Originally posted by wangao0316 wow, these are beautiful. how's tammy 18-200 compared to 18-250?
I've only used the 18-250. But the reviews suggest it is much better than the Tamron 18-200:
Tamron 18-200mm F3.5-6.3 AF XR Di II LD Aspherical IF Macro Lens Reviews - Tamron Lenses - Pentax Lens Review Database Tamron AF 18-250mm F/3.5-6.3 Di II LD Lens Reviews - Tamron Lenses - Pentax Lens Review Database
Photozone described the IQ of the 18-250 as "fairly amazing" for a lens of this type:
Tamron AF 18-250mm f/3.5-6.3 Di II LD Aspherical [IF] macro (Canon) - Review / Test Report (tested on a Canon body - so without image stabilization)
A number of reviews complain of "zoom creep" - that is, the lens slides out when you point it down. Yes it happens, but it's a non-issue in my view.
The Pentax DA 18-250 is the same lens as the Tamron 18-250 with a different badge (and higher price). The only advantage of the Pentax-badged version is that your K-50 can do in-camera distortion correction of jpgs. (It doesn't work with any third-party badged lenses.) But shooting RAW and correcting in PP is a much better option.
The later Pentax 18-270 (also based on a Tamron, although the Tamron itself was not released in Pentax K-mount) is said to be optically similar but adds HSM autofocus (ie driven by a motor in the lens and therefore quieter).
Tamron has since released a 16-300, but it is not available in K-mount (and is not likely to be).
Consider the Sigma alternatives too (18-200, 18-250, 18-300). The most recent C (Contemporary) series are said to be the best of them.
In head to head tests, good superzooms do better than the kit lenses.
All superzooms have similar disadvantages. They don't have the same level of sharpness as a good shorter-range zoom (especially in the corners), they have relatively high levels of barrel distortion at the wide end and pincushion at the longer end (both easily corrected in post-processing), and they are slow (ie maximum aperture at any given focal length is not wide compared with other zooms). Some are also prone to chromatic aberrations (e.g. purple fringing on high-contrast subjects) and vignetting (darker corners) - good to avoid although can often be fixed in PP.
Also, like almost all zooms, their best resolution comes from stopping down a stop or two (ie using a narrower aperture) - if you can shoot at f6.7 at the wide end and f8 at the long end you can get very good results, especially in the centre of the image. That means these lenses work best in good light (natural or flash). If the majority of your shooting is indoors, get something faster.
As a starting-out lens, a good used superzoom is a great buy. Bang for buck is great - even if (as you should) you buy some good post-processing software to go with it. Leaves you with some coin in the pocket for the Limited prime, or macro lens, or ultrawide, or ultra-tele, or portrait lens, or whatever, that you will come to crave.