Sigma 18-250 mm f/3.5-6.3 DC OS HSM
4. Image resolution
You can see immediately what I meant writing in the introduction about the good image quality of some contemporary megazooms. The lens doesn’t break any resolution records but nobody expects it after all. If you take into account the focal lengths range, available here, such results can be easily described as excellent (especially those in the 18-50 mm range). Firstly and foremostly the maximum relative aperture is fully useful in the whole focal range so you can get photos of good quality. It is important, especially at the longer end, where the lack of light makes it sometimes impossible to close down the aperture. We don’t have any reservations about the frame centre performance.
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However, if we want to construct a lens with a wide focal range, sensible dimensions, able to provide sharp images in the frame centre and sell it for a price which could be acceptable to amateur photographers, some weaknesses are unavoidable - there’s no getting away from it. Looking at the frame edge performance we see at once where these weaknesses are situated in the case of the Sigma 18-250 mm OS.
In the 18-50 mm range once again we shouldn’t complain at all. To tell you the truth the combination of the maximum relative aperture and the shortest focal length gives a result just a tad below the decency level but really only a little bit. On slight stopping down the decency level is exceeded and further on the results only improve.
The most serious crisis can be observed at longer focal lengths. At 120 mm and by f/5.6 the Sigma doesn’t reach even 20 lpmm. Then it improves but only with great effort, climbing slowly upwards to get near the area of usefulness only by f/11. At the maximum focal length and maximum relative aperture we see the value a bit above 20 lpmm for a change but in the case of 250 mm the frame edge will never be fully sharp; even after a significant stopping down we will never get a higher MTF value than 30 lpmm.
Frame edges at longer focal lengths are a visible problem of the Sigma - they are the price we have to pay for such a wide range closed in such a casing.
Below we present some of our test chart crops taken near the frame centre of JPEG files which were saved along RAW files, described earlier.
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