A couple of good precision engineering images I located:
Image by digitalpimp.
Raffles Spot, Singapore
Explore #54, January 18, 2011
SMC Pentax M Zoom 75-150mm 1:4
Image by Yumi Abe
"What a strange lens !
Mechanically it is a piece of really clever engineering. It is a push-pull zoom, but the lens does not truly extend at all more than its focal range. In common with most of the M series lenses it is beautifully built, exuding solidity and precision. The aperture ring has precise half stop detents. Subsequent comes the wide zoom/concentrate ring. This is uncommon in that 150mm is set with the ring nearest to the camera finish. The zoom/focus is nicely damped, just stiff enough to decrease any zoom creep. The front of the lens extends slightly at short concentrate distance and rotates when focusing. A short built-in sliding hood completes the design and style.
The general look and really feel is of a really heavy, ‘long and thin’ telephoto lens. It is truly not that extended, about the identical length as the DA50-200. The concentrate travel angle is fairly brief at about 120 degrees or much less, just a bit also short for actually precise focusing. Similarly, the aperture clicks are fairly closely spaced. The zoom travel is also brief, at about 2 cm. However, in use it handles well, greater than some other push-pull zooms of that era.
On APS sensor camera it is optically sharp even at f4 over most of the focal variety and becomes quite sharp when stopped down a couple of clicks. There is no geometric distortion to speak of and really small chromatic aberration. There can be just a touch of purple fringing in higher contrast areas. Bokeh is not negative, possibly slightly on the ‘busy’ side – the aperture has only six blades. All round contrast is affordable, contemplating that there are 12 glass elements in the way. Colour is slightly on the cool side.
In its day, on a full frame camera, it would have been a fine portrait zoom lens.
There is however anything not quite appropriate with this lens when employed on a DSLR, such as my K-7. Somehow the shots lack ‘bite’, requiring a fair bit of post-processing contrast tweaking. It is hard to tell whether it is a lack of microcontrast, flare, shortage of pixie dust or some thing else. I suspect that there is just as well considerably glass in the way.
Well, obtaining taken it out on a sunny day with plenty of contrasty light, I have changed my opinion of this lens. It is really extremely sharp and it appears to like a lot of harsh light – as long as you do not let it shine on the front element. Shoot from the shade into the sunlight and it genuinely starts to execute, as it the last 3 shots under.
Becoming an all-manual lens the aperture and focal length are not recorded. The shots beneath were taken largely at f4 (or possibly f5.6) and ‘various’ focal lengths."