Canon EF-S 24mm f/2.8 STM - Review / Test Report
Lens Reviews - Canon EOS (APS-C)
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Back in 2012, Canon introduced the Canon EF 40mm f/2.8 STM - a full format pancake lens. Pancake lenses have a short barrel in an attempt to minimize the size of the overall setup. Because of their size, pancake lenses tend to have a rather moderate speed and, for obvious reasons, they are also limited to a rather shallow range of focal lengths. Now it seems as if the concept was successful enough to release another one - the Canon EF-S 24mm f/2.8 STM. As you may notice, this is not a full format lens but designed for APS-C DSLRs only. In this scope it actually resembles its full format counterpart because the field-of-view is equivalent to a "38mm" full format lens.

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-S 24mm 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. In a silent environment there's a slightly 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 STM-optimized EOS cameras but for older cameras this is a bit of a bold statement - the progress from USM is rather marginal in this case. Just to put this into perspective, most mirrorless systems do a better job at contrast AF (thus "LiveView").

Equiv. focal length"38mm" (full format equivalent)
Equiv. aperturef/4.5 (full format equivalent, in terms of depth-of-field)
Optical construction6 elements in 5 groups inc. 1x aspherical element
Number of aperture blades7 (circular)
min. focus distance0.16m (max. magnification: ~1.3.7)
Dimensions 68.2x22.8mm
Filter size52mm (non-rotating)
Hoodbarrel shaped, optional
Other featuresSTM motor