SMC PENTAX-M MACRO 100mm f4
I haven't used one myself, but the Tamron SP90 adaptall gets consistently good reviews and there are plenty on Ebay for around £80-£100, maybe less if you are patient.
That's probably where I would go next unless I can find a Vivitar Series 1 105mm or clone at an affordable price.

Keep an eye out on here, there have been a couple of good value macros sold lately!
LennyBloke
Lennybloke..the f2.8 would be nice

The Tamron 90 and Sigma 105 also fit the bill with good reviews but again as "new" are probably out of budget at the moment.
If I was dipping my toe in the water with quality macro I wouldn't hesitate to buy this lens - my view is that if it doesn't suit you, you can always list it on here again. Just my tuppeneth

LennyBloke
...the Tamron SP90 adaptall gets consistently good reviews ...
the older 90/2.5 lens is a beauty
K-1 K-01 Q-7
some len
Close to the Edge
Down by the River

PENTAX-K5 | SMC PENTAX D FA MACRO 100MM F2.8 WR | SMC PENTAX-DA 18-135MM F3.5-5.6 ED AL [IF] DC WR | SMC PENTAX DA 50-200MM F4-5.6 ED WR | AF160FC AUTO MACRO RING FLASH | 2X KAX TELEPLUS MC TELECONVERTER
My collection :flickr | PPG
the_coach
Member
Any users of the 100mm macro f4 out there with advice one way or the other ?
I am exploring macro options without spending a fortune, would prefer a macro lens to extension tubes. Any advice welcome .