LensTip.com

Lens review

Pentax smc FA 31 mm f/1.8 AL

14 October 2011
Arkadiusz Olech

11. Summary

Pros:
  • Excellent build quality,
  • Very good image quality in the frame centre on slight stopping down,
  • Decent image quality on the edge of the frame,
  • Slight chromatic aberration,
  • Negligible distortion,
  • Low vignetting,
  • Well-corrected astigmatism,
  • Fast and accurate autofocus.

Cons:

  • Not satisfactory image quality near the maximum relative aperture,
  • Distinct coma already in the corners of the APS-C/DX sensor,
  • Loud autofocus,
  • Work against bright light should have been better,
  • Average price/quality ratio.

Comparing the results of the tested lens to those achieved by such lenses as the Nikkor 1.8/35 or the Sony 1.8/35 and taking into account the difference in price, it would be difficult to describe the performance of the Pentax in positive terms. Of course the question how representative the specimen we got for testing was might need some explaining as well – it was a new lens, from an official distribution channel, which had gone through the quality control process at the factory. The opinions of Pentax users describe this lens as an instrument which is very sharp already from the maximum relative aperture; our testing model was obviously not such a thing. It is also not completely clear whether the positive opinions concern mostly the lenses which had been produced previously in Japan and if they also concern the devices produced nowadays in a Vietnam factory. It certainly might be a problem because the laboratory results we got during the tests of other “Made in Vietnam” lenses such as the Pentax smc FA 43 mm f/1.9 Limited or the Pentax smc DA* 16–50 mm f/2.8 AL ED IF SDM also differed greatly from the opinions of users. It would be difficult not to go one step further and notice a significant decrease of control quality which took place after moving the production from Japan to Vietnam. Our statistics, based on a quite limited sample but still supported by the results of our tests, shows that one Pentax lens, produced nowadays, out of very small number of other specimens, is flawed; the fact that it also concerns the best and the most expensive products of this company is especially painful. Shame…


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While reviewing the Pentax smc DA 21 mm f/3.2 AL. Limited I praised Pentax for well thought out tactics, going against the common tides a bit and based on launching small, handy, fixed-focal instruments, adapted to contemporary APS-C sensors and fitting very well Pentax bodies. The mere existence of the Pentax smc FA 31 mm f/1.8 AL is, unfortunately, in complete contradiction with that tactics. When a company decides to build a system based on “primes” and a small sensor, it should give its users also a good standard lens as well. Seemingly with the Pentax company we’ve got a wide choice. Firstly there is the Pentax smc DA 35 mm f/2.4 AL, launched not so long ago on the market – a small lens, physically light and moderately priced. Unfortunately it is also relatively slow and its build quality - just average. If you care about the build quality you can think about the Pentax smc DA 35 mm f/2.8 Macro Limited model but it is even slower. The aperture fastness is quite important here because on a small detector it is more difficult to get shallow depth of field.

If you want to own a faster brand-name standard we are practically condemned to the quite elderly Pentax smc FA 31 mm f/1.8 AL, a lens which seems to be seriously out of place in this system. Firstly it was designed for analogue full frame, providing an image circle so wide that most of Pentax reflex cameras’ owner won’t be able to profit from it. When we want to use this lens along with digital APS-C reflex cameras we have to carry additional grams and centimeters because a lens designed for a smaller detector could have been noticeably smaller and lighter by definition and if you decided to keep the dimensions of the 1.8/31 model, you could get a noticeably faster and optically better device. It is not the end of bad news, there is still the price. It’s enough to mention that for the amount of money you have to pay for the Pentax smc FA 31 mm f/1.8 AL you can buy the Nikon D7000 with the Nikkor AF-S DX 35 mm f/1.8G lens…

To tell you the truth it is not really relevant to the subject whether the specimen of the Pentax, tested by us here, was a typical sample of the current production from Vietnamese factories or a bit worse. Even if its results were slightly better and closer to those represented by earlier lenses, produced in Japan, the conclusions would remain the same. On the small APS-C sensor the differences in image quality between the Pentax 1.8/31 and the 1.8/35 models of Nikon and Sony are so small that they don’t justify at all the sum of money you have to spend on the Pentax, tested here.

To sum up – if I wanted to engage in a system which pursues an idea of working with fixed-focal lenses on a small sensor, I would expect there was a standard lens available - something optically better, faster and cheaper than its full frame equivalent. Pentax likes going against the tide and here I grant it, they were consistent. The company offered its users a lens slower, optically worse, old and much more expensive. You must admit the price is quite steep for such uniqueness.


Sample shots

Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/1.8, exp. 1/800 s, ISO 200
File: JPEG, 4928×3264 pix, 8.4 MB
Camera: Pentax K-5
Parameters: 31 mm, f/1.8, exp. 1/500 s, ISO 200
File: JPEG, 4928×3264 pix, 8.66 MB
Camera: Pentax K-5
Parameters: 31 mm, f/2.5, exp. 1/400 s, ISO 200
File: JPEG, 4928×3264 pix, 7.92 MB
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/1.8, exp. 1/1000 s, ISO 100
File: JPEG, 4928×3264 pix, 7.27 MB
Camera: Pentax K-5
Parameters: 31 mm, f/2.2, exp. 1/1250 s, ISO 100
File: JPEG, 4928×3264 pix, 9.55 MB
Camera: Pentax K-5
Parameters: 31 mm, f/2.8, exp. 1/1600 s, ISO 100
File: JPEG, 4928×3264 pix, 7.54 MB
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/3.5, exp. 1/1250 s, ISO 100
File: JPEG, 4928×3264 pix, 9.12 MB
Camera: Pentax K-5
Parameters: 31 mm, f/8.0, exp. 1/320 s, ISO 100
File: JPEG, 4928×3264 pix, 11.03 MB
Camera: Pentax K-5
Parameters: 31 mm, f/3.5, exp. 1/640 s, ISO 100
File: JPEG, 4928×3264 pix, 10.21 MB
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/1.8, exp. 1/4000 s, ISO 100
File: JPEG, 4928×3264 pix, 9.63 MB
Camera: Pentax K-5
Parameters: 31 mm, f/4.5, exp. 1/800 s, ISO 100
File: JPEG, 4928×3264 pix, 10.83 MB
Camera: Pentax K-5
Parameters: 31 mm, f/6.3, exp. 1/500 s, ISO 100
File: JPEG, 4928×3264 pix, 10.26 MB
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/2.5, exp. 1/3200 s, ISO 100
File: JPEG, 4928×3264 pix, 9.19 MB
Camera: Pentax K-5
Parameters: 31 mm, f/5.0, exp. 1/800 s, ISO 100
File: JPEG, 4928×3264 pix, 10.24 MB
Camera: Pentax K-5
Parameters: 31 mm, f/1.8, exp. 1/500 s, ISO 100
File: JPEG, 4928×3264 pix, 9 MB
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Pentax smc FA 31 mm f/1.8 AL - Summary
Camera: Pentax K-5
Parameters: 31 mm, f/3.5, exp. 1/2000 s, ISO 100
File: JPEG, 4928×3264 pix, 9.42 MB
Camera: Pentax K-5
Parameters: 31 mm, f/2.0, exp. 1/4000 s, ISO 100
File: JPEG, 4928×3264 pix, 9.03 MB
Camera: Pentax K-5
Parameters: 31 mm, f/4.0, exp. 1/800 s, ISO 100
File: JPEG, 4928×3264 pix, 10.19 MB



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