Canon EF 40mm f/2.8 STM review

The EF 40mm f/2.8 STM was announced in June 2012, alongside the EOS 650D (Rebel T4i) SLR and EF-S 18-135mm f/3.5-5.6 IS STM zoom. These two lenses were Canon's first to use a linear stepper motor for focusing, giving rise to the 'STM' designation; this motor type allows silent refocusing during movie recording, and has therefore become widely-used in lenses for mirrorless cameras. This means that in principle the 40mm STM should be well-suited to working with the hybrid AF systems in Canon's recent cameras, including the 650D and its replacement the EOS 700D (Rebel T5i), the EOS M mirrorless model, and the diminutive EOS 100D (Rebel SL1).

The 40mm focal length may seem a bit odd - 35mm and 50mm are more familiar numbers to most photographers - but Canon has chosen it for good reason. Firstly it's relatively easy to design a small lens of this focal length for 35mm format SLRs, which means that it joins a distinguished line of compact 40mm primes from the likes of Olympus, Pentax and Voigtlander. Secondly, many photographers consider 40mm to be the 'perfect normal' lens on full frame, providing an extremely natural perspective to images.

The 40mm is the smallest EF lens Canon has made; it's just 22mm (0.9" thick), and weighs in at a mere 130g (4.6oz). It follows a recent trend for ultra-compact 'pancake' primes that are designed for maximum portability, encouraging users to carry their camera with them more of the time. But despite its size Canon has still managed to fit in a 6 element / 4 group optical unit that includes an aspherical element to help reduce aberrations. This makes it slightly more complex than its nearest equivalent, the Pentax smc DA 40mm F2.8 Limited that uses a 5 element / 4 group design in a lighter, even slimmer body.

The 40mm is also an inexpensive lens - indeed at around £150 / $199, it's Canon's second-cheapest for SLRs. But the elephant in the room is that one cheaper option - the EF 50mm f/1.8 II, which is half the price and offers substantially better light-gathering capability. Another option is the venerable EF 35mm f/2.0 - one of the earliest lenses made for the EOS system, although somewhat more expensive. In this review we'll see how the 40mm compares to these alternatives.

Headline features

• 40mm focal length, F2.8 maximum aperture
• Ultra-compact 'pancake' design: just 22mm thick
• Linear Stepper Motor (STM) focusing with full-time manual override
• 0.3m closest focus, offering 0.18x magnification
• Canon EF mount for full-frame and APS-C SLRs, or EOS M using adapter

Angle of view

The pictures below illustrate the angle of view on full frame and APS-C. On full frame the 40mm is a classic 'normal' lens; on APS-C cameras it behaves like a short telephoto.

Full frame	1.6x APS-C (64mm equivalent)

Canon EF 40mm f/2.8 STM specifications

Approx Price	• $199 (US)  • £149 (UK)  • €199 (EU)
Date introduced	June 2012
Maximum format size	35mm full frame
Focal length	40mm
35mm equivalent focal length (APS-C)	64mm
Diagonal Angle of view	• 56.8º (full frame)  • 37.1º (APS-C)
Maximum aperture	F2.8
Minimum aperture	F22
Lens Construction	• 6 elements / 4 groups  • 1 aspherical element
Number of diaphragm blades	7, rounded
Minimum focus	0.3m
Maximum magnification	0.18x
AF motor type	• Linear Stepper Motor  • Full-time manual focus  • 'Focus-by-wire' manual focus
Focus method	Unit
Image stabilization	No
Filter thread	• 52mm  • Does not rotate on focus
Supplied accessories*	Front and rear caps
Optional accessories	ES-52 screw-in metal hood
Weight	130 g (4.6 oz)
Dimensions	68 mm diameter x 22 mm length (2.7 x 0.9 in)
Lens Mount	Canon EF

* Supplied accessories may differ in each country or area

This lens review uses DxOMark data thanks to a partnership between dpreview.com and DxO Labs (read more about DxOMark and our partnership with DxO Labs). DxOMark is the trusted industry standard for independent image quality measurements and ratings. DxOMark has established this reputation with its rigorous hardware testing, industry-grade laboratory tools, and database of thousands of camera, lens and mobile test results. Full test results for this lens can be found at www.dxomark.com.

If you're new to digital photography you may wish to read the Digital Photography Glossary before diving into this article (it may help you understand some of the terms used).

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