The Sigma 18-200mm and Tamron 18-200mm are older generation lenses.
The Tamron 18-250mm (K-mount) and DA18-250mm are sibbling, manufacturered by Tamron. They are newer generation and
Camera lens tests, user reviews, camera accessory reviews - SLRgear.com! compared the Tamron 18-250mm to the Tamron 18-200m:
"
With the new AF 18-250mm zoom lens that provides enhanced image quality, [.. we] were pleasantly surprised by the Tamron 18-250mm f/3.5-6.3 Di II LD Aspherical IF Macro AF. It maintained good center sharpness across its entire focal length range, softness in the corners was average to better than average, and things flattened out nicely when we stopped it down just a bit."
"
Given its shorter zoom range, you'd expect the original 18-200mm lens to outperform the new 18-250, but that proved not to be the case. The 18-250mm beat its 18-200mm predecessor all hollow when it came to sharpness. Wide open, the 18-250 was much better behaved, and sharpness was markedly better across the board."
{http://www.slrgear.com/reviews/showproduct.php/product/1009/cat/23}
Welcome to Photozone! reviewed positively further the Da18-250mm: "
The Pentax SMC DA 18-250mm f/3.5-6.3 ED AL [IF] is a worthy representative of its species." {http://www.photozone.de/pentax/367-pentax-smc-da-18-250mm-f35-63}.
Overall the DA18-250mm adds a longer focal lenght range and tends to have better IQ than the older lenses. So it is better value IMO.
Hope that the comment will help.
Last edited by hcc; 08-23-2011 at 05:35 PM.