SMC Pentax-FA 31mm f/1.8 AL Limited Lens

SMC Pentax-FA 31mm f/1.8 AL Limited Lens: Performance

Of course the reason for a lens is making images and all that quality of finish would be to no avail if the optical properties did not match. The questions will be, what qualities does this lens have, and of course potential buyers will want to know how the price can be justified when compared to the very inexpensive SMC Pentax-DA 35mm f2.4, which could well be used for the same applications. Although this is not a comparison test, having within a few days tested both options some comment can be made.

The graphs reveal some interesting data. Digital cameras work best with lenses of telecentric design, where the exiting light from the lens is as parallel as possible. Film lenses have quite oblique peripheral light rays so may not be ideal. However, as we are using a smaller format than the lens is designed for, we are only using the centre of the field. This is where the lens performs best and the hope would be that this would make up for the lack of telecentricity.

Chromatic Aberration chart
		As regards CA, is very well controlled at the centre and reasonably well controlled over most of the APS-C field. In practice, this is not a problem and CA is not evident in any of the test shots taken. There is some slight barrel distortion, around 1.45%, but this is very acceptable for what was designed as a wide angle lens and much better than many zooms. Standard lenses tend to suffer from slight barrelling anyway, so the effect is not unusual and in any event not obtrusive for most subjects. Even architectural subjects looked good with no real distortion visible. A good result.

In terms of flare, the SMC coating does its job very well and in almost all circumstances flare is banished.

MTF50
		The MTF50 graph shows us the fine detail performance of the lens and the 31mm is very satisfactory. It has the classic rise and fall performance of a quality lens and is fully usable at all apertures. There is very even performance across the field, no doubt aided by using the sweet spot of the field for our digital sensors. Performance peaks at f5.6, but in practice I would be happy to use whatever aperture gave me the required depth of field.
MTF20
		The MTF20 graph shows us the performance with more gritty textures and the contrast of this lens is high. This is certainly an excellent result and the lens will produce sharp, contrasty images with plenty of fine texture and detail. Again, the peak is around f5.6, but any aperture will give good results. Even at f22 images will be satisfactory, although there is some fall off through diffraction effects. Again, this is a sign of a good lens. A poor lens will steadily improve towards f22, something that can be observed in older poor zooms. Skin tones are handled very nicely, with a certain smoothness that flatters the subject.

Beyond the graphs and measurements, the lens has a certain quality about it. The images look very good indeed in a undefinable way. This is realm of bokeh, character, feel and all the other more emotional aspects of the lens designers' art. To put it another way, this is a mature lens of character that produces images that just look extremely good.

The slight barrelling is not obtrusive in architectural shots, ISO200, 1/400, f/6.3, 31mm (35mm equiv: 46mm)		Not exactly a lens for wildlife, but sometimes we can approach quite closely with proper care, ISO200, 1/250, f/8, 31mm (35mm equiv: 46mm)
A standard lens is well named as a general purpose focal length. The 31mm is suitable for most general travel shots, ISO200, 1/30, f/11, 31mm (35mm equiv: 46mm)		Close portraits are possible, but care should be taken not to induce distortion by getting too close. Skin tones are well handled, ISO200, 1/60, f/8, 31mm (35mm equiv: 46mm)
Live view was used to capture this awkwardly placed subject. The colour is really well captured, ISO200, 1/250, f/8, 31mm (35mm equiv: 46mm)		The 31mm is capable of resolving lots of fine detail, useful for quality landscape work, ISO200, 1/250, f/11, 31mm (35mm equiv: 46mm)
This is not a macro lens, but moderate close ups are possible and quality is well maintained at closer distances, ISO200, 1/60, f/11, 31mm (35mm equiv: 46mm)		The 31mm is an excellent general purpose focal length, ISO200, 1/50, f/8, 31mm (35mm equiv: 46mm)

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