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Lens review

Pentax smc FA 31 mm f/1.8 AL

14 October 2011
Szymon Starczewski

4. Image resolution

The resolution test of the Pentax smc FA 31 mm f/1.8 AL. was based on RAW files from the Pentax K-5 reflex camera. In its case the best fixed-focal lenses reach results on the level of 52-53 lpmm and the decency level is situated around 34-35 lpmm. Let’s check how the tested lens compares here. The graph below shows its performance in the frame centre and on the edge of the APS-C/DX sensor.

Pentax smc FA 31 mm f/1.8 AL - Image resolution

It is clearly seen that in the centre the area near the maximum relative aperture doesn’t provide you with images of the satisfactory quality and to get it you have to close down the aperture to the value of about f/2.2. Fortunately the achievements of the lens are getting better very fast and already by f/2.8 the resolution reaches the level of 40 lpmm - you can have no reservation about it. Near f/4.0 and f/5.6 apertures the lens gets as high as the best system “primes”.

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On the edge of the frame, even after stopping down to f/2.8, you still don’t get full usefulness of image. Here, in order to get a sharp photo, you must stop down by an additional value of 0.5 EV.

What can be said…reminding ourselves how such lenses as the Nikkor 1.8/35 and the Sony DT 1.8/35 performed on similar detectors, lenses which are several times cheaper to boot, the Pentax didn’t impressed us to say the least of it.

The crops below, showing the centre of our test chart for two chosen aperture values, were taken from JPEG files.

Pentax smc FA 31 mm f/1.8 AL - Image resolution