[stevebrot]

With M lenses size was very important--many are reduced in number elements and filter size dropped from 52 to 49mm--as a result I believe it is fair to say most of us consider the M lenses to be weaker than their K counterparts...

I don't know whether that is a valid generalization regarding user opinion on this site. Overall, the Pentax K-mount primes are pretty decent lenses. Some are standouts and some are not so good. The K-series (plain SMC on the label) hold a sweet spot for many of us in that they are harder to come by and have uniformly excellent build with few optical compromises. With the M and A series lenses Pentax began offering more consumer-level options. The M 28/2.8 is a good example. It was price competitive with Vivitar back-in-the-day. The addition of cheaper lenses into the mix tends to color perception.

I have both the Pentax-K 55/1.8 and Pentax-M 50/1.7. The 55/1.8 is better optically, but not enough so that it finds a place in the bag over the 50/1.7. I shoot both these lenses on film cameras and the form factor of 50/1.7 wins out over the marginal increase in image quality.


Steve


(BTW...what does filter size have to do with optical quality?)

[gofour3]

The M series was also marketed as the line
for use on the LX professional body,
except for special cases like the 24/2.8, 30/2.8 and 50/1.2
which needed a 52mm filter size rather than the standard 49mm of the M series.
(Strangely enough, the 200/4 and 400/5.6, which also took bigger filters,
were nevertheless designated as M lenses.)

There were also “M” series zooms that had 52mm & 58mm filter threads.

Phil.

[lytrytyr]

There were also “M” series zooms that had 52mm & 58mm filter threads.

Indeed, or even 67mm for the 35-70.
although the 40-80 and 75-150 did manage to stick to 49mm.

[Digitalis]

what does filter size have to do with optical quality?

I suppose such an uncommon filter thread can make finding an appropriate lens hood for these lenses could be considered an impediment to obtaining the best IQ from a lens.

[6BQ5]

Oh, wow! I did not expect to see so many replies to my question! And I also didn't expect such a wide variety of replies too!

Here is what I was guessing the differences would be. It's a completely data-less guess though.

The "K" lenses, which are not specifically marked as "K", came out to coincide with the K series of bodies (K2, K1000, etc). All of the optics were based on the previous M42 lenses. There were probably some tweaks here and there to the design and a few extra focal lengths were added.

The "M" lenses came later with the release of the "M" series of bodies (ME Super, MX, etc). Based on what I read around the web and what others have said here it seems like the "M" series of products were designed to be slimmer, smaller, and targeted toward the larger amateur audience. These were the people who simply wanted to take pictures. They weren't so concerned with resolution as the enthusiast or professional of the day. These lenses weren't bad at all but compromises were made to make them smaller. The pictures produced were still "good".

The "A" lenses are an extension of the "M" lenses. They still retain the compact design of the past but the elements were redesigned so that the optical performance matches the "K" series of the past. Pentax must have learned a thing or two since 1975. Coatings were tweaked a bit too. These lens designs were the pinnacle of perfection and would not improve until the digital era came upon us.

So, in terms of desirability, the order would be A - K - M.

[lytrytyr]

Well, as I hope our posts have shown,
it's not as cut-and-dried as that.

[stevebrot]

...The "K" lenses, which are not specifically marked as "K", came out to coincide with the K series of bodies (K2, K1000, etc).

Yes

All of the optics were based on the previous M42 lenses.

Some, not all

The "M" lenses came later with the release of the "M" series of bodies (ME Super, MX, etc).Based on what I read around the web and what others have said here it seems like the "M" series of products were designed to be slimmer, smaller...

Yes

...and targeted toward the larger amateur audience. These were the people who simply wanted to take pictures. They weren't so concerned with resolution as the enthusiast or professional of the day. These lenses weren't bad at all but compromises were made to make them smaller. The pictures produced were still "good".

Not quite. Most of the M-series cameras, with the exception of the MX and later the LX (1980), were indeed aimed at the consumer and advanced amateur market. As for the lenses, the intent was to competitive (when used on the MX and LX) with the professional grade product line of compact cameras and lenses from Olympus. Some were definitely consumer grade, but most were not. The distinction at the time (not as obvious now) was based on price point. It is important to note that K-series bodies and lenses were on the shelves contemporary with the M-series product for several years. K-series bodies were even sometimes sold with M-series lenses.

The "A" lenses are an extension of the "M" lenses.

For primes very true, though not true for many A-series zooms.

They still retain the compact design of the past but the elements were redesigned so that the optical performance matches the "K" series of the past.

Nope. Most of the optical designs were carried over from the M-series lenses. The big difference was in terms of build and addition of the "A" contacts to the mount. The A-series introduced plastic build and full program auto exposure into the Pentax world.

These lens designs were the pinnacle of perfection and would not improve until the digital era came upon us.

Nope again, with the possible exception of A* premium line.

