I have the Tamron 18-250, which is the same as the Pentax DA 18-250. The Tamron-badged version is generally a little cheaper. As Photozone said (tested in a Canon mount), the image quality from this lens is "fairly amazing" given its range:
Tamron AF 18-250mm f/3.5-6.3 Di II LD Aspherical [IF] macro (Canon) - Review / Test Report It was regarded as a big leap from earlier superzooms. If you really want an affordable one-lens solution, the 18-250 is a good choice.
But (and there's always a but) all the superzooms tend to be slow (ie maximum aperture at any given focal length is not very wide), have a lot of distortion, are prone to vignetting at wide focal lengths and be rather soft in the corners unless stopped down. Some are also prone to chromatic aberration (CA), although the 18-250 is not bad in that regard.
Certainly some of these issues are software-correctable. You mention the advantage of the Pentax-badged version for in-camera distortion correction. That's true, but it will only correct jpgs, not RAW images. Nor will it achieve nearly as much improvement as you could get shooting RAW and post-processing. PP can really do wonders with images from the 18-250.
In particular, the slow maximum aperture will often drive you to use a higher ISO, which means more noise. There is nothing like good PP software (e.g. Topaz or DxO Optics Pro) for correcting noise. I speak from experience. Here is a shot taken with the Tamron (at sunset, from a moving boat) at 3200 ISO after processing with DxO OP:
I doubt the 18-270 would offer better IQ than the 18-250:
http://www.photozone.de/Reviews/412-tamron_18270_3563vc_canon The main advantage is that it has quieter AF.
You mention that you already have the 55-300. (Is that right or do you mean the 50-200 that comes in a two-lens kit with the 18-55?) If you already have the 55-300, if what you really want is more reach, and are not absolutely set on an all-in-one lens, you already have the answer. The 55-300 is faster and the IQ at the long end is significantly better than the 18-250. Also because it's not an internal-focus lens, you get the stated focal length at any distance from the subject. In other words, 300mm means 300mm. With the 18-250 or any other internal focus lens, the field of view only represents that for the stated focal length at infinity. For example, if your subject is 5 metres away (15 feet), shooting at 250mm the field of view might only be equivalent to 180mm.
If you don't already have the 55-300, consider the DA-L version: it is optically identical to the others and is great value at around $US150. If you want a really cheap alternative, try the Sigma 70-300 for about $100.
As for the wide end, that is harder. Yes there's a big difference between 16 and 18mm. Tamron now have a 16-300mm superzoom, but it isn't available in K-mount. If you really want the extra width your best cheap wide option might be the Pentax DA 16-45 f4. Better IQ than the kit zoom. Otherwise one of the good ultrawide zooms, like the Sigma 8-16 or 10-20 (two versions), Tamron 10-24, or Pentax 12-24, or one of the 10, 14 or 15mm primes. But none of these is really cheap - and none is WR, if that really matters to you. For WR options wider than 18mm, you would need to go to the DA*16-50 or the new DA 16-85, but both are quite expensive.
Last edited by Des; 05-23-2015 at 12:12 AM.