Film Photography

Optical design of the three SMC Pentax-M 50mm lenses

We’ve discussed here a few times the qualitative differences in the photographs Pentax’s manual-focus 50mm lenses take. The general consensus is that all three lenses — f/2, f/1.7, and f/1.4 — are very good. But the f/1.7 is probably the best-liked of them for how it renders color and for its bokeh. Nobody would scoff at the f/2 if the f/1.7 wasn’t available; it imparts a wonderful warmth to color photographs and has all the sharpness anybody could ever want. While the f/1.4 is a fine lens, it tends toward clinical accuracy and so lacks the endearing character of the f/2 and f/1.7. It is also a lot more expensive than the f/1.7 for only a fractional increase in aperture.

PentaxLensBooklet.jpg

While rummaging through my gear I found a little booklet from the 1970s SMC Pentax-M era that describes and shows the optical design of all the lenses Pentax sold then. It’s fascinating to see how these lenses are designed. All of them are some number of elements in two groups; all of them have similar front-group design. It’s the rear group that differs most among them. Check it out:

pentax 50-14
pentax 50-17
pentax 50-20

This book is about the SMC Pentax-M line of lenses, but I feel sure the corresponding SMC Pentax-A lenses are optically identical. The Pentax-As seem to be lighter, probably due to more plastic in the bodies, but they allow later bodies to control the aperture.

Here are three shots, one from each lens. Sure, film, lighting, processing, and scanning varied, all of which play into the final results. But really, can you tell which lens shot which? Probably not, and that’s the point: get any of these lenses and you should be quite happy.

Carmel Artomobilia 2017
Sweet Dog
GMC truck

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34 thoughts on “Optical design of the three SMC Pentax-M 50mm lenses

  1. Thanks for sharing the info about the optical design of these three little gems. Indeed, I cannot say which image was shot with which lens. I think that I cannot tell this with most of my lenses ;)

  2. There are actually 4 SMC 50s, the three you mention and the f/1.2 which they made in both M and A versions. The 1.2 lenses are big hunks of glass compared to these wonderfully compact and lovely lenses.

    • The book lists the 50/1.2! I should have included it. But it’s such a specialized and rarefied beast. The other three lenses are the ones everyone endlessly discusses.

    • Actually no, 50/1.2 is not a member of M-series lenses, it’s part of previous, K-series – it is named just “smc Pentax”, not “smc Pentax-M”. But you’re right, there is a fourth M-series 50mm lens – smc Pentax-M Macro 50mm 1:4.

  3. I have the 1.7! It is a really good lens but I tend to use my 28mm 2.8 more just because I prefer the width. I have the A lens equivalent of the 1.7 too. I should do a test to see comparisons of A and M lenses!

  4. I’ve had all three of these too, plus the A version of each, and there’s little difference in the final image to my eye at least. I did a test with some Yashica 50s, again 50/1.4, 1.7 and 2, shooting exactly the same scene with the same camera and same roll of film. My conclusions were again the same, I couldn’t tell much difference, sometimes I preferred the f/1.4, sometimes the f/2 but the majority I went with the f/1.7, so that’s the one I kept. For a while.

    I think the lens we choose comes down to other factors. Some people have a kind of snob factor about it and couldn’t possibly be seen with a lowly 50/2 or 50/1.7 when there’s a 50/1.4 or even 50/1.2 available.

    Others are simply looking for simplicity and low weight in which case the A version 50/2 is a great option, especially as you can use it on auto aperture with a DSLR too.

    I also found in my travels that in the M series some of the 50/2 lenses have a metal aperture ring, others are plastic, which makes the latter lighter still. For those looking for a compact 50mm lens SLR set up, a Pentax ME series body (ME, ME Super, MG, MV etc etc) paired with one of the plastic ringed 50/2 lenses is about the most compact, lightweight and capable combo there is out there.

    • I admit to feeling snobby when I put the 50/1.4 on. Look at meeeee, with my fiddy one point fo’! Oooooeee! But the 50/1.7 has character the other lens lacks.

        • The 50/1.2 does not balance well on M series bodies. I use mine on my K1000’s and the 50/1.7 on ME Super or MX.

          The 50/1.2 performs noticeably higher than the other Pentax 50’s even stopped down. But it’s bug, heavy and expensive compared to other Pentax lenses. Compared to other brands, however, the Pentax 50/1.2 is probably the smallest and certainly cheapest 50/1.2 available.

        • Not surprising to learn that the 50/1.2 balances better on the older K bodies. I have a KM around here someplace, I suppose for the day I splurge on that 50/1.2.

  5. Heide says:

    Isn’t lens design a fascinating topic? I wish I understood the technical aspects as well as you do — but at least I can thoroughly appreciate your photos.

  6. jon campo says:

    Hi Jim, An interesting post as always. I have all these lenses (not the f/1.2) And find that I like the 50/f2 a lot more than I thought I would. It’s light and can go without a lens hood and I like the look of it. Interesting that some seem to love the 28/2.8, I took that lens on a trip last year and was really disappointed with the results. Perhaps I just have a bad sample. I think my favorite Pentax lens is the 40mm pancake. It is tiny, and mine is very sharp.

    • The 50/2 is a lovely lens. I would have kept using it exclusively but I was doing some work in my church’s dim basement and the 50/1.4’s extra stop really helped. One day I’d like to have a 40mm pancake.

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      • M28/2.8 is the least sharp lens of the bunch, at least below ƒ/5.6, but it’s quite small. I’ve been skeptical towards M135, but the lens proved me wrong – it’s quite sharp even wide-open.

        • I’m not a fan of the 28/2.8, but it’s more for being too wide for me than for its softness. The 35/2.8 is a wonderful everyday lens.

  8. Brian says:

    I kinda gotta say that no one really has love for the 50mm f/2 that is not really part of the “nifty fifty” lineup. the f/2 is not a great lens. You didn’t mention the F/1.2 which is a great lens. The trinity of 50’s is the 1.2, the 1.4, and the 1.7. It’s funny, even in your comments here it’s like you don’t know that an F/1.2 exists in the M lineup. I guess I probably sound like a jerk (shame on me) but check out the 1.2 it’s a solid performer just like the 1.4 and 1.7 although it costs more than both of it’s counterparts combined usually.

    • Yeah, I didn’t know about the f/1.2 when I wrote this! I’m not sure my book lists it.

      The f/1.7 is my favorite of the three I did know about, and the f/2 is next. Both have characteristics that I enjoy and make them interesting. There’s nothing wrong with the f/1.4, but it lacks interesting character.

    • There’s just one problem with Pentax 50mm ƒ/1.2 lens – there is no 50mm ƒ/1.2 lens in the M-series; that lens is part of either earlier K-series, or later A-series, with an auto-aperture position on the aperture ring. The fastest, ƒ/1.2 version does not exist in the M-series probably because it was a high price/low production volume lens, that did not financially justify a complete redesign M-series lenses got.
      But, there are actually four 50mm lenses in the M-series – ƒ/1.4, ƒ/1.7, ƒ/2 and ƒ/4 Macro. That last one usually gets left out, despite being probably the sharpest of them all – albeit, it’s just an ƒ/4 wide-open, so (justifiably) not fast enough to deserve a “nifty fifty” moniker.

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