Originally posted by cprobertson1 C) Increased low-light performance - of my shots, 1/3 take place at night, 1/6 macro, and the rest in decent light. That's not including astrophotography, which, due to stacking takes up a huge proportion of my total shot-count! In fact, I am very happy with my K-50 for astro - though I am still a novice in that particular field!
While it is true that when already having a very well-performing camera, that a better one may not necessarily translate to a visually superior result, in answering a query such as yours here, I always take into consideration one's interests in photography. In your case, I definitely recommend saving up to get a good deal on the KP plus DA 18-135mm combo. Keep your K-50 as a secondary backup body.
As to the lens- the DA 16-85mm offers better edge performance particularly as zoomed into the tele range. So if one has a particular interest in more wide angle shots below 18mm, and having better sharpness at the edge of the frame in the tele range above 50mm, this is a very fine lens to consider. However, the special combination of virtues the DA 18-135mm offers should not be ignored-
1. Although not quite as wide at the shortest FL, the additional zoom into the tele range is considerable and very useful for not having to change lenses as often.
2. Its very fast and accurate AF, one of the very best, far better than your kit lens. This is very important.
3. For landscape shots mostly up to 50mm, its edge performance is actually quite good, and really good if stopped down even a little.
4. Its central-area sharpness is rated excellent by test labs through its zoom range, an outstanding result for such a range, and as one gets into more telephoto, the usual thing is the frame's edges become less important anyway. Even test labs are guilty of down-grading a lens for edge performance wide open in the tele range and through a huge zoom range and even worse, downgrading due to corner performance, when every other aspect is exceptional.
5. Speed. While not a pro-style f/2.8 fast zoom lens, or semi at f/2.8-4, it can keep to f/3.5-4.5 up to 70mm or around one stop off the pro-style but with greater zoom range.
6. Size. It is amazingly compact for what it is and is capable of, in keeping with the design concept of the KP, while the DA 16-85mm is substantially larger and heavier.
7. Its build quality is very good indeed, similar to that of the DA 16-85mm, including WR construction like the camera body. To get WR construction with Nikon or Canon, you can expect to shop in the $1,000+ USD category for each.
8. Closeup quality. Good bokeh (the smoothness of out-of-focus background, etc as opposed to a "nervous" characteristic) which is usually found on more expensive lenses. Actually, its less sharpness in telephoto settings at the frame's edges can add to this good effect.
9. Cost. For all that it is, and can do, price is quite reasonable.
Since a "picture is worth a thousand words" I direct you to a recent thread in the SLR lens section of these forums, by "bigyinn", entitled: Lens Choices.
He is mostly interested in wide angle, but also wonders about upgrading from his DA 18-135mm. On page 2, there is post # 20 by Des. A closeup of his dog. I've noticed closeups are one of your interests. No doubt taken well into the telephoto range. For getting huge closeups as in macro, of course a macro lens is needed, but that is another story. This example is a good one as to what this lens can do on a camera body with imaging quality of a KP. He also provides a link for further examples. My own post in this thread is on page one, #11. I might mention, the DA 18-135mm seems to have fewer issues with getting an off-copy than usual. Fewer complaints seen here than usual.
I've been into photography for some 45 years. I now have many lenses, some of top pro-style quality, also a number of camera bodies, and like Des, some lenses overlapping in FL- as in a pro-style Sigma 17-50mm f/2.8, the excellent DA 20-40mm f/2.8-4 Limited, as well as the DA 18-135mm f/3.5-5.6 DC WR, yet if I had to do with just one of these it would be the DA 18-135mm- it is that useful and satisfying.
The KP camera- There is no doubt of its superiority for low light/higher ISO use. I've come through numerous DSLR bodies. The K-5 flagship series was the first real breakthrough in this regard. It has a sensor similar to that in your K-50, performing even a tad better than your otherwise fine camera. I still have and use mine, especially the K-5 IIs. Wonderful controls set. Still very satisfactory imaging, so I understand your satisfaction with your K-50. The KP's image quality with a good lens, as well as its processor for out-of-the camera JPEG images, and its exposure metering, all outstanding.
I sometimes skip over some upgrading of models, while determining the advancement would not be particularly meaningful for my use. I did so with the K-3 series. But the KP turned out to be a whole new ballgame, about as great as the K-5 series over its predecessors. I also do quite a lot of low light work. It is also remarkable in its concept of being lighter and svelte, but with a pro-level build quality. I don't know about your area, but over here in the US the price has come down to what amounts to bargain-level for this type of build and controls set. Its controls set is way above that of your K-50. You can make changes more directly and efficiently without having to go to menus. Bracketing or changing metering types, for example, are more accessible. The pull-out screen can be useful for many situations. Doing work on a tripod where you don't have to bend over, or down low where you don't have to crawl on the ground, or up high over head. There is so much more. Sometimes having a better set of tools, which the KP has aplenty, will in the process of exploring those tools, be inspirational towards learning more in-depth methods to improve your photography. In that sense, the equipment can sometimes make a difference as well as being more convenient. A very powerful camera in a svelte body.
The superior higher ISO performance, while maintaining good detail, essentially makes your lens behave like a faster, more expensive lens on a lesser body. If you, say, use your DA 18-135mm lens set at f/5.6 for optimum results, by increasing ISO while preserving quality, you might actually get better results than a "better" lens on a lesser body set at f/2.8, since the lens you are using has no such aperture. In that case, it is also about shutter speed as well. I bought mine in silver and it is beautiful. Inspires one to use it even more.