Pentax HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR
4. Image resolution
It would be difficult to have any reservations concerning the MTF values, shown by the Pentax 20-40 mm. What’s important, all focal lengths’ MTF 50 values at the maximum relative aperture are on a level of 40 lpmm or higher, which guarantees you images of good quality. What’s interesting, at 20 and 30 mm the stopping down of aperture helps only a little with the limiting optical aberrations so the MTFs don’t get noticeably higher. As a result we weren’t bowled over by the maximum results of the lens. The maximum MTF values, reaching 47-48 lpmm, are a performance you can expect from a 18-55 or a 18-135 ‘kit’ lens. From a ‘limited’ device with a narrow focal range you have every right to demand more.
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The more or less good impression you might get after the assessment of the frame centre disappears immediately when you glance at the values at the edge of the frame.
The image quality in the area near the maximum relative aperture is weak. The lens has to be stopped down to f/5.6 in order to provide fully useful photos. It means that at the shortest focal lengths you have to stop down by as much as 2 EV. What’s more, the lens’s maximum results are once again hardly impressive. In a case of such an expensive device a level of 40 lpmm, which is reached near f/8.0, is simply not enough.
If you want to know what resolution level we expect from a lens bought at that price point you might consult our review of the Sigma 18–35 mm f/1.8 lens, which test was performed on the Canon 50D; its sensor provides practically the same MTFs like those you get using the Pentax K-5 so the values are directly comparable. Seeing those results it becomes clear that the more expensive Pentax 20-40 mm fares rather badly.
At the end we present crops taken from our resolution testing chart photos, which were saved as JPEG files (the lowest sharpening level) along with RAW files, used for the analysis above.
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