Originally posted by CreationBear The reason I ask is that I've been trying to educate myself on older Pentax lenses (I'm specifically looking for a mid-focal length, manual focusing lens) and my impression was that the K-series was a much warmer (almost Canon-like) lens, but with the M-series, you start to get the cooler tones that are closer to the modern DA Limited lines I'm familiar with.
It has been my opinion that the K lenses are somewhat warmer than the M and forward lenses, particularly the 50/1.4. In addition to coatings updates, the optical formula for the A50/1.4 was reportedly tweaked to be a bit sharper f/2 and above. I've always thought the M50/1.4 was an especially cool rendering lens.
There are so many cult classics in the K series, transitioning as they did from the metal and large element Takumar lenses to bayonet, but with more glass than the M's generally. Old guys like me love them. There were also some true Milestone lenses - K28/2, K85/1.8, K200/2.5 in the range.
K28/3.5 and K35/3.5 are extremely sharp and best or second best of their FL Pentax has ever made. K24/3.5 is the old Tak formula with a bayonet stuck on the rear. The K24/2.8 has more contrast and is a bit sharper but gives up some magic in the bargain.
In the mid-range you want to look for the 85, 105 120 and 135/2.5. The 85/1.8 is pricey (though I missed one here last month for 60% of market price by 5 minutes).
Any of them, well maintained, is beautiful and sublime to the hand - a real pleasure to hold and focus if you enjoy the experience of photographing over and above capturing images.