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Lens review

Sigma 18-50 mm f/2.8-4.5 DC OS HSM

8 March 2010
Szymon Starczewski

4. Image resolution

The image resolution of Sigma 18-50 HSM OS has been measure on the strength of RAW file from Nikon D200. The measuring errors oscillated within 0.3-1.4 lpmm. The graph obtained for particular focal lengths 18, 30 and 50 mm, for the center of the frame is presented below.

Sigma 18-50 mm f/2.8-4.5 DC OS HSM - Image resolution

To be honest, we want to applause loudly. The behavior in the 18-30 mm range is great. Even at the maximum aperture the lens obtains values reaching almost 40 lpmm, and at around f/5.6 an outstanding level of 45 lpmm. Sigma falls behind the prime Nikkor 17-55 f/2.8, but really not much. Additionally, it comes off better than the new stabilized kit lens of 18-55 mm VR parameters.

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Sigma’s weakest point in the center of the frame is the behavior at 50 mm. It’s hard to cal it bad or poor, as it certainly is not. Even at the maximum aperture the level of 30 lpmm is exceeded, the one we consider acceptable. 50 mm focal length is noticeably worse than the rest and here Sigma has troubles to surpass Nikkor 18-55 VR, even though it has better brightness.

So, is it a really nice surprise? A very good lens for little money? Unfortunately, there are no miracles. Let’s have a look at the edge of the frame performance. 

Sigma 18-50 mm f/2.8-4.5 DC OS HSM - Image resolution

There are no reasons to praise here. Especially weak is the shortest focal length, where the graph is very flat and obtained values never exceed 30 lpmm. At longer focal lengths it’s better, but not much. The lens needs to be stopped down to around f/5.6 for it to get quite sharp images. At the edge of the frame Sigma loses not only to more expensive lenses of Nikkor 17-55 f/2.8 class, but also to cheap stabilized kit lenses of 18-55 mm class, although in the case of the latter not a lot. 

Sigma 18-50 mm f/2.8-4.5 DC OS HSM - Image resolution