Canon EF 85mm f/1.8 USM (full format) - Review / Lab Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)
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Introduction
Analysis

Distortion

As expected for a fix-focal lens the level of distortion is basically negligible in field conditions (0.24%).

The chart above has a real-world size of about 120x80cm.

Vignetting

It is a well known fact that full format sensors aren't exactly easy beings when it comes to vignetting and the EF 85mm f/1.8 USM has its share of problems here. At f/1.8 you can experience an edge deterioration of ~1.7EV. That's not as bad as e.g. the EF 50mm f/1.8 II but not great either. However, the problem is already well controlled at f/2.8 and a non-issue by f/4.

Here's an illustration of the problem at f/1.8:

MTF (resolution)

There isn't really much to say in this chapter - the Canon lens produced very good resolution figures across the image field throughout the tested aperture range. At f/5.6 it even scratches excellent levels. The lens shows no field curvature problems whatsoever.

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 chromatic aberrations (color shadows at harsh contrast transitions) are basically a non-issue - this is a very rare characteristic.

Bokeh Fringing (Longitudinal Chromatic Aberrations (LoCA))

LoCAs (non-coinciding focal planes of the various colors) are a primarily problem with ultra-fast prime lenses and the EF 85mm f/1.8 USM suffers quite a bit here as well. At f/1.8 the greenish and magenta focus halos are very obvious. The situation improves slowly towards f/2.8 but you can still spot traces of bokeh fringing at f/4 although it's not disturbing anymore here. This is all not unusual but it's not desirable either.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/1.8 f/2.8 f/4

Bokeh (out-of-focus blur)

The quality of the bokeh is very good at f/1.8 with evenly rendered circular highlights and smooth contrast transitions. However, the mentioned bokeh fringing is also pretty obvious in our test scene - see e.g. the greenish halo in the sample crop taken at the statue. At the very outmost image portions highlights loose their circular shape towards an elliptical form. The lens does not feature circular aperture blades - this is no problem at max. aperture but you are already getting slightly octagonal highlights at f/2.8 and more so at f/4.

Purple Fringing

We've already reported the problem before but the EF 85mm f/1.8 USM is somewhat prone to purple fringing at extreme contrast transitions. The issue is primarily limited to f/1.8 so you can work around it in critical scenes.

Verdict

The Canon EF 85mm f/1.8 USM is a very desirable lens and, surprisingly, a very affordable one. Both contrast and resolution are very good straight from the max. aperture. The lens shows some hefty vignetting at f/1.8 but beyond it's not really a big problem anymore. Neither distortions nor lateral CAs are something to worry about in field conditions. However, bokeh fringing (LoCAs) can be visible in some scenes and in extreme situations you may also spot some purple fringing. The quality of the bokeh is generally very good at f/1.8 (apart from the LoCA problems) but the lack of circular aperture blades shows up in out-of-focus highlights when stopping down. The build quality of the lens is very good and the AF performance leaves nothing to be desired.

Optical Quality:    
Mechanical Quality:
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