Sigma C 18-200 mm f/3.5-6.3 DC Macro OS HSM
4. Image resolution
Let’s check first how the Sigma presented itself in the frame centre – the following graph, with results at 18,50,100 and 200 mm focal lengths, will make it easier to assess.
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In the case of mega zoom lenses it’s typical that their resolution decreases noticeably with the increase of the focal length. However we are pleased to say the new Sigma is free of that flaw. The highest result of about 48 lpmm the lens achieves at 50 mm focal length; still even the 200 mm doesn’t lag behind a lot, at the maxium relative aperture and on slight stopping down reaching near 46 lpmm, a value deemed to be very good. Even when wide open the lens can provide images we don’t hesitate to call sharp. Apart from a very consistent performance at all focal lengths you should emphasize the fact that the Sigma remains useful across the whole aperture range. Even by the fastest aperture, that of f/3.5 at 18 mm focal length, the MTFs reach 36 lpmm so safely above the decency level.
Those results show that the mega zooms evolution moves in the right direction. Not so long ago the manufacturers managed to eliminate weak points in the centre of the frame and now you can watch another good move – leveling out of performance at all focal lengths while maintaining a very high standard.
Let’s check now the situation on the edge of the frame.
So far, every zoom lens tested by us has showed a significant crisis on the edge. Usually the weakest focal lengths were included in a range from 50 to 100 mm, with a very weak image quality at the maximum relative aperture which hardly improved on stopping down. In the case of the new Sigma 18-200 mm OS the maximum relative aperture is the weakest at 18 and 100 mm focal lengths. Still the results you get there are only slightly under the decency level; it’s enough to stop down the lens just a bit to get fully useful images. What’s more near f/8.0 the image is of really good quality no matter what focal length you employ. Perhaps in a case of an expensive fixed focal length device such a performance wouldn’t be impressive but in a case of an amateur megazoom it is really something worth praising.
What can be written in the summary? Even a few years ago a 18-200 mm class megazoom lens without any weak points seemed to be a preposterous idea. Today it is an established fact - all because of the modest Sigma.
At the end let’s look at the crops taken from our resolution testing chart photos, saved as not sharpened JPEG files along with RAW files we used for the analysis above.
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