Lens review

Sigma A 24-70 mm f/2.8 DG DN

22 January 2020
Arkadiusz Olech

4. Image resolution

The resolution test (resolution meaning MTF50 function value) of the Sigma A 24-70 mm f/2.8 DG DN was based on RAW files from the Sony A7R II. In the case of that mirrorless camera the decency level we set about 39-41 lpmm and some of the best fixed focal length lenses have been able to exceed 70 lpmm; so far the Samyang AF 85 mm f/1.4 FE (76.5 lpmm) and the Voigtlander Apo-Lanthar 65 mm f/2 Aspherical 1:2 Macro (78.5 lpmm) have fared the best.

Let's check how the Sigma A 24–70 mm f/2.8 DG DN compares – its results in the frame centre at 24, 45, and 70 mm focal lengths presents a graph below.

Sigma A 24-70 mm f/2.8 DG DN - Image resolution

You can tell at first glance that the middle of the focal length range sticks out in a positive way – it exceeds distinctly 70 lpmm already up from the maximum relative aperture. It's a level many system fixed focal length lenses wouldn't be ashamed of so the tested Sigma should only be praised here.

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Both ends of the focal length range lag behind, however. Does it mean you have the right to criticize the performance? Definitely not. The lens reaches very sharp images all the same and it does so up from the maximum relative aperture. Of course you can point out that the Tamron 28–75 mm f/2.8 Di III RXD was slightly better at 28 mm than the Sigma at 24 mm but two other things should be takien into account as well. Firstly, it was the best focal length of the Tamron and, when it comes to the rest of them, it fared worse than the Sigma. Secondly, the 28 mm focal length still remains a tad easier to correct than 24 mm; the difference in the angle of view is significant.

Now let's check the edge of the APS-C sensor and how the Sigma fares there – an appropriate graph can be found below.

Sigma A 24-70 mm f/2.8 DG DN - Image resolution

Once again the middle of the focal length range is the best but this time 24 and 70 mm don't lag behind so much. Now it becomes clear what the Tamron 28-75 mm had to sacrifice in order to get a very good result in the frame centre at 28 mm. It is weaker than the Sigma at practically every focal length. The tested Sigma's constructors took proper care not only of the frame centre but also of good image quality on edges.

What about the very demanding edge of full frame? Did they manage to repeat their success? Let's have a look at the following graph.

Sigma A 24-70 mm f/2.8 DG DN - Image resolution

At the maximum relative aperture, throughout all focal lengths, the resolution lands a tad below the decency level. Still, it's enough to close down the aperture to near f/3.5 and the images become useful. The peak of its performance the lens achieves at 24 mm – it's obvious the constructors focused on that focal length the most and it is a very good choice. In its case we often care about sharpness evenly spread across the frame (landscape or architecture shots) and, in the case of longer focal lengths, the edges often don't fit the depth of field anyway. It's also worth adding that the performance of the Sigma is at this place distinctly better than the performance of the Tamron – that lens had problems with exceeding 40 lpmm everywhere.

At the end of this chapter, traditionally, we present crops taken from photos of our resolution testing chart which were saved as JPEG files along with RAW files we used for the analysis above.

A7R II, JPEG, 45 mm, f/4.0
Sigma A 24-70 mm f/2.8 DG DN - Image resolution
A7R II, JPEG, 70 mm, f/2.8
Sigma A 24-70 mm f/2.8 DG DN - Image resolution