In this article I'll pique your interest in 135mm lenses and give an overview of the Pentax-specific offerings.
"Back in the day" there were a number of popular focal lengths, after the ubiquitous 50mm:
35mm for street photography
28mm for wide angle, a nice field of view (FOV) for landscapes
85mm in a fast optic for portraits
100mm or so for macro lenses -- close-ups of bugs, flowers and such-like
All of these focal lengths were easy enough to make at a reasonable price point. They have been popular through the history of SLR photography, but with the introduction of digital something odd happened.
The 135mm disappeared.
Whither the 135?
I am not sure why this happened. I think it was mostly to do with the rise of zoom lenses. After all, almost every contemporary photographer has a lens that will shoot at 135mm, it just isn't a prime. I think it was also partially to do with the popularity of the "cropped" APS-C sensor. The field of view of a 135mm lens on these sensors is that of a classic 200mm. Though that can't be the whole story, since 200mm was hardly a useless length, even back in classic 35mm film days.
Regardless of the reason, there are only three 135mm primes on the market today, the Canon EF 135mm f/2.0L USM, AF DC-Nikkor 135mm f/2D and Zeiss Sonnar T* 135mm f/1.8. On the other hand, a trawl through eBay will reveal literally hundreds of older models for any camera system you care to name. So that's one reason to consider this focal length: availability.
The second is affordability. While you can pick up an old lens for fifty bucks or so, those I listed above will run you $1000, $1400 and $1500 respectively. No doubt they are made to the highest standards, but are we really losing that much by getting a classic alternative? Not if it's been well looked after. If there's one thing I have learned it's that old optics might well be better than contemporary optics. Once upon a time people engineered and built things to last. On the other hand, it is true that the newest designs can be optimised for digital sensors and might have fractionally better coatings on the front and back elements. These factors are real but negligible for most purposes.
What are the main compromises in getting a classic lens? First of all, it will be manual focus. That's not a big deal to anyone who enjoys photography as an art. Secondly, you likely won't get automated apertures. Be sure to have read my last article, SLR Aperture Control Mechanisms, to familiarise yourself with the alternatives. Lastly, a piece of gear that's been banging about since the sixties might need servicing to bring it up to standard. Be careful of what you buy sight unseen; you are gambling on its condition. (But I do this all the time.)
Now I'll put on my Pentax hat and have a look at their past 135mm models, just to get a feel for the choices out there. Here I am replicating the process I went through before buying a 135mm lens, in order to help out anyone who might follow in my footsteps. Pentax is an interesting case, since they do not currently have a offering in this class. But if you shoot some other system, keep reading. A lot of the choices are for M42 screw mount and can be as easily adapted for Canon as Pentax.
The Pentax Offerings
The earliest Pentax lenses are M42 screw mount, but can be used with many contemporary cameras via a simple adapter. The M42 mount was first developed by Carl Zeiss at their Jena plant in 1938 and was associated with brands like Pentacon and Praktica. But Pentax did the most to introduce this mount to the USA and so it was often named after that company. Their M42 lenses were branded "Takumar" and still have a devoted following. The build quality is excellent and manual focus a delight to use. Aesthetically they are black and "industrial" in appearance, often with large chunky rings.
No fewer than six variations on the Takumar 135mm f/3.5 were released from 1957 to 1973. To this we can add three different Takumar 135mm f/2.5 lenses. Of these the lens in the highest regard is the "Super-Multi-Coated TAKUMAR 1:2.5/135" (to give it the official name) from 1972. While you can spend only $50 for a Tak, that particular model will be significantly more.
When Pentax changed from M42 to their K-mount they kept the Takumar brand for the first while. The "Takumar (BAYONET) 1:2.5 135mm" is a lens to avoid. But the next generation "smc PENTAX 1:2.5 135mm", made from 1975 to 1985, uses the same optical formula as the best Takumar and is considered to be an excellent "sleeper" lens. There is also a smaller and slower "smc PENTAX 1:3.5/135" which does not have the same following.
By the way, this first generation of lenses are called "K" after the mount, even though the designator appears nowhere in their names. They are still manual focus and manual aperture in operation. If you read my previous article you might wonder why such lenses are still completely manual even on the newest bodies. The answer is that Pentax has "crippled" the KAF (and subsequent) mounts. They do not have the mechanical stop-down coupler so they cannot be used automatically with older lenses. Instead one must manually "stop down" by pressing the Green button. I still hope some newer digital body will correct this oversight, which is the most glaring mistake Pentax has made with their cameras. (Though of course it only affects those of us looking to use old lenses.)
The next generation, "M", were very similar to the K's and in fact overlapped them in production. The "smc PENTAX-M 1:3.5 135mm" is supposed to be decent, but I haven't found anyone raving over it. Following this was the "smc PENTAX-A 1:2.8 135mm", which unfortunately repeated the optics of the Takumar Bayonet, despite offering automatic aperture for the first time. This is perhaps because it was a deliberate attempt at a cheap lens to contrast with the "smc PENTAX-A* 1:1.8 135mm", likely the pinnacle of all Pentax offerings. This comes up for sale only once a year and attracts as much money as any contemporary lens, despite being manual focus. The superb optic and wide aperture are the reasons, though it needs to be mentioned that it is more than twice as heavy as its junior version. Not everyone wants to carry close to a kilogram of prime lens.
The first auto-focus series was labelled "F"; today these are relatively obscure compared to the "FA" that followed. Both these series offer similar lenses with a more plastic feel, though still made of metal underneath. Personally I dislike their loose focus and cheap looks despite the fact that the "smc PENTAX-F 1:2.8 135mm IF" and the "SMC PENTAX-FA 1:2.8 135mm IF" are well-regarded. But with prices in the vicinity of $500 we are now out of bargain territory.
The obvious conclusion to reach is that if you want a manual Pentax-compatible 135mm lens then look for the K 135/2.5. But this overlooks two facts. First, its reputation has caused prices to increment to the $300 range. Second, there are many other brands of compatible lenses that might be better deals.
If you want a very good Pentax lens you are in competition with lots of other like-minded photographers, and so will pay a hefty (though fair) price. As an alternative, in my next article I will turn my eye to third party brands.
For specific information on each model mentioned, see:
* Bojidar Dimitrov's Pentax K-Mount Page
* Pentax Forums Lens Review Database