Finally managed to get my hand on one of these. For a lens unit of its characteristics, they are absurdly cheap these days. Curiosity that drives me, is whether it would be able to beat my all-times fav, the SMC 50mm f1.7, which for my taste and preferences beats all of my lenses ever, be they manual or electronic. I have found the ideal set up for me when travelling under luggage restrictions: the 50mm is the ‘longer’ option that I carry with me (I shoot a crop of x1.5), and then I take my Sigma 10-15mm f3.5 as an ultra-wide angle. That’s it.
The SMC-M f1.4 at the times when been in manufacture (late 1970s till 1984 somewhere) has been the widest aperture offering among the normal lenses offering of Pentax. It has 7 lens inside, and 8 aperture blades, differing from to the six ones of the f1.7, and as a size is heavier and bulkier. Still it is much under the size of the majority of modern lenses. My copy is in excellent condition. The focus ring has a very long focus throw which I have accustomed to with the f1.7, actually I prefer this over the short one, as it gives a tight precision control over the focusing process. The aperture has clicks from 1.4 to 22, which is normal as well, the ring clicks tightly and firmly in a pleasant way. It clicks from 1.4 to 2 and then there are half clicks along the whole range of indications between 2 and 16. Anyway, I almost never shoot using apertures narrower than 4, so that does not quite matter to me. Minimal focus distance is around 45cm, which makes it a good companion for shooting objects. Ideal for portraits. All of things here make it equal to the f1.7, as the only factor that distinguishes it over the darker version, is the wider aperture which is never to be underestimated, indeed. Bokeh with the f1.4 is king, large, smooth and circular, contrary to the f1.7 one which is sometimes exotically jagged and angular due to the construction of the blades; sharpness of f1.4 is very good even from 1.4. The glass of both lenses is perfect.
The cons that I see for my 2 days’ walk around, are a few. First – and for me, the greatest one – is the range of aperture clicks. If I need a wide aperture lenses, I’d appreciate more stops from 1.4 to 2 (my Revuenon M42 55mm f1.4, for example, has a stop at 1.4, then 1.8, then 2). The SMC-M f1.4 goes from 1.4 to 2 directly, which diminishes its advantage over the f1.7 that also goes to f2 directly. The purple fringe in place at f1.4 is usual for these types of wide aperture lenses, so I’d not be over-complaining that. Of course, focus is difficult to nail down at f1.4 [but I’d not consider that a disadvantage, as one buys an f1.4 for the shallow depth of field; don’t do that, if your hands and eyes are being challenged by the idea and practice of manual focus].
So, if you can find it for a bargain deal, get a copy. At the same time, if you already have the f1.7, I’d see no huge reason to move to the f1.4. Yet I intend to take it with me on my next trips, replacing the f1.7.