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.
Possibilities as Wide as its Zoom Range
Going from an 18mm wide-angle lens in the park, to an 85mm portrait lens at a wedding, and later attaching a heavy 250mm lens for wildlife or sport photography means you'll have to carry pounds of gear around with you, by hand or on your back, all day long. With the 18-250mm F3.5-6.3 DC Macro HSM, you won't have to risk dust entering your Pentax camera body, dropping one of your lenses as the bride's aunt runs into you with a glass of wine, or fumbling to switch to your zoom lens as you watch an eagle soar away--missing opportunities because you had the wrong lens at the time
In Close, or Wide Open
Not only is the zoom range excellent for a variety of situations, but the focus range is also conducive to macro photography--with the added benefit of zooming in on the tiniest textures or creatures. With a maximum magnification ratio of 1:2.9 you can get an up-close and personal shot of the petals of a flower, then immediately zoom out and catch a group photo before anyone blinks
Incorporating double-sided aspherical lenses, and redesigning the lenses construction, Sigma has made improvements in keeping this lens at a relatively compact size, yet still achieving an outstanding zoom range. The filter thread diameter has been reduced to 62mm--while, with a 13x zoom range, the length of the lens still remains a mere 3.4" long
Tough Exterior, yet Quality Interior
While the design and construction of the lens has kept it relatively compact, the new Thermally Stable Composite (TSC) material has been used to keep it strong and resistant to weathering effects. In addition, a brass made bayonet mount reinforces the strength applied to the surface of the lens. The combination of these features leads to reduced contractility with changing temperatures, a higher quality of lens construction, and allows for slimmer parts to be used--keeping it compact and strong
Flare, Aberrations, and Ghosting Reduction
Everybody says you should invest in your glass, and Sigma's 18-250mm provides just the way for you to do that. Three aspherical glass elements offer correction for distortion, four Special Low Dispersion (SLD) glass elements provide optimum color correction and sharpness, and a Super Multi-Layer lens coating reduces flare and ghosting. This array of optical features assure high image quality throughout the entire zoom range, and widely varying lighting situations.