Optical quality
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The Canon lens has a slight edge over the Tamron optic at its widest aperture and at its sharpest f/5.6 aperture. However, the differences are minimal and both lenses are impressive.
We shot our resolution charts using the Canon EOS 5D Mark III, with both the Tamron lens and the similarly priced Canon 24-70mm f/2.8L USM, at 24mm, 50mm and 70mm settings. The charts indicate the sharpness of the lens in the centre of the frame. Detail produced by the two lenses at each of the three settings is fairly similar. It is also impressively similar to the Sigma 105mm f/2.8 Macro EX DG, which is our standard lens for shooting the charts, up to the 32 marker. As expected, there is a drop-off at f/2.8 and f/22, where image detail is softer. The Tamron lens is at its sharpest between f/5.6 and f/11.
Vignetting is obvious at f/2.8, with reduced brightness covering a large area of the corner portion of the frame. By f/5.6 the effect has all but gone, and by f/8 it has disappeared completely for consistent edge-to-edge brightness. This is not unusual for a lens of this type. Unprocessed raw files show chromatic aberrations in details like the edges of tree branches in the form of red and green fringing. This is more severe at wider focal lengths and towards the edges of the frame.
Images taken in our studio to test the level of distortion indicate that there is the expected barrel distortion at 24mm, but this is not drastic by any means. In real-world images, one can see the effect of barrel distortion in the corners of the frame where objects are stretched, but detail here is still crisp. At 50mm there is virtually no distortion across the entire frame, while at 70mm there is just a minor amount of pincushion distortion.
As a lens that will inevitably be used for portraits and shallow-depth-of-field work, I have looked at the quality of out-of-focus areas. Tamron claims that the nine diaphragm blades are rounded to give pleasing out-of-focus areas (bokeh), which mainly proves to be the case. In certain images shot at f/2.8, however, such as backlit gaps in between leaves in a tree, out-of-focus areas have a strange orb-like appearance.
We spoke to a representative of Tamron Japan, who states that this ‘onion’ effect is not unusual or specific to Tamron, but I took the same image on Canon’s equivalent lens and it does not show the same effect. Other than this specific scenario and exposure, though, I am pleased with the bokeh.
Image: The Tamron lens has pleasant bokeh, although in this situation, at f/2.8, the out-of-focus areas have an orb-like appearance
