Pentax S.M.C M 50/1.7

[Ben]

Intro

 

I read many enthusiastic reviews on Pentaxforums about this prime and since it was generally cheap and has the same focus direction as my other Nikon Legacy glass, I gave it a try. I found a copy for 30€ and the seller assured me, that it was optically flawlessly working and just had some cosmetic tokens of time. 

 

Well so far for seller’s honesty. The lens came to me with fungus behind all elements, I was unable to turn the focus to infinity and it was taken apart unprofessionally to declick the aperture. The small metal ball that served as friction point for the aperture ring was missing entirely. So after taking this lens almost completely apart twice over the course of 6 hours, molding a substitute ball for the aperture clicks from hard plastic and cleaning the elements with distilled water and q-tips, I now have a clean, clicking and focusing lens.

 

So with all the good things I have to say about this lens in the following, you have to keep in mind that this lens was in really bad shape and good copies may perform even better than mine. I use a cheap cnc-machined adapter that only acts as an extension tube and has not moving parts except for the locking pin.

 

Haptics & Mechanics

 

The lens is made of metal and plastic and has a solid feel to it. The aperture clicks. Neither the focus nor the aperture ring wobble. The focus ring is rather small due to the very small overall size of the lens but it only takes some time to getting used to it. Focus on my copy was very smooth and the focus throw is quite long.

 

Sharpness & Contrast

 

Center sharpness is good from F1.7 but the corners need F3.4 to F4.0 to catch up. Contrast makes a big jump from F1.7 to FF2.0 and improves only gradually from there on. Diffraction shows it’s first signs at F6.7 and is noticeable above F11 in regards to sharpness. Contrast decline starts at F9.5 due to diffraction.

 

For the corner crops I chose the sharpest corner as I am not very confident my test setup was very precisely aligned. The the corners having different degrees of sharpness could also mean that my copy is decentered in some way. I need to check this at an infinity target to be certain.

 

center and cornder crops:

 

 

Aberrations & Vignetting

 

At F1.7 to F2.8 the lens shows a rather strong amount of axial(some call it lateral) chromatic aberrations that manifest in purple fringes in front of the dof and green fringes behind the dof. 

 

 

At F1.7 and F2 there is a small amount of tangential and a fair amount of sagital astigmatism in the corners. This is unfortunate for astrophotography as the point shaped light sources make this defect easy to spot.

 

 

Distortion in general is well controlled (very minor barrel distortion) but I think there is some curvature of field, as flat surfaces have not a linear sharpness degradation from center to edge but have some radii (if that is the plural of radius) that stand out in unsharpness.

 

Grid vs. distortion:

 

Full size radius to show curvature of field:

 

Vignetting is mostly gone by F2.8.

 

Flare & Colors

 

I haven’t noticed any color shifts in the edges that could stem from to steep incident angles. There is also no harsh cast as can be seen in some legacy lenses. The weather has been very bad where I live and I cannot say much about flare resistance or ghosting. 

 

Conclusion

 

I bought this lens in order to have a smaller substitute for my Nikon 60/2.8 micro AF-D as I rarely used it for macro anyway. The 60/2.8 has a very short focus throw on the range from 1m - infinity that makes it hard to use on everything except macro although it has a nice rendering in this range. After using the Pentax 50/1.7 for some time I definitely think that this lens is a keeper for my use cases. It is usable for amateurish widefield astrophotography at F2 with little compromise and from F2.8 without any sacrifices. The contrast kicks in at F2 and F2.8 as does the biggest boost in sharpness. As mentioned in the Intro this is not necessarily the best copy yet I still think this lens delivers. I would not hesitate to recommend this lens if the SEL5518Z is out of reach as this would be the only upgrade (optically) I consider a step up from the Pentax SMC M 50/1.7.

 

I will try to post more real world examples later on.

[Ben]

I attended a CSS conference in Amsterdam but time was limited, so only a few shots. 

[glitterball]

Thanks for a really excellent and studied post.

 

I also have this lens - also bought after reading the reviews on PentaxForums!

I'm about to sell my copy though, as I don't find it as good as the Canon FD 50mm f1.4.

The small size of the lens is very impressive, but not small enough for me to sacrifice quality.

My copy seems to be quite soft wide open, but a match for the FD when stopped down to f8. Admittedly, my tests were nowhere near as scientific as yours.

 

The colours from the old Pentax lenses seem better that the Canon FDs (to me anyway), and this is the first time that I've stuck with an FD over a Pentax (the Pentax K 28mm f3.5 is great and the Pentax-M 135mm f3.5 is probably the biggest bargain legacy lens out there).