LensTip.com

Lens review

Pentax smc FA 31 mm f/1.8 AL

14 October 2011
Arkadiusz Olech

3. Build quality

It is not clear what we should compare the tested Pentax to because, on the one hand, it is a full frame lens but on the other, in the digital word you can only use it on the APS-C/DX detectors. Apart from that, because of its untypical parameters, it would be difficult to find other full frame devices, similar to it. That’s why in the following chart we compare it to the full frame Pentax 2/35 and to full frame f/1.4-1.8 standards from other systems as well.

In the photo below the tested lens is positioned next to two Limited devices (but DA versions, not full frame), namely the smc DA 15 mm f/4.0 AL Limited and the smc DA 21 mm f/3.2 AL Limited. It would be difficult to call the tested lens small even if its aperture doesn’t make it a fast standard (as such we consider f/1.4 and on full frame). Here you would wish for more…

Pentax smc FA 31 mm f/1.8 AL - Build quality


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The lens starts with a metal bayonet mount inside which you can find a narrow, black ring made of plastics; you can see an inscription on it: „Assembled in Vietnam”. It surrounds a rear element which is 21 mm in diameter. The element is movable – it shifts with the whole optical system, hiding about 0.5 cm inside the barrel when you set the focus at minimum.

Pentax smc FA 31 mm f/1.8 AL - Build quality


Moving to the black and metal casing of the lens (you can also buy a silver version) the first thing that you encounter is an aperture ring, 7 mm wide and ribbed. It works with 0.5 EV step. The 0.3 EV step is available only from the level of the body.

Pentax smc FA 31 mm f/1.8 AL - Build quality

Next you can find a depth of field scale with markings for the following apertures: f/22, f/16, f/11, f/8, f/5.6 and f/4.0.The next part of the lens consists of a focus ring on which there is a distance scale expressed in feet and meters. The ring features some ribs which consist of two rings, every of them about 5 mm wide. The focus ring works lightly with only slight resistance but smoothly and without any unevenness. When you shift it you can hear a slight grinding. Running through the whole scale takes a turn through 90 degrees. It is not an impressive value and by f/1.8 it doesn’t guarantee any extremely precise settings when working in the MF mode.

The front part of the lens consists of an extended inner tube on which you can find first an inscription with the parameters and the name of the lens and then an built-in petal-type hood. The owners of square filters won’t be delighted for sure. Normal filters, which, in this case should be 58 mm in diameter, won’t create any problems though because their thread doesn’t rotate. The front element has a diameter of 42 mm.

The lens consists of 9 elements positioned in 7 groups. Inside you can also find a circular aperture with nine diaphragm blades which can be closed down to the value of f/22.

Pentax smc FA 31 mm f/1.8 AL - Build quality


The buyer gets both caps and a soft pouch in the box. The hood is attached permanently.

Pentax smc FA 31 mm f/1.8 AL - Build quality