Like others have said, with a 28-105mm lens you'd have no wide angle at all. Especially since you are just getting into photography, you would be doing yourself a disservice to be without wide angle capability. It is really hard to beat the choice you have made, and that includes quality.
The DFA 28-105mm DC WR lens was designed for a full-frame (FF) sensor camera body, namely the K-1. On it, there is plenty of wide angle. Your DA 18-135mm lens on the K-1 would be nearly unusable due to its being designed for a smaller sensor, APS-C, so its smaller-cut elements would cause vignetting (a darkening the image corners) when used with the larger sensor, except Pentax is again unique in that their K-1 can be switched to APS-C format, using only that portion of its larger sensor, to accommodate APS-C lenses, but with resolution reduced to about the same as 16mp APS-C sensors. Pentax did a fine job with their FF-oriented DFA 28-105mm lens, which even on a FF body does not match the zoom range of your DA 18-135mm lens on your APS-C type body, and especially since having a very fine 24mp sensor! A DFA zoom lens on a FF body would have to be one of 28-200mm to (almost) match the image size you can get with your lens and camera. Your lens zoomed to 70mm provides an image size about the same as 105mm on a FF body. But your lens can deliver a wider aperture of f/4.5 at 70mm, which the DFA lens cannot do at 105mm. Wide open at 105mm it would be at f/5.6 instead. As you gain experience, you will come to see how this can be advantageous for you in certain important applications.
If you want to visually see how very capable your lens really is, there are only a few test comparisons to confirm what you have already seen posted on this forum. But don't pay attention to test chart shots- unless your hobby is photographing test charts. And many reviews are simply the results of machine evaluations like DXO, with which there are too many variables, with different results with different sensors, etc. to reflect real-world use. I believe more in looking at comparative images. Just google the name of your lens: Pentax DA 18-135mm DC WR review, and in the listings will appear a test review by Imaging Resource. Pay no attention even to their opinions. You have two eyes of your own and presumably good vision. Go directly to the still life scene- test shots of real world objects! Offered are wide-open aperture at different focal lengths, where the lens will be at its greatest disadvantage (least performance), and again at mid-aperture f/8 where it will be at its best. Click on a category. Then click on the center of the image, which will be the edge of the basket. You will get a huge blowup, bigger than you would view in normal viewing of an image. You can move around the image from there. Check the detail and sharpness of the figures on the Hellas and Fiddler's Elbow bottles, among others, as well as the writing on the Samuel Smith bottle. This covers a broad central image area. To check edge performance, go over to the far brush and circular numerical scale.
I'm sure you will find that comparisons with other fine lenses proves your lens is doing very well, indeed. One very highly-rated zoom lens by Pentax is their HD DA 20-40mm f/2.8-4 DC WR Limited, which costs far more than your lens, with a great build quality but a much reduced zoom range. You can google it and bring up the same test. Of course your lens can't do f/2.8 at any FL, and its edge performance is not as good, but otherwise, it gives a very good accounting of itself in the same range, especially if you are not shooting wide open. That combined with all it can do, and its very good build for the cost, makes it very special.
Last edited by mikesbike; 01-08-2019 at 01:53 PM.