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

Features
Handling
Performance
Verdict
Specification
This is the widest of the three Pentax FA Limited lenses, all of which were designed for use on film cameras. They are hand assembled to the highest standards in high grade aluminium and expected to have optical performance of the highest order. This is well recorded as having been well achieved for film cameras, but now we shall look at how these designs fare with the current demands of the APS-C sensor. This first test is the 31mm, a 35mm-equivalent of 46.5mm, a wide standard lens on DSLRs.

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

The 31mm is beautifully constructed using high grade materials, with impeccable finish and engraving. The lens cap is deep and secures firmly, aided by its plush green velvet lining. There is little chance for this to fall off. The art of lens making has been given full rein here and there is no doubting that we have a quality lens in terms of its manufacture.

The construction is 9 elements in 7 groups, including a moulded glass aspherical element. This is an expensive way to make a lens and this is reflected in the price.

The lens hood is built in, but the petal construction still makes it possible to fit 58mm filters if required. There is an aperture ring, making the lens suitable for all Pentax bayonet film cameras as well as the digital range. For the DSLRs the aperture ring should be set to “A” to allow all functions to operate.

This compact lens is not overly heavy at 345g, but certainly more substantial than a modern plastic construction would be. Being from the FA series, there is no Quick Shift focusing adjustment available, but manual focus is smooth and well damped when needed.

The focusing scale is viewed via a cut out, as with the older M series lenses, and there is a depth of field scale and infra-red focusing mark.

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

In general, Pentax lenses are compact, nicely designed in terms of ergonomics and very hard to fault in their handling. This 31mm is no exception and it balances beautifully on the K-5 DSLR. The viewfinder image is of course bright and clear, thanks to the f1.8 maximum aperture, and a pleasure to see.

The manual focusing is smooth and nicely damped. It feels very different to the classic manual focus lenses, but this is different technology despite the classic appearance.

A lens like this is always a pleasure to handle for those who like fine engineering and we are very much in Leica territory here in terms of finish. Even the lens cap is a precision item and I would not like to have to find out what a replacement cap might cost. Built in lens hoods are also an excellent idea, making operation faster and far more convenient.

The 31mm focal length would be a moderate wide angle on film, but on the DSLRs is becomes a standard lens. 43mm is the actual theoretical standard for 35mm film, being the measurement of the diagonal of the format, so ironically this 31mm is to APS-C what the 43mm Limited is to film. It is a good, useful length and makes an excellent digital standard 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)

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

Here we have a very fine lens indeed. It is an ideal fast standard for DSLRs, but then so is the 35mm f2.4 and that is offered at a fraction of the price. The less expensive lens is also excellent, but not quite as good as this 31mm. Casual users may not appreciate the difference, but keen users will. Not in isolation, but when given the chance to compare there is a difference. There is also a difference between a plastic construction and the gorgeous aluminium finish that we have here.

If we compare this 31mm with Leica prices, then we have tremendous value for money, but less so perhaps for those who are happy with a kit lens or one of the already very well made plastic lenses. For photographers who make large prints for exhibition then this 31mm will add a new level of pleasure in its rendering and subtle qualities.

An expensive but beautifully made lens with lovely optical qualities.

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

Impeccable manufacturing quality and finish
Robust Aluminium construction
Fast maximum aperture
Excellent optical quality
Built in lens hood
Full frame design

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

High cost
No Quick Shift MF option

FEATURES
HANDLING
PERFORMANCE
VALUE FOR MONEY
OVERALL

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

Price	£1039.00
Contact	www.pentax.co.uk
Focal Length	31mm
Maximum aperture	f/1.8
Minimum aperture	f/22
Angle of View	70 degrees
Construction	9 elements / 7 groups
Focusing Range	0.3m - infinity
Magnification	0.16X Max
Filter size	58mm
Mount	PentaxK-AF
Weight	345g
Dimensions (Length x Diameter)	68.5 x 65mm
Kit Content	Front cap Rear cap Soft case Special Features Highest picture quality Built-in lenshood Aluminium construction