The Sigma 18-35mm f/1.8 DC HSM Art Lens for Pentax is a wide-angle to normal-length zoom lens that features a fast, bright constant f/1.8 maximum aperture. It is specifically designed for use with APS-C-sized sensors and provides a 35mm-equivalent focal length range of 27-52.5mm.
The lens integrates four aspherical elements into its construction as well as five Special Low Dispersion (SLD) glass elements to help minimize various aberrations and distortions throughout the zoom range as well as provide greater image sharpness and clarity. A Super Multi-Layer Coating has also been applied to lens elements to reduce surface reflections, lens flare, and ghosting to produce images with higher contrast and color fidelity. The lens barrel design is constructed from a Thermally Stable Composite (TSC) material that is akin to metal in texture and resistance to temperature and environmental changes while also remaining lightweight and compact. An internal focusing and zooming mechanism also helps to maintain a compact and consistent overall length and non-rotating front ring also better enables the use of circular polarizing filters. For control of focus, a Hyper Sonic Motor (HSM) is used to ensure fast, precise, and quiet focusing capabilities that are well suited to continuous shooting and movie recording.
This lens is part of Sigma's Art line; deeming it well-suited to creative image making due to its large, constant f/1.8 maximum aperture and ability to produce aesthetic bokeh by way of a nine-blade circular diaphragm. The range of focal lengths it covers, from wide-angle to normal-length, is nicely situated for everyday and artistic shooting applications.
Four aspherical lens elements, as well as the incorporation of five SLD glass elements, help to minimize the appearance of distortions and aberrations throughout the zoom range and provide clear imagery with enhanced sharpness. A Super Multi-Layer Coating also benefits the image quality by reducing reflections and flare and contributing to greater overall contrast and fidelity.
The Hyper Sonic Motor provides fast, quiet, and smooth auto focusing that is well-suited for use with video applications. This AF motor can also be manually overridden at any time by rotating the focusing ring, giving you the ability to fine-tune image sharpness through manual focusing.