Excerpt: The Canon EF 40mm f2.8 STM is the company's first 'pancake' lens, designed to be as small and light as possible. Measuring just 22.8mm thick, it protrudes to roughly the same degree as the grip on many DSLRs, and at 130g you'll hardly notice it when mounted. This in turn makes even larger DSLRs feel much more portable than before, and allows you to squeeze them into smaller bags or even certain pockets.
Summary: The EF 40mm F2.8 STM offers excellent image quality in a tiny, well-made package. For APS-C users it provides a great carry-everywhere complement to a kit zoom, and for full frame shooters it's an excellent general-purpose 'normal' lens. The STM motor means that autofocus is almost completely silent, but it's not super-fast.
Pros: Very good optics, Tiny size, Silent autofocus, Responsive electronic manual focus, Decent construction, Low price
Cons: Can be susceptible to flare, No lens hood supplied, AF speed isn't the fastest
Excerpt: Handling and features The Canon EF 40mm f/2.8 STM is a slim, compact pancake type lens, which is compatible with Canon EF and EF-S camera bodies and costs around £230. A fast f/2.8 aperture should make this lens ideal for taking pictures in reasonably low light conditions and focusing is powered by Canon's new STM focusing motor.
Excerpt: The diminutive Canon EF 40mm f/2.8 STM Pancake Lens immediately inspires the high-pitched talking-to-a-baby voice from its first-time viewers, with short drawn-out words and phrases such as "awwwwe" and "it's sooooo cuuute" being especially common. It is of course the size and weight of this lens that triggers this response.
Excerpt: Canon has launched two new lenses with the EOS 650DRebel T4 DSLR - the EF 40mm f/2.8 STM and the EF-S 18-135mm f/3.5-5.6 IS STM, priced at £229.99 (about $355) and £479.99 (about $740), respectively. The STM stands for Stepper Motor, and indicates that the lenses use a new type of focusing motor designed to give a smoother automatic focusing action.
Excerpt: Given its tiny size, the Canon EF 40mm f/2.8 STM lens understandably has a very narrow focusing ring. Manual focusing is of the electronically-driven, 'focus-by-wire' type, more typically found on compact system cameras. There are no hard stops at either ends of the range, making it more difficult to set focus at infinity. Polariser users should be pleased that the 52mm filter thread doesn't rotate on focus.
Excerpt: 40mm might appear to be a bit of an odd focal length for a lens, but after spending a week with Canon's first pancake lens, it makes complete sense on our crop-factor EOS 60D. Pancake lenses are, quite simply, low in profile - attached to my camera it adds little to the width past the grip area. This means you can easily pop it into a small shoulder bag.
Canon EF 40mm f/2.8 STM - APS-C Format Review / Test Report
26 September 2012
Conclusion: The Canon EF 40mm f/2.8 STM represents a bit of a debatable concept on a DSLR - at least when compared to the much smaller mirrorless camera system. Honestly speaking there is also not really much of a size and weight advantage when thinking of its closest cousin - the Canon EF 50mm f/1.8 - which is also a faster lens. However, we wouldn't dare to state that it's not an attractive lens. It is just too cute for that.
Conclusion: The STM design for quieter video autofocus is only compatible with the 650D and 5D Mark III, but as a standard fixed 40mm lens it works really well with all Canon DSLRs, with a size and price that are hard to ignore.
Pros: Versatile, small and light, Sharp images, Silent autofocus in video mode on 650D and 5D Mark III
Cons: Auto/manual focus switch a little fiddly, Quiet autofocus feature only with the above models