This lens surfaced as a rumor only a couple of days ago.. Well, It just got real!
My personal opinion is that this lens will be a real winner if it beats the 24-105L as much as the Sigma 35 beats the 35L.
A few words from Sigma:
This new zoom fulfills Sigma’s uncompromising pursuit of high resolution.
The image sensors of digital SLR cameras continue to offer higher and higher resolution, and photographers are looking for high-performance lenses that leverage these sensors’ full power. In the case of zoom lenses as well, which are a dependable go-to in a wide variety of photographic conditions, photographers want convenience combined with exceptional optical performance. The new SIGMA 24-105mm F4 DG OS HSM zoom lens more than fulfills these requirements, offering exceptionally stable high resolution from wide-angle to medium telephoto photography.
While featuring a large-aperture 82mm filter size, this lens minimizes optical aberrations and avoids reduced peripheral brightness, an issue that tends to affect lenses with similar specifications. Each lens is checked with Sigma’s proprietary A1 MTF (modulation transfer function) measuring system to ensure it offers exceptional quality for the high-megapixel era. Try this new high-performance zoom for yourself, and experience Sigma’s uncompromising pursuit of high resolution.
Source: http://www.canonrumors.com …-sigma-24-105mm-f4-dg-os/