The Canon EF 40mm f/2.8 STM Lens is a prime (single focal point) lens made for the Canon EOS SLR cameras. It's interesting that Canon have decided to release this lens, because it already had three other prime lenses in the 50mm focal length range. But unlike the 50mm lenses, the Canon 40mm lens is actually a "pancake" lens (very thin compare to other prime and zoom lenses). No other Canon lenses could come close to its thinness. So if you are looking for the smallest lens for your Canon EOS camera, look no further than the Canon 40mm lens.
The Canon EF 40mm f/2.8 STM lens is a thin lens. It's so thin that it's barely holdable in our hands. It feels more like an 1.4x teleconverter than an actual lens. From the front, the lens element is so small and surrounded by so much plastic that it looks almost like a web cam. But it's sturdy, heavy, metal build reminds you that it is a real lens. A metal EF mount surrounds the rear lens element.
Because the lens element is so small on this lens, you wouldn't want to use it as a loupe. That task still belong to the Canon EF 50mm f/1.8 II lens.
The AF/MF switch has a "click" feel to it. It provides excellent tactile feedback and it's fairly easy to push.
The focus ring is made out of heavy-weight plastic. It motion is smooth and fluid. With the lens so thin, the only place for the manual focus ring is at the front of the lens. There is no space for a window to show focus distance and depth-of-field (DOF).
Instead of going with the established and highly acclaimed UltraSonic Motor (USM), Canon decided to use its new STepping Motor (STM) technology in this lens. The STM technology is to complement Canon's new digital cameras that allows auto focusing during video shooting. While focusing during video recording, the STM provides smoother auto-focus than USM and quieter operation than DC motor.
Whereas the USM is a combination of mechnical and electrical mechanism, the STM is practically an electrical system. With USM, the manual focus ring is always mechanically engaged to the focus mechanism. That means you and manually adjust focus after AF is achieved without changing the mode switch.
On the other hand, with STM, you are either in AF or MF mode. Rotating the focus ring in AF mode does absolutely nothing. Switching to MF mode does not engage any mechanical connection between the focus ring and the focusing motor. In MF mode, the focus ring controls the motor "by-wire", similar to the concept of "drive-by-wire" in modern cars. This means when you rotate the focus ring, the camera computer senses your adjustment, then adjusts the focus motor to match. But even with this mechanical disconnect, manual focus feels precise and responsive.
The STM is almost as quiet and fast as USM, but not quite. You can hear motor noise similar to the old Canon DC motor, but much quieter; almost at the noise level of USM. Out if the box, the 40mm pancake lens makes a really high pitch whining noise. But this noise disappeared after a few days of use.
The Canon ES-52 lens hood is the standard lens hood for the Canon EF 40mm f/2.8 STM lens. It is small and thin to match the pancake lens. Almost seems like it's a filter without glass.
When I first read the Canon ES-52 lens hood model number on the Canon 40mm lens box, I thought it shared the lens hood from the Canon EF 50mm f/1.8 II lens. Both lenses have 52mm filter thread and uses a screw on lens hood. But when I took my Canon EF 50mm f/1.8 lens out, I found that the Canon 50mm lens actually uses the Canon ES-62 lens hood. But I decided to try this lens hood on the Canon 40mm lens anyway.
The Canon ES-62 lens hood has an adapter that screws onto the front of the Canon 40mm lens. After screwing the adapter onto the lens, the lens hood can be clipped onto the adapter.
The Canon ES-62 is rather huge compared to the Canon 40mm pancake lens. It doubles the size of the Canon 40mm lens and more. But it provides a very secure feeling and seems to do a good job blocking flair. It doesn't cause any vignette either.
Therefore, the Canon ES-62 is well-suited for the Canon 40mm lens. If you feel the Canon ES-52 is too small for your liking, you can try the Canon ES-62 lens hood.