So, in terms of desirability, the order would be A - K - M.

Conventional wisdom around here:

Optical quality: K over M or A (with a few exceptions)
Build: K or M over A (again, with a few exceptions)
Ease of use on Pentax dSLR: A over K or M

Steve

---

Last edited by stevebrot; 06-25-2013 at 12:10 PM.

[6BQ5]

Well, as I hope our posts have shown,
it's not as cut-and-dried as that.

No kidding!

stevebrot,

Thanks for picking my guess apart. I think the next logical step in this thread would be to start ranking lenses of the same focal length and aperture to see which is the best. From a past thread I learned that this is almost a pointless exercise because it would require people to take unnatural pictures of lines, bricks, and fence posts. Then, after zooming in to a level that nobody would even dare to try in normal circumstances we can make attempts at sharpness, distortion, and color rendition. I think something like that works when there are extremes in quality, like when a bad lens is really, really bad so that almost any "good" lens looks flat out excellent. I was kind of hoping that there would be some massive general rifts between all the series of lenses so that it would be easy to say "bad", "good", and "great" when used with a digital sensor. Then again, Pentax probably didn't make a "bad" lens. They simply made "good" and "better".

I think it would be interesting from a historical point of view to take a lens design and track its design progression to see what new-at-the-time technologies and ideas Pentax incorporated. Pentax probably had 10 design variations for each one that was released. I think you would have to have worked there for 30+ years to do something like that though.

[stevebrot]

I think it would be interesting from a historical point of view to take a lens design and track its design progression to see what new-at-the-time technologies and ideas

I might make an interesting idea for a new thread of discussion. There are some pretty strong opinions on this site regarding the "best" for a particular focal length in the full range from the early '50s vintage to present product.


Steve

[lytrytyr]

I think the next logical step in this thread would be to start ranking lenses of the same focal length and aperture to see which is the best.

The problem there is that different users have different priorities.
As I mentioned earlier, I like the M50/1.4,
because performance at infinity is important for me,
whereas other users with other priorities prefer the K or A versions.

If you do want to reduce everything to ranking numbers,
you can check out the user l e n s r e v i e w s here on PF,
although there have been several threads questioning the rankings.

[stevebrot]

it would be easy to say "bad", "good", and "great" when used with a digital sensor.

What I have found is that it is hard to make a generalization based on brand or lens series. That is with a few possible exceptions such as the Pentax "*" and "Limited" lenses. There are so many cheap lenses that exceed expectations and so many high end that are not as good as advertised. The idea is to have fun and use the resources on this and other places on the Web. The lens clubs on this site are a huge resource providing evidence of what a particular lens can do in the right hands. The reviews (here and elsewhere), though biased as to excellence, are a good resource regarding build and potential problems.

Enjoy!


Steve

[gofour3]

I think it would be interesting from a historical point of view to take a lens design and track its design progression to see what new-at-the-time technologies and ideas Pentax incorporated. Pentax probably had 10 design variations for each one that was released. I think you would have to have worked there for 30+ years to do something like that though.

As well as some Pentax lenses were designed in collaboration with other manufactures.

Like the two Pentax/Carl Zeiss classics:
K&T15/3.5 AL
K28/2

Phil.

[carrrlangas]

As well as some Pentax lenses were designed in collaboration with other manufactures.

And more to date, with Tokina, the DA*16-50, DA12-24 and DA10-17

[Kerrowdown]

I'm a tad bias, but A primes serve me well , even to this day.

[northcoastgreg]

The "K" lenses, which are not specifically marked as "K", came out to coincide with the K series of bodies (K2, K1000, etc). All of the optics were based on the previous M42 lenses.

Most "K" lenses, not all, are based on previous m42 designs. As has already been noted, the K 28/3.5 is a new design (optimized for edge to edge sharpness), as is, I believe, the K 20/4.

Based on what I read around the web and what others have said here it seems like the "M" series of products were designed to be slimmer, smaller, and targeted toward the larger amateur audience.... These lenses weren't bad at all but compromises were made to make them smaller.

That's an interesting theory, and I suppose it's probably at least partly true. But I've sometimes suspected that the "compromises" in the M series lenses (or at least in some of them) resulted, not so much from their size, but from the fact that the M series was a rush job. I don't believe Pentax ever came out with so many new designs in so short a period. Of course, we can't be certain exactly when the M series was planned, but it seems likely it was planned after the switch to the K-Mount in 1975, in which many of the K series lenses were released. Most of the M series was released just two years later, in 1977. It's as if Pentax designed a whole new series of lenses in just a year or so.

When Pentax devoted more time to designing a compact lenses, the results were usually very good, with very little evidence of quality lost due to compromise. That's true of two of the best of the M series lenses, the M 300/4 and the M 28/2, which came out in 1980. And of course it's true of the DA limiteds as well.