The Canon EF40mm f/2.8 STM single-focal-length lens, measuring a mere 22.8 mm in depth and weighing only approximately 130 grams, is the thinnest, most lightweight lens in the EF series—an achievement made possible by the incorporation of an STM.
Employing special optical lens elements such as a glass mold aspherical lens, the pancake lens corrects for various aberrations while delivering a f/2.8 maximum aperture and high-resolution, high-contrast performance across the entire imaging area. In addition, optimized lens positioning and coatings greatly minimize the occurrence of flare and ghosting.

By incorporating an STM in the AF drive, the EF40mm f/2.8 STM makes possible even smoother, more silent AF operation compared with lenses that employ a DC motor, such as the EF-S18-55mm f/3.5-5.6 IS II (released in March of 2011). The new lens offers a minimum focusing distance of 0.3 m and includes such features as a full-time manual focus function.