In our first comparison, with an overall DxOMark Score of 24 the Tamron matches the high-regarded $1,900 Nikon AF-S Nikkor 24-70mm f2.8G ED but can’t quite match the category leader, the optically awesome but costly $2,300 Canon EF 24-70mm f2.8L II USM.
Looking at the Lens Metrics Scores for Transmission, Distortion, Vignetting and Chromatic Aberration it would seem the Tamron should be ahead, but they don’t tell the whole story.
Look at the overall DxOMark Score and the linear scale and gauge behind it. The score is an average result based on the lens performance over the various focal lengths (when tested on a specific camera), and the blue-color gauge shows the range of results.
If the gauge is wider, or larger, as it is for both the Tamron and Nikon lenses, it means the performance isn’t consistent at each focal length. Notice the narrow range of results for the Canon lens, it's a simply a superb performer throughout the zoom range. In the case of the Tamron, and Nikon in this instance, both lenses show a noticeable loss of sharpness in the mid-to-longer focal lengths, and not only in the centre of the frame but also at the edges and corners.
One area where the Tamron surpasses both the formidable Canon and the Nikon lens is in the control of troublesome lateral chromatic aberration. Like the pricier Canon and Nikon zooms, the Tamron has a complex optical formula including multiple low-dispersion (SLD in Tamron parlance) glass elements, three molded aspheric and one-double sided aspheric element but the result is the Tamron has the lowest chromatic aberration of the three. Perhaps not surprisingly, the heaviest effects when visible occur at its widest setting over the f/2.8-8.0 range.
Despite taking 82mm filters it’s the most compact at 108.5mm in length but falls between the two in weight at 825g (the Canon is 113mm in length, weighs 805g, and takes similar size filters whereas the Nikkor has a 77mm filter thread measures 133mm front to back and weighs in at 900g).
In our final comparison, we’ve pitched the Tamron with the recently announced Canon EF 24-70mm f4L IS USM. Although it has stabilization and an ultrasonic motor it’s more compact at 93mm in length, takes 77mm filters and weighs just 600g. However, it loses a stop of light and retails for $1,499. With low levels of CA, vignetting and distortion the new mid-speed Canon is a good performer optically, but it has some off-axis softness wide-open particularly at 50mm.
Our other contender should be no stranger to Tamron, Sigma is largest independent lens maker in Japan and has offered a couple of 24-70mm f2.8 designs. The latest, the 24-70mm f2.8 EX DG IF HSM has the usual exotic mix of glass, standard 0.38m minimum focus distance and has an ultrasonic motor for fast near silent AF and takes 82mm filters, but it lacks any form of image stabilization. However, at 790g it’s the lightest of the f2.8 models and at $899 it’s the most modestly priced. It has very high central sharpness at the wider end at maximum aperture that improves across the frame on stopping down but sharpness drops off quite dramatically at 50mm. It has also has heavier barrel distortion and higher levels of chromatic aberration.