The Canon EF 17-40mm f/4L USM is a durable, weather-sealed, professional L-series ultra-wide-angle lens known for being a relatively affordable option in Canon's lineup, featuring fast, quiet USM autofocus and a constant
aperture. It is highly regarded for landscape and architecture photography on full-frame cameras, though it is often considered a great, sharp option for crop-sensor APS-C cameras as well.