Sigma 17-50 mm f/2.8 EX DC OS HSM
4. Image resolution
Let’s start our analysis with the frame centre performance. The results obtained at 17, 30 and 50 mm focal lengths are presented in the picture below.
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It’s worth reminding here that the best „primes”, tested on the 50D, reach the level about 52 lpmm and results near 34-35 lpmm we consider to be the decency level. The performance of the tested Sigma makes us clap our hands and cheer. 17 and 30 mm focal lengths are practically indistinguishable. At the maximum relative aperture we see excellent – maybe even outstanding, when we take into account the fact that we deal here with a zoom lens- results at the level of 49 lpmm. By f/4.0 and upwards it would be difficult to differentiate between the Sigma and a good prime lens. A bit weaker performance we saw at 50 mm. The differences are not big; even there the lens fares very well indeed because at the maximum relative aperture it reaches the result above 46 lpmm and by f/5.6 it goes as far as 50 lpmm.
Now let’s see what happens at the edge of the frame.
Here there will be no cheering. The lens fares the best at 50 mm and we express no reservations about it. Even at the maximum relative aperture the image is sharp and on stopping down it becomes very sharp. At shorter focal lengths there are more problems, though. The most serious issue is the performance at the maximum relative aperture where the MTFs go near 30 lpmm so below the decency level. Fortunately in the important apertures range from f/4.0 to f/11 the decency level is reached or significantly exceeded. Overall the tested Sigma fares relatively well at the edge but its performance contrasts sharply with the outstanding results in the frame centre. We must honestly admit, though, that even more expensive competitors didn’t manage better here. For example the Canon 17-55 mm by f/2.8 didn’t met the decency level at any focal length.