Pentax DA 16-85mm f/3.5-5.6 Lens
This is my standard zoom lens, which gives me a nice range from wide angle to short telephoto, weather-sealing, and excellent image quality. It's not the widest aperture, but that means that the lens is relatively compact and lightweight, which I usually benefit more from than I would from an f/2.8 lens.
Pentax FA 50mm f/1.4 Lens
This is a beautiful lens for portraits, landscapes and detail shots, especially when utilising the shallow depth of field that it offers, but even at smaller apertures, it's a lovely, sharp optic.
A 50mm lens is a great first step into 'prime' (non-zoom) lenses, as they're generally very good value, with wide apertures that allow a lot of flexibility with regard to depth of field.
Pentax DFA 100mm f/2.8 WR Macro Lens
After several years using an old Sigma 105mm macro lens, I've changed to this Pentax one, as it offers weather sealing, and better resistance to flare (see blog post here
). It's also more compact, and lighter in weight, but when asked what macro lens people should buy, my reply generally is that it's very hard to buy a bad macro lens, and any model from any of the main brands, including Sigma and Tamron, will serve you well.
All true macro lenses will give you 1:1 or lifesize magnification, but the different focal lengths will dictate how close you need to be to your subject and what field of view you get. Something around 90-105mm is often the first macro lens people end up with.
Pentax DA*300mm f/4 Lens
I used to have a Tamron 300mm f/2.8 lens, but I found it too heavy to carry around all the time, so when Pentax brought this lens out, I had to have it. It's so sharp, and focuses quite closely, so I use it for all sorts of shots, from birds and other wildlife, to fungi and flowers.
This lens is also weather-sealed, so I can happily plod about in a downpour and get pics I'd otherwise miss.