Canon EF 200 mm f/2.8L II USM
5. Chromatic and spherical aberration
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The situation of lateral chromatic aberration, which is negligible on the edge of the APS-C sensor and full frame alike. The level of 0.04% is not anything you should worry about.
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We would like to introduce in this chapter an evaluation of another axis aberration - the spherical one. You can assess it by looking at out-of-focus image of a light point. When the aberration is corrected in a perfect way the layout of light inside the circle should be even, no matter whether you get an out-of-focus image in front of or behind the focal point. If the aberration is overcorrected or undercorrected the image on one side of the focus will have a light rim around the edge and on the other side it will be lighter inside and darker on the edge.
Let’s check how the Canon EF 200 mm f/2.8L USM II fares in this category.
As you see both images are very much alike although some very slight differences can be still noticed. In both cases there is a lighter rim on the edge which intensity is a bit higher in the case of front focus image. The front focus image also has much more homogeneous surface in the centre whereas the back focus image shows the centre a tad lighter than the rim. You can assume that the spherical aberration is corrected very well but not in a perfect way.
