Canon EF 40mm f/2.8 STM Lens Review

Canon EF 40mm f/2.8 STM Performance

At maximum aperture, sharpness in the centre of the frame is already excellent, with clarity towards the edges approaching very good levels. Stopping the lens down improves sharpness across the frame, with clarity across the frame peaking at f/4.

Resolution at 40mm

How to read our charts The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges. Averaging them out gives the red weighted column. The scale on the left side is an indication of actual image resolution. The taller the column, the better the lens performance. Simple. For this review, the lens was tested on a Canon EOS 1Ds Mark III using Imatest.

Chromatic aberrations are virtually non-existent, remaining under half a pixel width in size at all apertures. This extremely low level of CA, should pose no issues, even in large prints, or harsh crops from the edges of the frame.

Chromatic Aberration at 40mm

How to read our charts Chromatic aberration is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software. Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more. For this review, the lens was tested on a Canon EOS 1Ds Mark III using Imatest.

As is quite common for lenses of this type, falloff of illumination towards the corners is quite pronounced. At f/2.8 the corners of the frame are 2.1 stops darker than the image centre and visually uniform illumination isn't achieved until f/8.

Distortion is well controlled, with Imatest only detecting 0.5% barrelling. This low level of distortion should pose few issues for normal shooting. If absolutely straight lines are paramount, then you'll be glad to hear that the distortion pattern is uniform across the frame, which should make corrections relatively straightforward to apply.

No issues with flare were encountered during testing, with only a slight loss of contrast being noticeable when shooting directly into the light at wide apertures.

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