The DA 18-135mm has been lab-tested and user- proven to have excellent resolution in its central area throughout its zoom range, which is exceptional. Most zoom lenses will diminish in quality gradually towards the longer end of the zoom range. With this lens, that diminishment takes place at the edges of the frame, starting around 55mm or so. usually, however, the more tele being used, the less important the edges become as a general rule anyway. I have that lens, and am happy with its performance and versatility as a general-purpose lens, and when I need more zoom range without having to change lenses often. The 16-85mm lens represents an improvement in edge performance.
There is a good reason many of more modern lenses tend to have a shorter focus throw- not because of being for digital, but because AF can be achieved faster. The K-5 is a fine camera, but its AF performance is not one of its high points. When I got my K-5 IIs, I noticed an improvement in AF with some of my lenses. My KP's AF is better yet. The old film-era lenses you list that you are most fond of are MF lenses, and are specifically designed for MF, hence the greater focus throw.
If you should develop an interest in telephoto, do not get the newer Pentax DA 55-300mm f/4.5-6.3 PLM lens (which is not usable on a K-5 anyway)- it will have a very short focus throw. Get the older DA HD 55-300mm f/4-5.6 instead, as it has a quite long focus throw, which is better for fine-tuning by using MF. Its optics are very good and comparable to the newer lens.
As to image quality, the DA 17-70mm was rated pretty high by lab tests, though I'm sure there are others still higher. The sensors of the K-3, K-70, and KP will all have yet more resolution capability than your K-5. The K-5 IIs will provide more detail because, like the affore-mentioned current models, it has omitted the AA filter. Even though it does not have much zoom range, the DA 20-40mm Limited is indeed a fine lens. I love it on my KP, which is capable of showing what it can do. I can recommend it for a very high-quality kit when teamed with the DA* 50-135mm f/2.8 lens- another great lens, and it does have plenty of focus throw as well as a MF/AF switch right by your thumb where you hold it. It also does not change its size as you focus or zoom it. Very compact for what it is. To get the last bit of quality, and for low-light faster-aperture uses, it is usually necessary to break up the zoom range into smaller segments. You just have to accept giving up some versatility. As to wide angle- only you can decide your needs. You can operate your DA 17-70mm lens only from 20-40mm to get an idea. The reason for the 20-40mm Limited not being 16-40mm instead- it was designed with the goal of offering a very compact, higher quality lens, especially for compact camera bodies- specifically the KP. For wide angle, I also have a Pentax DA 12-24mm f/4 which has a very high rating for resolution. I am very happy with it. 3.5 stars is a very good rating from photozone. Very few get 4 or above. But you can't always go by just that. They will sometimes downgrade a lens for things that are not that relevant- they sometimes exaggerate the importance of certain factors.
I often have my camera in a holster-style belt/shoulder-strap case. With the KP, it is most often the DA 20-40mm Limited on the camera, and the little DA 15mm LTD is so small it can fit into the accessory front pocket of the case. Then, in a lens belt case on the other side, resides my DA* 50-135mm f/2.8, or in a jacket pocket I have my 77mm LTD in its own case. This makes for an easy-carrying top-quality kit of pretty good range.
Otherwise, for a single lens having a wider angle, and a good zoom range along with very good performance, the DA 16-85mm is highly-rated if you get a good copy.
Last edited by mikesbike; 01-09-2018 at 09:52 PM.