Review by Klaus Schroiff, published September 2012

Introduction

The Canon EF 40mm f/2.8 STM is a so-called pancake lens - the meaning of the designation is easily obvious from its size- and weight specs. In fact it is the first of its kind (by Canon) ever since the EOS system was introduced back in 1987. In the global scope Canon is a late follower though. Such lenses are immensely popular in the mirrorless camera segment but they are also a center product in the Pentax K lineup. So small is beautiful these days ... and now also in Canon land.

The EF 40mm f/2.8 STM has another feature which is new to the Canon EOS system - rather than taking advantage of a conventional micro-motor or USM AF, Canon has introduced a stepping AF motor (STM). It is meant to provide a smoother auto-focusing experience in movie or Live-View mode. However, the camera has to be optimized to take advantage of this.

There have been discussions in the usual communities whether a 40mm f/2.8 is really all that attractive. This is also an issue on APS-C format cameras where the lens has a field-of-view equivalent to a 64mm thus it's somewhere in between of a standard and moderate tele lens here. However, it'll surely find many fans e.g. among the street photographers and its coolness factor alone will certainly also make it a success in the market anyway.

Pancake lenses are easy beings in terms of build quality - they are about as simple as it can get so it is not surprising that the Canon lens has no issues here. It has a tightly assembled plastic body based on a metal mount. The focus ring operates smoothly. An inner lens tube which extends a little when focusing towards shorter distances. The focus ring is decoupled from the actual focus gear so you cannot retract the inner tube after detaching the lens from the camera.

As already mentioned, the Canon EF 40mm f/2.8 STM uses a stepping motor for AF. AF operations are fairly fast in phase detection AF mode but, frankly, not quite as speedy as a ring-type USM AF - at least based on the EOS 5D II. In a silent environment there's also a noticeable amount of AF noise although it remains unobtrusive.
Manual focusing works "by wire" so you are actually controlling the AF motor when turning the focus ring. This works pretty well actually. Canon advertises the STM as an innovation for movie taking. This may be true on the new EOS 650D which has been optimized for STM lenses but for older EOS cameras this is a bit of a bold statement - the progress from USM is rather marginal here. Just to put this into perspective, most mirrorless systems do a better job at contrast AF (thus "LiveView").