Size, Weight, Build

Both are small and light lenses, although as previously mentioned the M 50/2 is very nearly a pancake lens. As such, the rubberized focus ring is necessarily small at less than one centimeter. Coupled with the tight manual focus, this can make the M 50/2 sometimes hard to focus. The FA 50/1.4 has an even smaller focus ring, which isn't made for manually focusing, but as such it has less resistance and is therefore easier to move.

The M 50/2 also has a cheap-ish plastic aperture ring, which doesn't click into place at each stop like the older metal lenses. Of course, the FA 50/1.4 has an even worse plastic aperture ring, but it isn't intended for use at all.

The FA50/1.4 is made of a dark grey plastic, thankfully a bit more tasteful than its F-series forebears. The focus scale is placed behind scratchable clear plastic and interestingly has a mark for infrared photography. The M 50/2 is black metal, plastic, and rubber, with blue and yellow painted numbers on the distance scale. (Like all the M- and A-series lenses, it too has a mark for infrared light on the focus scale.)

In your hand, the FA 50/1.4 does feel like a much more substantial piece of glass - even though at 220g it is still by all means a small lens.

Physical Usage

The M-series lenses, along with their SMC Pentax (commonly known as the K-series) predecessors, are the worst-implemented manual focus lenses on modern Pentax bodies. Pentax released its first digital SLR, the *ist D, based on their *ist film camera. Neither supported metering at all with the M and K lenses because there is no mechanical coupling between the lens and the camera. Gone completely are the days of shooting without electricity, I suppose.

Since then, Pentax has implemented a work-around, using the 'Green Button' while in manual exposure mode to 'automatically' stop the lens down and take a reading. If you want to shoot with M and K lenses in aperture priority mode, modern Pentax cameras will only shoot them wide open.

The FA 50/1.4 supports all the modern accoutrements you would expect of new cameras. Sigh.

Side view of both the SMC Pentax-FA 1:1.4 50mm, left, and the SMC Pentax 1:2 50mm, right.
Click here for to see the front elements.

In addition to the typical Pentax axial color, this series of 100% crops (click on the photo for the full-size version) of out-of-focus crops at different apertures shows a clear difference between the two.
Click here for a gallery of samples of these two lenses.

Sharpness

The M 50/2 was surprisingly (to me, anyway) sharp. It has a decent amount of resolution, but what it obviously lacks compared to its modern foe is contrast. Examining the shots of the pool table, for example, yielded dark grays instead of deep blacks, and in general the lack of contrast made the images look more flat and bland. Without shooting a test chart I can't say if the M 50/2 is resolving more than the FA 50/1.4 but it is going to come out of the camera looking less 'sharp'.

The FA 50/1.4 is definitionally sharp enough for me, being the standard against which I base Normal lenses.

Bokeh

In addition to contrast, there is a considerable difference in the out-of-focus rendering of these two lenses. At wider apertures, the FA 50/1.4 shows edges around bright highlights, but the M 50/2 is much more pronounced. At smaller apertures, the hard edges of the blur disk dissipate, but the 6 straight aperture blades of the M 50/2 start to appear much more unseemly than the 8 (also straight) blades on the FA 50/1.4.

To be fair, for many subjects, the difference between these two can me minimal (check out the purple cone flower shots here and here). But under not unusual shooting conditions the FA 50/1.4 can produce a pleasant background with less worry than the M 50/2.

Flaws

Both of these lenses suffer from some typical Pentax flaws. The first, and most obvious, is longitudinal chromatic aberration, or axial color. The M 50/2 suffers slightly more than the FA 50/1.4 from these green or blue artifacts showing up in the bokeh.

Both lenses also suffer from color fringing at high contrast areas, as the samples of the Galileo thermometer show clearly (see the crops here).

Also in the Galileo samples, the M 50/2 seems to suffer from flare around some of the specular highlights more so than the FA 50/1.4 does.