its head – although it is not as ugly as with Tamron’s 35/1.8 VC.
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.
As the 50% crops above show: The Tamron 45/1.8 VC produces only a small circle of confusion. It exhibits marginal onion rings, a green outline which stems from the longitudinal CA of this lens, and an uneven light-distribution across the circle outside the optical axis. There’s a clear cat’s-eye effect towards the borders/corners of the sensor but there is no clipping from the mirror-box.
Following is the same shot done with the Nikon AF-S 50mm f1.4G for comparison:
As you can see the 50mm f/1.4 lens produces a 50% larger circle of confusion near the center which stems from the 2/3 larger maximum aperture and the 10% larger magnification from the 50mm focal length. The “Bokeh balls” are also smoother with no onion rings as the Nikon does not have any aspherical elements which typical produce this effect. But at the border of the APS-C/DX image circle and beyond the advantage of the Nikon fades a bit as the diameter of the Bokeh balls shrink pretty fast and develop some nasty hot-spots. Still in the most important area of an image the Tamron 45/1.8 VC cannot match the Bokeh of the Nikon 50/1.4G.
Now let’s see how this analysis of out-of-focus point-light sources translates into Bokeh-performance shooting a book-shelf.
While the (less important) foreground is nicely blurred, the middle ground suffers from longitudinal CA coloring the black letters green. The lens even introduces some magenta color inside the “a”s and “e”s. The background produces a strong blur but shows some outlining which makes the Bokeh somewhat nervous.
Catching a strong light-source shining directly into the lens is a real risk with wide-angle lenses – even with the lens-hood attached. That could produce strange colorful ghosts-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 Tamron for these artifacts I went through a series of well calculated shots against a strong light source to provoke glare and ghosting.
The results are pretty good: As long as the light was clearly outside the image-frame I found no artifacts – although the light was shining directly into the lens. When the light was near the image corner but still outside the frame it provoked a flare:
So you have to watch out a bit for strong light-sources directly outside the frame. But other than that the flare-resistance of this lens is good. The following image shows the amount of ghosting that you can provoke:
Considering the extreme test-conditions ghosting is pretty moderate.