If you have a Pentax camera with a smaller-size APS-C sensor and K mount, then the 18-250mm F3.5-6.3 DC Macro HSM from Sigma is an incredibly versatile lens that is a real jack-of-all-trades lens you can carry in any situation. With a focal length spanning from a wide-angle 18mm up to a 250 mm telephoto zoom, with macro capabilities to boot, Sigma's 18-250mm could easily be your camera's first all-in-one lens, or a great replacement for several smaller lenses that you're looking to consolidate.
Sigma redesigned the optical and structural design of this lens in order to achieve a relatively compact size, at 4.0" long, as well as a close minimum focusing distance of 13.8" for macro photography. Their Thermally Stable Composite (TSC) construction maintains the structure of the lens, with little variation despite changing temperatures. The lens also incorporates a brass-made bayonet mount, to further reinforce the lenses durability and resistance to wear and tear over time.
The minimum focusing distance is the same throughout the focal length of the entire zoom range, and provides a maximum magnification ratio of 1:2.9. As an added benefit over fixed focal length macro lenses, the size and framing of subjects can quickly and easily be adjusted by rotating the zoom ring. For even further convenience, at each focusing distance on the side of the lens barrel, the maximum magnification ratio is also displayed.
For high image quality, you know you've got to have high quality glass, and Sigma has incorporated their Special Low Dispersion (SLD) glass element into the 18-250mm. In addition to the SLD element, three aspherical lenses, including a double-sided aspherical lens, have been used in the lens' construction--offering excellent correction of color aberration, for better clarity and quality of images throughout the zoom range.
A Super Multi-Layer Coating has also been employed to reduce flare and ghosting with this lens. This makes it resistant to strong incident light, such as backlight behind a subject, providing sharper, high contrast images even shooting into a light source. To prevent internal reflections from occurring, the included petal-type lens hood can be attached in order to block out extraneous light.
For a lens that offers a wide-angle to telephoto focal length, and the ability to zoom-in with macro photography, Sigma's 18-250mm F3.5-6.3 DC Macro HSM made for Pentax cameras is more than a match. Whether you're on a macro product shoot, a walk in the park, or have a wedding to go to, this lens will help you shoot high quality images in any situation.