For the final part of our Know Your Camera assignment we looked at chromatic aberration.
Chromatic aberration is also sometimes known as colour fringing and is the result of light rays not meeting at one focal point, due to limitations and defects in the lens. Light is defracted by different amounts depending on its colour and is accentuated when it hits a positive or convex lens. It can be countered by having a negative or convex lens close to the positive lens but the more glass within a lens makes it heavier, more expensive and more complicated to make.
I took this photograph of holly in the garden and at this size it looks like a perfectly ok photograph. But, on closer inspection, particularly where the dark leaves are next to the bright blue sky, you can see a blue line around some edges of the leaves and a purple line around other edges.
This could have been corrected by stopping down the lens and is also much more obvious on wide aperture prime lenses.
Chromatic aberration can be corrected in Lightroom using the defringe sliders.