This is a very short review or, rather, a comment on this lens. Nothing systematic, or scientific. However, my comment will be applicable to all copies of the said lens, not only of mine. Here is why.
SMC Pentax – F 50mm f/2.8 is a true macro lens; it allows 1:1 magnification without any attachment. Like almost all true macro lenses, its performance is optimized for close focusing distances. The lens does “shine” as a macro. You can easily find macro images taken by this lens if you make a simple Internet search. Or click here to see one of mine. It was taken probably at f/4. The background is blurred a bit during post processing. The camera (Pentax K3) was on a tripod. I have almost nothing to say about the use and performance of this lens as a macro; it is either perfect (optically) or excellent for all practical purposes.
My comment is on its weak point. Most, if not all, macro lenses are also optically very good tools for their respective focal length. For “full frame” cameras, 50mm lens is usually considered the standard lens. For digital cameras with APS-C sized sensors, 50mm is widely used for portraits. A typical lens to subject distance for a portrait is beyond macro range, unless your subject is a spider!. It is here the practically insurmountable problem of this particular lens becomes apparent: You can not focus properly AND consistently whatever you do; autofocus or manual focus! Unbelievable? Surprising? Below is the reason :
From 2 meters to infinity you only have about 2-3 millimeters of barrel rotation available. At f/2.8 or say, f/4 a less than a millimeter rotation of the lens barrel (by the autofocus mechanism or by your hand) moves the focus plane drastically. You can still shoot a proper portrait as intended, of course, IF you are lucky! It is optically / technically / practically impossible to see the change in your focus plane at the viewfinder of any of the digital SLRs. So you can not adjust the plane of sharp focus “consistently” at portrait range using this lens.
This is by design. There is nothing you can do. Simple but unfortunate.
On the other hand, the lens is a perfect macro lens for all practical purposes.
You just need another lens for portraits.