Lens review

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM

10 June 2009
Arkadiusz Olech

4. Image resolution

The lens’s resolution, understood as a MTF50 function value, was tested by analyzing RAW files from the Pentax K20D. It is worth reminding that this camera sensor performance is similar to the performance of 10-megapixels bodies, used by us in the tests (Pentax K10D, Nikon D200 and Sony A100) so the results can be compared directly.

The frame center results are shown in the chart below.

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Image resolution

The main question to be asked is if there is a point of changing the kit lens 18-55 mm AL II for the tested Pentax 17-70 mm. The comparison between those two lenses’ graphs doesn’t show any big differences able to bowl you over. The results are similar, however, because the kit lens is so good, not because the Pentax 17-70 is so weak. The results for the lens stopped down by about 1 EV compared to the maximum relative aperture are very alike for both instruments. The quality difference can be only perceived wide open, where Pentax 17-70 mm fares significantly better.

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It is worth noticing though, that in the 17-70 mm model the maximum aperture at 50-70 mm focal length doesn't give very good results. There the lens reaches just 30 lpmm - the traditional decency limit. On the other side the kit lens ends at 55 mm with f/5.6, a point where Pentax 17-70 mm increases to the 37 lpmm level.

The second comparison worth doing concerns Sigma 17-70 mm. Here you must remember that the test was made on the 8-megapixels Canon 20D sensor, giving lower MTF50 values by definition so making the direct comparison more difficult. Taking into account the fact, that the tested lenses quality results differ by a 10% factor (you might consult the Sigma 1.4/30 test to assess it properly) we can come to a conclusion that the Pentax 17-70 mm and Sigma with the same parameters perform almost similarly in the frame center.

Let’s move to the frame corner results. They are presented in the next chart.

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Image resolution

Here, the situation is not as pleasant but you can hardly expect any records, taking into account the big focal lengths range, good fastness, wideangle presence and a moderate price to boot. One thing is worth notice: the bad performance results for the wideangle, which, even on quite significant stopping down, is slightly worse than in the case of the kit lens 18-55 mm. At 30 mm both lenses fares alike, at 50-55 mm for the common focal lengths range the results are similar too. At 70 mm Pentax performs especially weakly by f/4.0 and f/5.6 and only on stopping down to f/8 the situation gets definitely better. I would like only remind you that the frame corner results for Sigma 17-70 didn’t look more optimistic.

We finish the chapter by presenting the excerpts of our test chart, based on JPEG files saved along with RAW files.

Pentax smc DA 17-70 mm f/4.0 AL [IF] SDM - Image resolution