Studio Tests (APS-C)

The Canon 40mm f/2.8 STM performs very well on APS-C in the studio, especially given its tiny size. Sharpness is very decent across the frame even wide open, and improves further on stopping down. Distortion, CA and vignetting are all very low too, as is usual for a full-frame lens on APS-C. Overall, compared like-for-like it closely matches the EF 50mm f/1.8 II or EF 35mm f/2.0 in these tests.

Sharpness Sharpness results are very impressive - the 40mm gives very good results right across the frame wide open. There's little need to stop down further for sharpness alone, but the lens continues to give excellent results on stopping down further until diffraction softening starts to become noticeable at F11 and smaller apertures.
Chromatic Aberration Lateral chromatic aberration is pretty low. There's a little red/cyan fringing towards the edge of the frame, but you'll have to look very closely to see it.
Vignetting Vignetting is negligible, as usual for a full-frame lens used on APS-C.
Distortion Distortion is extremely low, with just a little barrel-type visible if you look closely.

Macro Focus

Macro (APS-C) - 120 x 80 mm coverage
Measured magnification: 0.19x
Distortion: Very slight barrel

Minimum focus distance*: 29.2cm
Working distance**: 22.0cm
Focal length: 40mm (64mm equiv)
* Minimum focus is defined as the distance from the camera's sensor to the subject
** Working distance is measured from the front of the lens to the subject

We wouldn't expect a 40mm pancake to be ideal for close-up work, but the Canon acquits itself reasonably well. Its measured maximum magnification using manual focus is 0.19x at a minimum focus distance of about 29cm. Note though that SLRs won't generally autofocus quite this closely.

Image quality is perfectly acceptable. The centre of the frame is sharp at F2.8, but in our flat-field chart test the corners are somewhat soft. But they sharpen up well on stopping down, and match the centre at F8. There's a little colour fringing towards the corners from lateral chromatic aberration, and a tiny bit of barrel distortion, but chances are you won't notice either in normal use.

Specific image quality issues

As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. Here we're looking at issues specific to APS-C users; for a fuller picture, be sure to read the next page too.

Lateral Chromatic Aberration

The 40mm's CA profile means that colour fringing towards the corners of the frame is slightly more visible on APS-C compared to full frame, but most of the time you'll struggle to see it. All recent Canon SLRs can correct it in their JPEG processing too, but older models won't; however it's easy enough to correct in post-processing if you shoot RAW.

The example below illustrates this using RAW conversions from an EOS 650D file. Without CA correction there's a little red/cyan fringing in the corner of the frame, although it's only just visible even in a 100% crop. Turn on CA correction and the fringing is essentially eliminated.

Canon EOS 650D, F5.6
RAW + ACR, no CA correction RAW + ACR, CA corrected
100% crop, bottom left corner 100% crop, fringing removed

Refocusing during movie recording

The stepper motor used by the 40mm should in principle allow silent refocusing during movie recording. Canon's latest touchscreen EOSs (the 650D/T4i, 700D/T5i, 100D/Rebel SL1 SLRs, and mirrorless EOS M) aim to make the most of this by enabling touch-controlled focus 'pulling' in video - you simply tap the screen during recording to tell the camera where you want it to focus.

The company's 'Hybrid' focus system works by using phase-detect elements on the imaging sensor to determine the direction the lens should start moving to focus. Then as it reaches the target point, contrast-detection AF takes over to fine-tune correct focusing. The examples below give an idea of how well this system works, using the EOS 100D (whose sensor offers a wider array of phase detect elements compared to the other cameras).

Video example 1

This example is shot in a quiet indoor environment, with close range subjects, and in M mode with the aperture set to F2.8. Focus is switched from the foreground to the background, then back. This illustrates just how quiet the lens's focus motor is - it's barely audible at all. But refocusing isn't especially impressive here - the contrast detect 'fine tune' stage is particularly visible.

1920x1080 25fps, MOV, 20 sec, 47.3 MB Click here to download original file

Video example 2

This example is again shot at F2.8 on the EOS 100D, but in an outdoor environment. The foreground subject distance is about 1m, and the background around 100m. Focus starts on the foreground, and switched to the background, then to the foreground, then the background again. Any focus noise is entirely drowned out by ambient noise, but again the focusing itself isn't particularly decisive.

1920x1080 25fps, MOV, 16 sec, 105.7 MB Click here to download original file