Canon EF 40mm f/2.8 STM
Category: Normal Lens
Conclusion - Pros
- Very good optics
- Tiny size
- Silent autofocus
- Responsive electronic manual focus
- Decent construction
- Low price
Conclusion - Cons
- Can be susceptible to flare
- No lens hood supplied
- AF speed isn't the fastest
The EF 40mm f/2.8 STM is a fairly unprepossessing little lens, and its tiny size and relatively low price might make you wonder whether significant compromises have been made in its design and construction. But the moment you start shooting with it and looking at the images it produces, any such thoughts rapidly disappear - it's actually a very fine lens. The main trade-off against other primes is its relatively slow maximum aperture, but that's really a function of its size, and the resultant diminutive dimensions mean that it's a lens you can carry around with you all the time and barely notice.
Optics and image quality
Optically, there's very little to complain about in our studio tests; the 40mm is impressively sharp wide open, and very sharp right across the frame at normal working apertures (F5.6 - F11). Chromatic aberration and distortion are both very low too. About the only possible criticism is vignetting wide open on full frame, which measures 1.7 stops, and is more than you'll see when shooting faster lenses at F2.8. But the broad falloff profile means it's not often objectionable in real-world use, and it's trivial to correct in post-processing when necessary. All Canon SLRs made in the last 5 years or so can correct it automatically in their JPEG processing too, if you desire.
Moving on the real-world use, the 40mm continues to impress. It gives consistently fine results on both full frame and APS-C cameras, with an attractive rendition of out-of-focus ares of the image. Its rather exposed front element means that it can be somewhat susceptible to flare in bright conditions, so that's something to look out for while shooting. Sadly Canon refuses to supply a hood with the 40mm, but you can use a generic 52mm screw-in hood to combat stray light in bright conditions.
Autofocus and manual focus
Autofocus performance is fine - it's not super-fast, probably because the entire optical unit moves back and forward for focusing, but it is very smooth and quiet. The electronically-driven manual focus works very well too, and arguably rather better than the relatively loose mechanically-coupled mechanisms found on many other inexpensive lenses such as the EF 50mm f/1.8 II. One thing to be aware of, though, is that the focus can't be adjusted when the lens is removed from the camera. This means it's entirely possible to find yourself putting the lens in your bag with the optical unit protruding 8mm out of the barrel, making it more vulnerable to damage (and reducing the size advantage too).
One attraction of the STM focus motor is the ability to refocus smoothly and silently during movie recording - which Canon makes particularly easy to control on its latest touchscreen SLRs. The 40mm delivers on this promise only partially - it's extremely quiet, but refocusing during recording isn't quite as decisive as Canon's latest kit zoom, the EF-S 18-55mm f/3.5-5.6 IS STM (click here for an example with this lens). In particular the lens's relatively slow autofocus can make the contrast-detect focus confirmation step of Canon's hybrid system a little more obvious, resulting in a visible 'wobble' around the point of focus.
Some people may well question the lens's focal length: 40mm is a relatively unfamiliar number on full frame, as is the 64mm-equivalent it offers on APS-C SLRs. We struggle to see it as an ideal focal length on APS-C for general purpose photography, and certainly wouldn't recommend it as the only lens to own; it's best used to complement a kit zoom. But the short telephoto effect can be useful for head-and-shoulder portraits, and the larger aperture compared to kit zoom offers better background blur and low-light performance.
On full frame, though, we're of the camp that sees 40mm as an excellent general purpose option, and close to the ideal 'normal' lens. It's a great pairing with cameras like the EOS 6D and 5D series, whose low-light performance goes a long way to negating any perceived disadvantage of the 'slow' maximum aperture (for a prime). It's never going to be the first choice if you want a lens to give shallow depth of field and highly-blurred backgrounds, though.
The big question, perhaps, is how it compares to the older EF 50mm f/1.8 II and EF 35mm f/2 primes, or the more expensive EF 50mm f/1.4 USM. Our studio tests reveal that there's little to choose between them optically, with all performing very similarly across the shared aperture range. To an extent, this helps to narrow the choice down to size, price and features. If size is your priority and you don't need a faster aperture, then the 40mm is the obvious choice. If you're looking to experiment with selective focusing and shallow depth of field, one of the 50mm lenses would be a better bet (however the 50mm F1.8's 5-bladed diaphragm results in distracting pentagonal out-of-focus highlights when it's stopped down a bit).
The Final Word
The EF 40mm f/2.8 STM is a lens whose impressive image quality belies its low price and tiny size. For APS-C users it's a tiny, inexpensive companion to a kit zoom, and on full frame it offers a great option for general shooting. It's also nicely built, and on the whole the STM focusing works pretty well. Compact pancake primes have undergone something of a revival in recent years, with such gems as the Panasonic Lumix G 20mm F1.7 ASPH and Samsung NX 30mm F2 Pancake showing that small size doesn't necessarily have to mean compromised quality. It's taken a while for Canon to come to the party, but the 40mm takes its place alongside its peers as one of the best examples of its type.
Ergonomics and Handling
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.
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