Sigma A 28-45 mm f/1.8 DG DN
4. Image resolution
Let’s check how the tested Sigma A 28-45 mm f/1.8 DG DN compares here. A graph below presents the results in the frame centre, at 28, 36, and 45 mm focal lengths.
Just one glance at the performance of the tested Sigma is enough to clap and cheer, especially if you focus on the 28-36 mm range. In this case the lens is hovering around a very high level of 70 lpmm already at the maximum relative aperture. On stopping down the aperture, at 28 mm, the lens is able to reach 83.7 lpmm and at 36 mm its resolution amounts to even 84.5 lpmm.
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The situation is very interesting indeed. Not so long ago the Sony FE 85 mm f/1.4 GM II threw off the podium the Sigma C 2/65; now another Sigma managed to win the third place superseding the Sony FE 20 mm f/1.8G. Taking into account the fact that the Sigma is, after all, a zoom lens, it's no mean feat.
The results at the 45 mm focal length are noticeably weaker but, of course, it doesn't mean that they should be described as bad. They are weaker just because you compare them to truly outstanding values from the 28-36 mm range. Overall they remain very good – after all at the maximum relative aperture you deal with a level of over 56 lpmm and on stopping down the aperture the lens has no problems whatsoever to exceed a value of 70 lpmm.
This performance allows you to answer the question why Sigma decided to apply a maximum focal length of 45 mm, not 50 mm. Probably at 50 mm the image quality would deteriorate to a level not acceptable for the producer. Of course you could try to prevent it but only at a cost – by increasing the weight and physical dimensions of the lens. Sigma didn't want to do that; after all the 28-45 mm model can hardly be called small and lightweight even as it is.
Now let's check how the Sigma A 28-45 mm f/1.8 DG DN deals on the edge of the APS-C sensor – an appropriate graph can be found below.
Here the middle of the focal range fares the best and the results from both ends are lower and they are similar to each other within the margin of error. Still there are no reasons to complain, no matter where you look. Even at the maximum relative aperture you see results on a level of at least 50 lpmm at every focal length.
Now let's check how the situation changes on the demanding edge of full frame.
The situation changes very quickly because now the 28 mm focal length is the weakest and the 36-45 mm range fares a bit better. The differences aren't that huge and once again the fact that, no matter what focal length you employ, even at the maximum relative aperture, you get results from 44 to 49 lpmm so safely above the decency level, is the most important one.
To sum up: not only the unique Sigma A 28-45 mm f/1.8 DG DN has no weak points in this category but also it proves to be one of the sharpest lenses we've ever tested. It is a huge achievement if you take into account the fact that you deal with a zoom lens. A round of applause! A round of applause again!
At the end of this chapter we present crops taken from our testing image in the frame centre. It’s worth remembering that those are JPEG files (with the lowest sharpening level applied) saved along RAW files, used for the analysis presented above.
| A7R IIIa, JPEG, 36 mm, f/2.8 |
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| A7R IIIa, JPEG, 45 mm, f/1.8 |
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