Lens review

Sigma 18-200 mm f/3.5-6.3 DC OS

8 January 2010
Arkadiusz Olech

4. Image resolution

At the beginning once again I must state clearly that I am not a big fan of megazooms as I think they are contrary to the very idea of having a reflex camera. My view was a bit changed by the Nikkor 18-200 VR which, with its versatility and quite good optics, impressed me very positively. How the Sigma 18-200 mm fares against such a background, tested on RAW files from a Canon 20D? Let’s have a look at the graph below, showing MTF50 values for the frame center.
Sigma 18-200 mm f/3.5-6.3 DC OS - Image resolution

The Sigma 18-200 OS surprised us very nicely indeed. The results for the 18-50 mm range are simply excellent! The lens gives us exceptionally sharp pictures up from the maximum relative aperture. What is the reason of such a good performance? It seems that the front elements system is responsible - it must be big enough to guarantee a sensible light at 200 mm and a sufficiently wide angle of view reproduction at 18 mm. At 200 mm and by f/6.3 the active light-gathering aperture is almost 32 mm wide and uses most of the lens’s space so you can expect a worse image quality there. With the 18 mm and f/3.5 settings the aperture is only 5 mm wide and at 50 mm and by f/5.0 – only 10 mm wide so mainly the middle part of the optics, aberrations-free, is used there.

The results for the 100-200 mm range are also very good. We really can’t find fault with f/8 and wider, only the maximum relative aperture results make us feel unsatisfied a bit. Taking into account the fact that we deal with a megazoom here such a performance is nevertheless very good.

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Let’s look at another chart which shows how the lens fares at the frame edge.

Sigma 18-200 mm f/3.5-6.3 DC OS - Image resolution

Here the situation is definitely worse. The faults and physical limitations of a megazoom are clearly visible. Good border performance is noticed only in the 18-50 mm range and just on stopping down by at least one stop. At 200 mm we get average result. What’s the most interesting thing, the worst performance we notice at 100 mm where the results by f/5.6 and f/8 can be only described as weak.

Conventionally we show some test chart crops, obtained from JPEG files, at the end of this chapter.

Sigma 18-200 mm f/3.5-6.3 DC OS - Image resolution