The Canon 40mm f/2.8 STM is not much larger than a lens cap but it offers unbeatable optics. The less weighs around 4.6 ounces and is less than an inch deep. The build is of superior quality, coming in with a metal lens mount that will outlast any plastic mount. It is optimized for full-frame digital SLRs and it works with every EOS camera ever made.
Why go for a 40mm focal length? This figure is quite the novelty, as most prime lenses are 35mm, 50mm, 85mm, or 100mm. However, a 35mm lens on a full-frame camera might be too wide. Additionally, the 50mm might provide a little too long results. And that’s when the necessity for a 40mm comes in – the middle ground length that many are looking for.
It’s one of the first lenses to be equipped with an STM focusing motor – a faster and less noisy alternative to the conventional micro motors. The motor can continuously autofocus during both video recording or live-view photography.
Another thing worth mentioning in terms of focusing is that the minimum focusing distance of the Canon 40mm lens is less than 12 inches. This means that photographers can get very close to their subjects and still keep a sharp focus.