Manual Focus Lenses...

You would end up paying 1000+$ for this lens today...if you can find one.

I have a K135 f2.5 somewhere. Nice lens but a lot of CA and they won't go away when stopping down.

As Dangermouse said : the 50mm & the 135mm were THE standard lenses in the film era and so there are lots of superb 50s and 135s out there.

I have two MF 135s :
SMC Takumar 135/3.5 - and I love this lens, maybe just a shade more than my Carl Zeiss Jena Electric MC Sonnar 135/3.5 (which I guess actually more people would appreciate) but both are wonderful lenses.

One other thing to bear in mind: You can fit M42 (screw mount) lenses to your K20D but you need an adapter, the best one is the genuine Pentax (as it's quite tricky to machine the bayonet fitting properly and 3rd party ones often don't fit too well) but this can be quite expensive. Worth factoring into your costs! It can also be fiddly to remove. If you want a specific Takumar then most of these lenses were redeveloped for the K mount with the original SMC Pentax range of 1975-77. I leave the Takumars on the Spotmatic/SV and buy the K series for my later film and digital bodies.

Are all the Zeiss-badged 135mm lenses Sonnars? I've been keeping an eye open for a cheap one as I fancy having a go with this different lens design and I now have a reliable Spotmatic to use M42 lenses on. The K-m seems to have major problems with the adapter and underexposes horribly no matter what you try.

Zeiss Sonnars : sorry not the man to ask, some very knowledgeable lads on MFLenses though.

M42 adapters : I have 3 and luckily haven't had any of the horrible (shavings in the camera) issues some people have experienced. However I now use the Pentax made adaptor exclusively - it is the highest quality one and just takes 4 or 5 secs to put on / take off instead of the messing around for 20 - 30 secs of the 3rd party adapters.

CA is easy to eliminate via software these days. My Vivitar Series 1 28-90 really suffers from it, but otherwise is a great lens. I have a lightroom preset that zaps the CA in milliseconds.

Really that's expensive, looks like a great lens though... but for that money

I wonder how it would preform against the Carl Zeiss Makro-Planar T* 100mm f/2 that goes for slightly more new.

Is their a website that explains all the different Carl Zeiss manifestations? Some I've seen a cheap and probably cheerful while some I've seen are expensive and sought after.

Smeggypants Posts the Number of the Beast

Is the old LBA flaring up Smeggy

It's positively raging

I wondered why I suddenly felt a deep chill.

If you meant with manifestations lenses then the MFlens forum might be useful, they have some reviews up or ratings.
http://www.mflenses.com/lens-review

You might find more feedback on the forum there.

I also have the mentioned Voigtlander 125. No need to fix any CA with this one

To answer Stefan question: the Zeiss is supposed to be better wide open in the extreme corners on 24x36. On the other hand, the Zeiss is "only" a 1:2 macro lens while the Voigtlander is a 1:1. Color rendition seems to be different as well.

Thanks Zav, they are both too expensive for me at the moment though.
1:2 is actually good enough for me and it's f/2 lens but nah, I've the DFA 100mm i'm happy with.

Maybe I'll keep my eye open for something like a Carl Zeiss Jena Olympic Sonnar 180mm f/2.8 have heard good things about that lens and it will give me more reach.

The CZ 135mm f/3.5 are all Sonnar's as far as I know. They usually have a Red S on them. Older ones actually say Sonnar. I don't think the performance varies that much although a lot of people think the earlier Zebra lenses were better for bokeh. The grease used on them wasn't very good and they are usually quite stiff. Beware of any f/2.8's some were rebadged Japanese lenses imported by the UK importer and not an official CZ lens (only sold in the UK and Serial No's never sent to Jena. So CZ Germany have no records of them).

Test here

I prefer the Russian lenses.