Canon EF 28mm f/2.8 USM IS - Full Format Review / Test - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

Our image analysis software struggled a bit with the distortion characteristic of the lens. It suggests a barrel distortion of 2.4% but visually we would estimate it to be around ~1%. There's a slight mustache sub-frequency which may have confused the algorithm. We would certainly classify this as a slight distortion for sure.

Vignetting

Typical for most lenses used on full format cameras, the EF 28mm f/2.8 USM IS shows some light falloff at large aperture settings. A vignetting of ~2.1EV (f-stops) will be very noticeable at f/2.8. However, the issue is substantially reduced at f/4 and not really a problem anymore from f/5.6 onward.

MTF (resolution)

As to be expected from a prime lens, the resolution characteristic is impressive. The center sharpness is great straight from f/2.8. The border quality is already very good fully open aperture whereas the corners are slightly softer here albeit still good. The peak performance is reached between f/4 and f/8 with an excellent image center and a very good outer image region. Diffraction effects start to have a limiting factor from f/11 onward although this setting is still good enough for very high quality images. The field curvature (flatness of the focus field) is marginal.

The centering quality of the tested sample was very good.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Lateral CAs (color shadows at the image borders) stay around an average pixel width of 0.7px at the image borders. This is still very low and usually nothing to worry about.

Bokeh

A moderately fast wide-angle lens may not be an obvious candidate for a bokeh test but you can achieve a shallow depth-of-field when choosing short focus distances.

The out-of-focus highlight rendition is pretty clean with a slight outlining effect.

The situation changes a bit when moving towards the corners. The highlight shape deteriorates towards an ellipsoid shape and a more emphasized outlining here. However, this is still fairly moderate for a full format lens (this is a vignetting effect).

The quality of the general blur in the focus transition zone is pretty good. The foreground blur is a bit busy whereas the more critical background blur is quite smooth and better than most zoom lenses for instance. This is a bit unusual for a lens featuring aspherical elements.

Bokeh Fringing / Longitudinal Chromatic Aberrations (LoCA)

The so-called bokeh fringing refers to colored halos in the focus transition zone. Hard contrasts can have a purple color tint in the foreground changing to greenish beyond the focus point. The EF 28mm f/2.8 USM IS shows a medium amount of bokeh fringing at f/2.8. There're only traces left at f/4 and the issue is gone from f/5.6 onward.

When browsing through the sample crops below you may also notice that the focus point remains constant (=no so-called residual spherical aberrations) and that the results are pretty darn sharp here despite the close focus scenario.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/2.8 f/4 f/5.6




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