I have experience with only a very small percentage of Pentax's current lenses, so I cannot even begin to offer my views on particular lenses. However, what I can do, is sort. I took all the lenses on Pentax's hardware roadmap and put them into a spreadsheet and sorted them, to try to understand what's going on. I did this for me, but I'm sharing it with you. Note: for simplification, I've ignored the alphabet soup Pentax puts into the names of all these lenses. I've tried to pull out the important bits into the third column, but for the full explanation, this website has an excellent guide. Note #2: Pentax has a "Limited" series of lenses which has nothing to do with their production run and everything to do with their physical size: Limited lenses are very compact, but also come at unusual focal lengths and maximum apertures. So, with no further fanfare, here's the chart.

K-mount Standard Zooms

These lenses are the jack-of-all-trade variety that are the highest sellers for all camera. Pentax seems to offer several upgrade paths for those looking to improve upon the kit lens, with some have a slightly wider focal length range, some featuring constant apertures throughout the zoom range, and some giving full-frame coverage.

Focal Length Aperture Why???
16-50mm f/2.8 Pro-level crop frame choice
16-85mm f/3.5-5.6 More zoomy than the kit lens
17-70mm f/4 Slightly better than the kit lens
18-50mm f/4-5.6 Very compact
18-55mm f/3.5-5.6 Kit lens
20-40mm f/2.8-4 Limited series (physically small)
24-70mm f/2.8 Full-Frame Pro option
24-105mm f/3.5-5.6 Full-Frame budget option

K-mount Super Zooms

For people who want an interchangeable lens camera but don't want to interchange the lens, there exist super-zoom lenses: lenses which combine the focal lengths of a standard zoom with those of a telephoto zoom.

Focal Length Aperture Why???
18-135mm f/3.5-5.6 Like the kit lens but zooms in more!
18-270mm f/3.5-6.3 Like the first one but even more zoomy!

K-mount Telephoto Zooms

For those new to photography, usually the first lens they buy after the one that comes with the camera is a telephoto zoom – it is common to find an entry-level lens or two in this category. But because this range is so heavily used by working professionals for everything from portraits to photojournalism to wildlife to sports, there tends to be an ever-increasing list of selections available, with the sky the limit on quality, weight, and price.

Focal Length Aperture Why???
50-135mm f/2.8 High-end crop-frame option
50-200mm f/4-5.6 Slightly smaller entry level option
55-300mm f/4-5.8 Entry level
55-300mm f/4.5-6.3 Retractable entry level option
60-250mm f/4 Bird lens on a budget?
70-200mm f/2.8 Full Frame pro choice
150-450mm f/4.5-5.6 Full Frame wildlife choice

K-mount Wide Angle Zooms

"Hey," you say to yourself, "I have a standard zoom and a telephoto zoom, why not a wide angle zoom?" Some people do indeed think this way, and so Pentax makes these lenses to sell to them.

Focal Length Aperture Why???
10-17mm f/3.5-4.5 Fisheye distortion
12-24mm f/4
15-30mm f/2.8 Full frame

K-mount Prime Lenses

Ok, finally, the good stuff. Prime lenses are the best, but probably also the most confusing part of Pentax's lineup. They're also the part I'm most interested in.

Focal Length Aperture Why???
14mm f/2.8
15mm f/4 Limited series pancake
21mm f/3.2 Limited series pancake
31mm f/1.8 Full frame limited series
35mm f/2 Full frame
35mm f/2.8 Macro Limited series
35mm f/2.4
40mm f/2.8 Limited series pancake
40mm f/2.8 Super pancake
43mm f/1.9 Full frame limited series
50mm f/1.4 Full frame
50mm f/2.8 Macro full frame
50mm f/1.4 Full frame older version
50mm f/1.8 Relatively compact lens
55mm f/1.4 Uncertain
70mm f/2.4 Limited series pancake
77mm f/1.8 Full frame limited series
100mm f/2.8 Macro full frame
200mm f/2.8 Mostly full frame
300mm f/4 Mostly full frame
560mm f/5.6 Basically a telescope

End

Writing this blog post has helped me understand Pentax's lineup – something I wanted to do before spending any money on it. Hopefully, if you're reading this, this post is helpful to you, too.