There are some obvious differences between the 30-year-old Asahi Optical Co. SMC Takumar 1:2 55mm and the half-as-old SMC Pentax-F 1:1.7 50mm. The SMC Takumar 55/2 is a manual-focus, manual aperture M42 screw mount lens. The fully-automatic bayonet Pentax-F 50/1.7 is four lens generations more advanced. The SMC Takumar 55/2 has a 5mm longer focal length and is half a stop slower in maximum aperture. The Pentax-F 50/1.7 has a minimum aperture of f/22 whereas the SMC Takumar 55/2 only stops down to f/16.

However, as different as the two lenses are, they share some important similarities. They both are roughly the same size. Both weigh 200g. Both focus to nearly the same point. They both have 6 straight aperture blades. The front lens elements are similarly sized, both having Pentax's Super-Multi-Coating, which provides excellent light transmission and protection against flare.

Build

The SMC Takumar 55/2 is metal with a rubberized focus grip. The grip is necessarily made for manually focusing, so it is wide and focuses smoothly across the range. The metal aperture ring moves smoothly and clicks into place smartly.

The lampshade-shaped SMC Pentax-F 50/1.7 was the start of the end of aesthetics at the Asahi Optical Company. It is made from dull gray plastic with the focus scale behind scratchable clear plastic. The aperture ring has a build proportionate to its lack of intended use. The notched focus ring is better than the later-generation FA lenses, but is not really intended for manual focusing.

Physical Usage

Using both of these lenses on DSLRs, the obvious advantage goes to the Pentax-F 50/1.7. As a modern lens, the F series supports all the automated features of the K10D, including open-aperture metering, exposure compensation via e-dial, MTF, and auto focus.

The SMC Takumar 55/2 is an M42 screw mount lens, and as such requires an adapter to fit K-mount cameras. It has an Auto/Manual switch that, when set to the Manual position, allows the lens to stop down as the aperture is changed. With the lens in this stopped-down mode the K10 can then meter in Aperture Priority, with the obvious trade-off of the darkened view. However, as with all M42 lenses, the DSLRs will only support center-weight or spot metering, Aperture Priority or Manual modes, and exposure compensation requires pressing two buttons simultaneously. For the duration of this matchup, the SMC Takumar 55/2 required +1EV compensation compared to the Pentax-F 50/1.7 to produce similar shutter speeds.

Side view of both the SMC Pentax-F 1:1.7 50mm, left, and the Asahi Optical Co. SMC Takumar 1:2 55mm, right.
Click here for to see the front elements.

This series of 100% crops (click on the photo for the full-size version) of both lenses between f/1.7 and f/8, shows demonstrates the uncanny similarity in out-of-focus rendering between the two.
Click here for a gallery of samples of these two lenses.

I was able to deduce only two differences between the images from both these lenses. The first difference is that the Pentax-F 50/1.7 produces slightly cooler (temperature-wise, more towards blue) images than the SMC Takumar 55/2. In the digital age, and shooting RAW, color corrections of this type are much easier to do than in the film era, so the slight temperature differences in images does not weight into my liking or disliking a lens.

Color Fringing

The second difference, and the only pronounced one, was color fringing at wide apertures. High-contrast subjects produced a purple/blue color bleeding in the Pentax-F 50/1.7 at f/1.7 and f/2 at light/dark transitions. The SMC Takumar 55/2 produced a slightly less noticeable red fringe in the same scene. Neither result is particularly disturbing or intrusive, and both can be addressed by the same post-processing.

Sharpness

I could detect no difference in sharpness at equivalent apertures between the two lenses. I would suspect most noticeable differences would be from focusing errors rather than the lenses themselves.

Bokeh

There is virtually no difference between these two lenses in their out-of-focus rendering. The crops attached to this matchup were taken from a variety of subjects, apertures, and distances. The undetectable differences between the two hints that perhaps the Pentax-F 50/1.7 inherited its optical identity from the SMC Takumar 55/2.