The Canon EF 40mm f/2.8 STM has been released simultaneously with the Canon 650D in 2012. It is the smallest and lightest lens for Canon SLR cameras. In terms of size, you might not think about the combination of a professional Canon EOS 1D X with a Canon 40mm lens immediately. However, this lens is indeed suitable for a camera with a full frame sensor and the Canon 1D X is the right partner for a hefty Canon 40mm STM review.

A lens with a focal length of 40 mm on a camera with a full frame sensor is ideal for creating landscape, documentary and street photography. It is also attractive that the shortest focal length is only 30 cm, enabling you to get close to a subject. The maximum aperture allows for taking sharp pictures without flash in low light or playing with depth of field by choosing aperture f/2.8, allowing you to limit the depth of field and to isolate a subject from a troubled background, as shown in the picture.

Construction and autofocus

In this compact lens, Canon has still found space for an AF switch. It is nice that you do not have to control this through a menu. The Canon 1D X focuses smoothly and quickly in combination with the Canon 40mm STM, but not as fast as some professional lenses with a USM motor. When focusing manually, the ring rotates when achieving the smallest focal length or at infinity. Not everyone will like this “Focus by wire” with manual focusing, also used at various micro-43 lenses.

The stepping AF motor (STM) provides improved AF during live view video. The lens is quiet during focusing, but not silent. Due to the fast aperture and responsive AF sensors of the Canon 1D X, this lens is accurate in low light too.

Image stabilization

The Canon EF 40 mm 2.8 STM has no built-in image stabilization. Given the relatively low focal length and maximum aperture of f/2.8, this is acceptable. However, the current trend is to provide the (expensive) lenses with a short focal length with image stabilization too, such as the recently introduced Canon EF 35mm f/2 IS USM.

Vignetting

Viewed from the front, it clearly shows how small the Canon 40 mm front lens is (with a lens diameter of 19 mm). All the light falling on the 24 x 36 mm sensor comes through this little lens. The risk of vignetting in the corners seems big therefore. Normally, we use all in-camera corrections possible for jpg files that the test camera offers. The Canon 1D X test camera (firmware 1.1.1) had no correction data available though. The measured vignetting goes from 1.5 stops at full aperture to more than half a stop at the smaller apertures.

Vignetting can be corrected well, like a few other lens aberrations, by a correction profile for the Canon 40mm STM in Canon’s DPP (Digital Photo Professional), which is free with the purchase of a camera, or by using the correction profiles in Lightroom or Photoshop. The automatic correction for vignetting in DPP has appeared to be slightly better than in Lightroom in our review. After correction in DPP, the mediocre score for vignetting of this Canon lens changes to a grade higher than 9. Such a correction can definitely be recommended.

Distortion Canon 40 mm

The distortion of Canon 40mm STM files, both in jpg and RAW, exhibit a low distortion. You will never see it in practice, even in subjects with many horizontal and vertical lines as in architecture photography. All right.

Bokeh

Initially, you would not think about bokeh immediately with a standard lens. Nonetheless, the large f/2.8 aperture combined with a camera with a full frame sensor delivers a beautiful background blur/bokeh, which happens in this case too.

For the true bokeh lovers: The aperture blades are not rounded, so the bokeh of a point source changes from round at f/2.8 to seven angled at f/8. In addition, at aperture 2.8, a ring is visible on the outside.

Flare

With a lens design with little lens elements (only 6 elements in 4 groups), a 18mm front lens and the Super Spectra coating on the lens elements, there is almost nothing on which internal reflection can take place. In practice shots, we encountered no ghosting or flare, but we should also mention that many gray skies and fog were present during the review period.

As in our test of the Canon 40mm STM on a camera with an APS-C sensor (Canon 650D), we have not encountered ghosting in this review either, even under the most extreme conditions in the studio. In exceptional situations with very bright backlighting, you may see some ghosting. A very good performance of this lens.

Resolution Canon 40 mm STM

Starting at full aperture, the resolution is very high, from outer corner to outer corner. The measured resolution across the aperture range of 2.8 to 11 is very constant and the resolution in the extreme corners is as high as the resolution in the center. Also in this section, this lens proves to be one of the best Canon lenses that we have ever reviewed. Rather, at the time of the review, the Canon 40mm STM is the best of all Canon lenses tested so far in terms of resolution.

Chromatic aberration

With some lenses, the refraction of light is visibly dependable on the color of light, because of which you may encounter purple and green edges in the corners in sharp contrast transitions, such as tree branches against a bright sky. This is called lateral chromatic aberration. The Canon 40mm lens is absolutely not troubled by this phenomenon, as evidenced by our Canon 40 mm review. At all apertures, the chromatic aberration in both RAW and jpg files is so low that it will not even bother you at very large magnifications. Because no in-camera correction profile was available for this lens at the time of this review, these very good performances are not flattered by any in-camera correction of chromatic aberration.

Canon 40 mm review: conclusion

Pros

• Small and light
• Superior image quality of the whole image for almost all parameters.
• No ghosting and very little flare
• Largest aperture of f/2.8 for low-light shooting
• Short focal length up to at least 0.3 m
• Very attractively priced

Cons

• Small ring for manual focus
• Not everyone will like “Focus by wire” with manual focus
• Compact design: lens offers no support for holding camera
• Slower aperture than the professional 35mm and 50mm Canon lenses

The combination of a very compact and lightweight Canon 40 mm STM lens on a large, heavy professional camera may seem odd at first glance. Perhaps you expect that this very inexpensive lens does not deliver such a good image quality as the big and heavy professional lenses. However, the Canon 40 mm STM lets the high image quality of the Canon 1D X come out very well, according to our second Canon 40 mm review.