[chongmic]

Is there significant different between optical quality of both series? Let say on a 135mm lens.

[Lowell Goudge]

if there is a difference it is likely mechanical build with the K series "seeming" (in my opinion) to have slightly better build.

Note also, some lenses are simply not available in the M series, or at least not at the same maximum aperture,.

those which changed or disappeared

K28F2.0 (M series max was 2.8)
K50F1.2 - gone totally
K105F2.8 - m series was 100F2.8
K85F1.8 (M series max was F2)
K135F2.5 - gone totally
K150mmF4 (M series was F3.5 - one of the few places a lens got faster)
K200F4 - M series was much lighter and smaller.

Many other lenses also had smaller filter rings on the M series, typically 49mm vs 52 mm for the K series

[hoanpham]

K200f2.5 - gone too.

[boriscleto]

The K35/3.5 disappeared, replaced by the M35/2.8.

[Lowell Goudge]

K200f2.5 - gone too.

totally forgot that one

The K35/3.5 disappeared, replaced by the M35/2.8.

yes but this is really an improvement of sorts as the 35F2 remained

[chongmic]

Good info. Thanks guys. I read in another forum that the K series share the same optical formula with the Takumar. Any truth to it?

[Lowell Goudge]

Good info. Thanks guys. I read in another forum that the K series share the same optical formula with the Takumar. Any truth to it?

some do, some don't

there are quite a lot of different optical formulas for hte different lenses,

what I do know, is that for the macros, the S-M-C Tak through SMC-M series lenses, both 50mmF4 and 100mmF4 have the same optical formulas, the 50mmF1.4 is the same through out, but the 300F4 for example is quite different.

the 200F4 is the same SMC Tak and K, but different for M.

so sometimes it changes, some times not.

[axl]

those which changed or disappeared
K50F1.2 - gone totally

Not really true, K50/1.2 v2 (with M style lettering) was produced alongside M series all the way until 1984 when it was replaced by A version

To OP:
K series are valued more because they are rarer. Some say Ks are better build. Ms changed optical formulas and in some cases sacrificed speed to get smaller.
Ks are bigger and in all cases heavier. Use bigger filter too. Personally, I'd pick K over M anytime

[boriscleto]

The K55/1.8 & K55/2 disappeared as well. Replaced by the M50/1.7 & M50/2.

[northcoastgreg]

Is there significant different between optical quality of both series? Let say on a 135mm lens.

It all depends on which lenses you are comparing, and also what is meant by "significant" differences. Generally speaking, the K series tend be better optically than the M series, but the differences aren't necessarily all that "significant." The K 135/2.5 may be "significantly" better than the M 135/3.5; but the M series 135 did not replace the K 135/2.5 (which remained in production until the A series), but the K 135/3.5, and there's probably not a huge difference between those lenses. However, if Yoshihiko Takinami's resolution tests can be trusted, it would seem that the K series are short teles might be significantly sharper than the corresponding M series short teles. But I would expect that a telephoto lens like the M 300/4 would outclass the K 300/4. Nor would I expect to find any significant differences among the 50/55mm lenses.

[Lowell Goudge]

something I would like to see, is a complete product range in a matrix, something like focal length and max aperture on the left, and lens series on the top, with perhaps one or two technical details in each grid. Maybe optical formula and weight for example.

it would give a really good snapshot of what was out there.

[m42man]

Taking the M and K 135/3.5 specifically, the M is totally different from the K. The M is 5 elements in 5 groups, K is 4/4. I suspect the extra element enabled Pentax to make the M much smaller than is usual with these lenses.

Incidentally, the K is the same as (I think) the S-M-C Tak 135/3.5, which in turn is the same as the later version of Super Tak - just the coatings will differ. (The early version of Super Tak was 5/4.)

Regarding IQ, the K 135/3.5 is superior to the M (but it isn't quite as nice to use as either the M or the Tak - for different reasons).

[hoanpham]

Regarding IQ, the K 135/3.5 is superior to the M

The M is lighter, I use it in daylight and at f5.6-8. It should not be an issue. The K135f3.5 may be better at f3.5 (maybe f4 too), but K135f2.5 step down one click will be much better.

Get both if possible. (They're still cheaper than AF counter parts, eg DA*50-135)

[Stone G.]

I would also refer to Bojida Dimitrov's Pentax K-Mount Page and the resolution tests shown there (also quoted by northcoastgreg above):
Measured Resolution Numbers

My conclusion from studying that page is that it is essentially impossible to tell - in general terms - whether a "K" or an "M" (or "A" for that matter) lens is the better, optically speaking.

My own personal experiences is limited but I do have the 35/3.5 in K version - and it is unbelievably sharp, as the table also indicates. On the other hand, I "only" have the 100/2.8 in M version, and according to the same table the 105/2.8 in K version should have a centre resolution about 40% above that of my humble M-counterpart - but in practice, when comparing my M-pictures with a friend's K-pictures, I cannot tell the difference. They are both excellent performers!

So, my personal conclusion is: If the focal lenght and aperture of any of (the better of) those lenses suit your needs, just buy it whether "K" or "M". (And if you regret, you can almost certainly sell it again without any significant losses).

B. R. / Steen G. B.

[Marc Sabatella]

My own personal experiences is limited but I do have the 35/3.5 in K version - and it is unbelievably sharp, as the table also indicates. On the other hand, I "only" have the 100/2.8 in M version, and according to the same table the 105/2.8 in K version should have a centre resolution about 40% above that of my humble M-counterpart - but in practice, when comparing my M-pictures with a friend's K-pictures, I cannot tell the difference. They are both excellent performers!

Once upon a time, someone posted a comparison of short telephotos that included both the K105 and the M100, shooting the same subject at a variety of different apertures, looking at both center and corners. If anything, I thought the M100 looked better than the K105. I kind of suspect something went wrong in the Yoshihiko Takinami resolution tests of the M100/2.8, as well as the M85/2, as they seem out of step with what practical experience suggests is true.

For me, the difference between K & M in size weight for any of the focal lengths that interest me is much more significant than any difference in optical quality that I've seen demonstrated, and any differences in build quality (most say the K series is slight better) comes in a distant third.