Lens announcements are not normally a reason to get excited, but this model from Sigma should change that.
Sigma is a third-party lens manufacturer who makes glass for Canon, Nikon and Sony cameras, just to name a few. The company has just announced an 18-35mm lens with a constant aperture of f/1.8. This means that, regardless of which focal length you choose, you are still able to access the maximum aperture of f/1.8.
A fast maximum aperture of f/1.8 allows photographers to gather more light, which is particularly useful for low-light situations, as well as obtain a shallow depth of field in photos. The lens is designed for APS-C or crop-sensor SLRs, and will have 9 diaphragm blades to deliver smooth bokeh.
What sets this Sigma lens apart from others on the market is that there hasn't yet been a first-party (that is Canon, Nikon and so on) zoom lens with the same constant aperture. Previously, to reach f/1.8, photographers have had to switch to a prime lens. The fastest first-party zoom lenses generally have a maximum aperture of f/2.8.
The lens weighs approximately 810 grams and has a 72mm filter thread. It zooms and focuses internally, which means no changes to the dimensions of the lens. Photographers who shoot in unconventional conditions are catered for with a special Thermally Stable Composite (TSC) material, which helps it to withstand extreme temperatures. Sigma hasn't yet announced pricing or availability, but given the specs, it will likely cost a pretty penny.