SMC Pentax M 28mm f/3.5 on Sony A7

SMC Pentax M 28mm f/3.5 on Sony A7

Another old wide angle that is about the same size as the OM, is it as good?

This is a baby sized prime, with fine build quality. It has a front aperture ring with 1/2 stop steps from wide open to f/22 (1 stop step between f/16 and f/22). The focus ring is smooth, and the front element is nearly flat. The lens is small, but with adapter on the A7 it is nearly twice as long. The two combined are about the same size as the E mount 30mm macro.

The SMC Pentax M, M for miniature, is their compact series of lenses introduced in the late 70s. This lens is a 6 element 6 group design. With its modest 0.3 m minimum focus distance, the magnification should be similar to the kit zoom. The filter threads are standard 49mm and the lens is fairly light at 180 g.

Spec wise this lens is nearly identical to the Olympus OM 28mm f/3.5 lens, more like the later MC model than the G model I tested recently (

SAM_0266 SAM_0268

Optical Performance

DSC01585__0.4 sec_YBR52_18_multi_cpp
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 (4000 LW/PH for A7). Anything above that is meaningless. The MTF50 chart is set based on a ratio of where f/22 appeared for several lenses and the vertical resolution.
Note: 4k video is 2160 LW/PH
Note: HD video is 1080 LW/PH
For video, you want your MTF20 values to be at or above those numbers. The green region and up is where a good photographic lens will lie, but decent is anything better than f/22 performance, so mid yellow and up. If you can’t see the colored borders, for MTF20 Red: ~1500 LW/PH, Yellow: 2700, Blue: 3500. MTF50 values are 900, 1620, and 2160 LW/PH.

Comments on the Results

The resolution and sharpness are excellent in the center at all settings, but the corners are a bit weak until f/8.

Distortion is better than average at 0.75% barrel.

Lateral CA is noticeable in the corners/midfield, but is not severe.

Flare resistance: Haven’t checked.

Measured Focal length is 30mm at 1:35 magnification

Pros and Cons

Bottom Line

Wide open the lens is comparable to the Kit 28-70mm, and stepped down it is sharper, as sharp as my ZEISS 24-70mm f/2.8 at f/8 and f/11. This is saying that this lens is very usable stepped down across the frame. This old lens is a winner on the A7. I am going to test this lens more since it does pretty well. The Olympus is a bit better in the corners based on my samples, but this isn’t bad either.

This is just a quick review to get the basic performance up. Too many lenses to test them all thoroughly, although I think this one moves onto the next round.

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

1 Comment

  1. memetph says:

    You should try the M 35 f2.8. I bought this lens though a so so reputation . I suppose it is based on APS-C sensor. On my A7 it is very very nice lens for landscape .