Design

The 40mm may be lightweight and super-slim, but it doesn't feel at all cheap. The mount is metal and the barrel moulded from rigid plastic; the focus ring rotates with a smooth, well-damped action. The overall impression is of a well-made, precision product. This is all in stark contrast to the EF 50mm f/1.8 II, a good lens but one whose construction quality reflects its bargain basement price.

In terms of design, the 40mm is resolutely conventional. It has just two controls, the slim focus ring and the focus mode switch, both of which are exactly where you'd expect them. The lens focuses by moving the entire optical unit back and forwards as a unit. Note though that the slimline pancake design doesn't find space for a bayonet mount to fit a lens hood; instead Canon offers the slimline screw-in ES-52 metal hood, or you can use any other with a 52mm thread.

Compared to Canon EF 50mm F1.8 II

This comparison gives an idea of the relative sizes of the 40mm f/2.8 STM and the EF 50mm f/1.8 II - previously the smallest and lightest in Canon's lineup. This emphasizes just how slender the 40mm really is: it's little more than half the size of the 50mm. The two lenses weigh in the same, though, despite the extra glass needed for 50mm's brighter maximum aperture; this is offset by its lightweight all-plastic barrel construction.

The table below compares the lenses in more detail, along with the more expensive EF 35mm f/2.0 (which is very similar in size to the 50mm). The 50mm's lower cost is reflected in its cheap construction and unrefined operation, while the penalty for the 40mm's compact size is its relatively slow maximum aperture. But even this is slightly offset by the 5-bladed diaphragm used by the other two lenses, which means that out-of-focus highlights are rendered as pentagons when the lens is stopped down.

 
EF 40mm f/2.8 STM
EF 50mm f/1.8 II
EF 35mm f/2.0
 Focal length  • 40mm  • 50mm  • 35mm
 Maximum aperture  • f/2.8  • f/1.8  • f/2.0
 Minimum aperture  • f/22  • f/22  • f/22
 Focus motor type  • Linear stepper motor  • Micromotor  • Micromotor
 Full time manual focus  • Yes  • No  • No
 Construction  • Plastic barrel, metal mount  • Plastic barrel, plastic mount  • Plastic barrel, metal mount
 Aperture diaphragm  • 7 blades, rounded  • 5 blades  • 5 blades
 Minimum focus  • 0.30m (11.8")  • 0.45m (17.7")  • 0.25m (9.8")
 Maximum magnification  • 0.18x  • 0.15x  • 0.23x
 Dimensions  • 68mm x 22mm
   (2.7" x 0.9")
 • 68mm x 41mm
   (2.7" x 1.6")
 • 67mm x 43mm
   (2.7" x 1.7")
 Weight  • 130g (4.6 oz)  • 130g (4.6 oz)  • 210g (7.4 oz)

On the camera

The 40mm really is a tiny lens, and this is emphasized on larger bodies like the EOS 5D Mark III shown left. Its size makes it an excellent complement to a general purpose zoom such as the EF 24-105mm f/4L IS USM or, at the other end of the scale, the EF-S 18-55mm f/3.5-5.6 IS II - you can carry it around all day and barely notice. On the miniature EOS 100D / Rebel SL1 it makes for a remarkably compact combination - so much so that Canon are bundling them together as a package in some regions.

The EF 40mm f/2.8 STM can also be used on the mirrorless EOS M model via the mount adapter EF-EOS M. Autofocus and electronic aperture setting are retained, and the combination isn't overly bulky.

But while the 40mm is one of the few EF lenses that can focus smoothly on the EOS M during movie recording, its autofocus is rather slow for stills shooting.

Autofocus

The 40mm uses Canon's Stepper Motor (STM) for autofocus. For normal eye-level shooting we've found it to perform reasonably well; focusing isn't especially fast, but it's positive and very quiet. As such it performs somewhere between Canon's excellent ring-type USM lenses and the slower, noisier micro-USM or micromotor designs. We've used it on bodies ranging from the entry-level EOS 100D up to the high-end EOS 5D Mark III, and saw no problems with focus accuracy or consistency. As always, though, it must be noted that focus speed and accuracy are dependent upon a number of variables, including the camera body used, subject contrast, and light levels.

Switch the camera to Live View, though, and the lens slows down considerably. In fact it's disappointingly sluggish with every camera we've tried so far, suggesting the problem lies at least as much in Canon's contrast-detect AF algorithms as anything else.

The 40mm is, however, one of the few Canon lenses that works acceptably if you want to refocus during movie recording. It's not quick enough to track a moving subject, by any stretch of the imagination, but for focus 'pulls' between subjects at different distances from the camera, it works OK. The best results in this respect come on Canon's Hybrid AF and touchscreen-equipped cameras - see the following page for a couple of example videos. The lens's lack of IS means that you'll need to use a tripod (or some other solid support) to get acceptable footage, though.

'Focus by wire' manual focus

The STM motor comes with a feature that's relatively unfamiliar to Canon SLR users at this price point: electronically-driven, 'focus-by-wire' manual focus. This means that the focus ring isn't mechanically-coupled to the lens, but instead instructs the camera to drive the lens's focus motor. This arrangement is almost ubiquitous on mirrorless system cameras, and also used by a few of Canon's top-end lenses such as the EF 85mm f/1.2 L II USM. It's also implemented on both of Canon's latest kit zooms, the EF-S 18-55mm f/3.5-5.6 IS STM and EF-S 18-135mm f/3.5-5.6 IS STM.

Once upon a time focus-by-wire systems had a reputation for poor precision and 'feel' that wasn't entirely undeserved, but times have changed. Canon's version on the 40mm STM is really rather good, and enables precise manual focus in a way the comparatively 'loose' design of the 50mm f/1.8 II struggles to match. The overall user experience is in fact really rather good, although one quirk is that manual focus won't work unless the camera's metering has been activated by a half-press of the shutter button. Note also that you can't change the focus without attaching the lens to a powered-on camera, so if you remove the lens when it's set to a focus close distance with the barrel extended, it's stuck there.

Lens body elements

The 40mm uses Canon's all-electronic EF mount, and can be used on both full-frame and APS-C cameras. All communication between the lens and camera is via the gold-plated contacts.
The filter thread is 52mm, and it doesn't rotate on focusing. This means filters such as polarisers and neutral density gradients are much easier to use.

Note that there's no bayonet mount for a lens hood (not an unusual omission from pancake lenses). But you can use a 52mm screw-in hood if you want to protect the front element and shield the lens from glare.
The focus ring has a 4mm wide, ribbed hard-plastic grip. It rotates very smoothly, but without any end stops.
Canon has included a standard focus mode switch towards the top of the barrel.

The 40mm features full-time manual focus, meaning you can adjust focus manually when set to AF. It doesn't work in quite same way as on USM lenses: you have to keep the camera's shutter button half-pressed for the focus ring to be active.
The lens focuses by moving the entire optical unit back and forward within the barrel, extending by 7mm at minimum focus as shown here.

Because even manual focus is motorized rather than mechanically-coupled, you can't retract the lens back into its barrel unless it's attached to a powered-on camera.
A 6mm-wide ridged grip runs around the sides of the barrel beside the mount, and provides positive handling when changing lenses.