SMC Pentax A 28mm f/2.8 on Sony A7

SMC Pentax A 28mm f/2.8 on Sony A7

How does this classic Pentax wide angle lens hold up on the Sony A7?

The Pentax A series lenses are great on Pentax K mount bodies since they have electronic contacts that the camera body can read aperture setting of the lens and set the camera accordingly, and the camera can also control the lens aperture setting. It is really handy to have most of the functionality of an AF lens, but only have to manual focus it. Adapted on other bodies this function is wasted since the lens is used in manual mode.

That aside, the Pentax A lenses remind me a lot of the Pentax FDn (new FD) lenses which are cheapened, but still good quality lenses. They are starting to use more plastics in the construction, but image quality was still top notch.

The lens is fairly compact, weighing in at 170 g. It has 5 slightly curved aperture blades, and a standard for the class minimum focus distance of 0.3m for a maximum magnification of only 0.13x. It has a 7 elements in 7 groups. It has standard 49mm filter threads.

SAM_0611 SAM_0610

Optical Performance

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Please click on the tab you want to view. Only 1 tab may be active at a time. MTF, Distortion, and Lat CA data acquired using Imatest
MTF20 Chart maxes at vertical resolution of the sensor which is 4912 LW/PH for A7R. Anything above that is meaningless. The MTF50 chart is set at the value where the best lenses perform in the center, or about 3850 LW/PH on the A7.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH

Comments on the Results

Central resolution and sharpness is very good or excellent at all apertures, with the rest of the frame improving nicely as the diaphragm closes.

Distortion is pretty typical at 1.4% barrel.

Lateral CA is noticeable and a bit worse than average.

Flare resistance: Haven’t checked.

Measured Focal length is 30mm at 1:35 magnification

Pros and Cons

Effective Resolution

The colors are selected based on overall MP, don’t compare the colors between systems. So relative to max sensor resolution a lens doesn’t do as well on the A7R as it would on the A7 relative to its (lower) sensor resolution, but it still resolves more detail on the A7R then it does on the A7.

Aperture Weighted Center Partway Corner
f/2.8
15.3 MP
19.8 MP
12.1 MP
6.7 MP
f/8
20.4 MP
23.2 MP
18.7 MP
14.6 MP

Bottom Line

This lens is not bad, but surprisingly almost all the 28mm lenses I have tested are very similar in performance. This one had one bad corner. It wasn’t really bad, but tended to show worse results than the others so it might drag down the corner average slightly, especially at larger apertures.

I have heard complaints about this lens on full format film cameras, but I am not seeing the issue. It appears to do about the same as the older designed f/3.5 lenses (although the M 28mm f/3.5 does have less distortion), and also very similar to the competitors.

Consider donating, it is about $50 a lens to rent.





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