SMC Pentax K 24mm f/2.8 on A7R

SMC Pentax K 24mm f/2.8 on A7R

How does the Pentax compact wide perform on full frame?

The SMC Pentax K 24mm f/2.8 is a nice wide/ultra wide lens for full frame. It is compact and well built. Smooth focusing typical of other lenses of this vintage.

The lens is fairly compact and dense, weighing in at 195 g. It has 5 slightly curved aperture blades, and a standard for the class minimum focus distance of 0.25m for 0.15x magnification. It has a 9 elements in 8 groups. It has standard 52mm filter threads (K lenses are mostly 52mm, and later M and A lenses are mostly 49mm). Aperture adjustments in half stop increments.

Lens curtesy of Lloyd Shell. Check out his great work at: http://lloydshell.zenfolio.com.

SAM_0998 SAM_0999

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 sharpness and resolution are excellent from f/4. Even from wide open the lens is good in the central region. However, corners trail notably, and it isn’t until f/8 or f/11 that the corners are good enough for most.

There is some field curvature, but nothing dramatic.

Distortion is class average with complex mustache distortion which is predominately 1.65% barrel.

Lateral CA is noticeable in the corners.

Flare resistance: Haven’t checked.

Measured Focal length is 26mm at 1:35 magnification

Similar to the Olympus 24mm f/2.8, this lens has a slight color shift in the corners on the A7R. Corner fix or Adobe Flat Field can help you fix this, or some vignetting correction can help as well.

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 this doesn’t do as well as the lens does 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
12.5MP
19.6 MP
6.1 MP
3.6 MP
f/8
22.4 MP
31.6 MP
14.5 MP
9.2 MP

Bottom Line

This lens is a typical 24mm f/2.8 from this vintage. It is pretty strong in the center, but weaker towards the corners. Shooting at f/8 or f/11 and you will do usably well for landscapes, not great, but not bad either. As good or better than the ZEISS 24-70mm f/4, but the ZEISS will be better at larger apertures. So for full frame I would be using f/8 or f/11 mostly, but on APS-C where it makes a great environmental portrait lens I would probably make use of its sharp central region.

However, my opinion with all of these 24mm lenses designed for 24×36 mm full frame but used on APS-C is usually a good kit lens will do as well. Remember, this was designed as an ultra wide lens. That aside, many people prefer using manual focus lenses for landscape work, especially when infinity focus is involved, since it is easier to set infinity focus on a manual lens than on a modern AF lens.

Lloyd has a gallery devoted to the 24mm here: http://lloydshell.zenfolio.com/p193848727
Remember, these photos are all copyrighted.

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





Crops

I know some people like comparison crops, but remember, real images have things like field curvature and focus variations so it is sometimes hard to make these match perfectly. The full image is underneath and the crop areas are from top to bottom: center, 24×36 corner, and APS-C corner (24×36 Partway).

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Above: f/2.8

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Above: f/4

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Above: f/5.6

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Above: f/8

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