Sigma 18-200 mm f/3.5-6.3 II DC OS HSM
4. Image resolution
Let’s check first how the lens fares in the frame centre. A graph, shown below, will help you to asses it properly.
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As you see, even at the maximum relative aperture and at all focal lengths the lens gets good results, situated in a safe distance from the decency level. It is a performance very similar to that of its predecessor. You must remember, though, that the older version was tested on the Canon 20D sensor, which provided MTFs by 2-3 lpmm lower. In the case of the newer model the results, reached on the D200, are not significantly better which might suggest a slightly worse performance. The fact is that the differences are slight – not exceeding the margin of error values, so overall we can assume that the lens fares similarly well in the frame centre as its older version and we don’t have any reservations. Quite the opposite is true – such a result can be only praised.
Let’s see how the lens performs on the edge of the frame.
The result is surprising but in the positive sense. Usually when a producer decreases dimensions of their equipment the image quality on the edge decreases as well. Here the situation is different. The previous Sigma 18-200 mm OS at the maximum relative aperture had weak results throughout the whole focal range and had to be closed down by 1-2 EV to improve. The new model doesn’t have any problems with delivering a decent image at the maximum relative aperture and the shortest focal lengths. The biggest crisis, as in the case of its predecessor, can be observed near 100 mm but this time the said crisis is somehow less bothersome. The performance at 200 mm changed for the worse, though – in the new model it is a tad weaker than that of the older lens.
To sum up: for such a range of focal lengths as offered here, the lens presents itself very well. Compared to its predecessor the physical dimensions decreased but the results, on average, remained more or less the same. In the new model they even got slightly more even. The middle of the frame is a bit worse than in the previous model but still very good; the performance on the edge of the frame improved significantly for a change.
Below we present some crops of our test chart, taken from JPEG files.