Chromatic aberration is the tendency for the three basic components of the color spectrum, red, green and blue, to focus at different points on the sensor. This is a function of the physical properties of glass and light, and the result of different wavelengths bending at different amounts when the glass refracts them. It’s one of the more basic challenges of designing optics where full-spectrum light is to be focused, and a problem that will significantly impact the appearance of an image. 

In the following charts, we’re looking for concurrent lines, showing the three primaries essentially lining up for the fine details of the test chart.

Canon EF 85mm f/1.2L II USM @ f/1.2

Canon EF 85mm f/1.2L II USM @ f/1.8

Canon EF f/1.8 USM @ f/1.8

The Canon EF f/1.2L shot at wide-open shows a slight amount of chromatic aberration that almost completely disappears when stopped down to f/1.8. The f/1.8 lens, on the other hand, shows a marked amount of aberration when shot at wide-open, as shown below.

When the lenses are stopped down to f/7.1 we’re seeing them perform at fairly equal levels. Where the f/1.8 lens suffered at full aperture, it has completely recovered as it has been stopped down.

Canon EF 85mm f/1.2L II USM @ f/7.1

Canon EF f/1.8 USM @ f/7.1

It’s interesting to go back to the test results throughout the aperture range and see where the EF f/1.8 starts to pull it together. It turns out, at f/3.5 the lens shows a much improved curve. Even at f/2.8 we’re still seeing some pronounced aberration.

Canon EF f/1.8 USM @ f/3.5