Optical design
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Optical design

The lens comprises 17 elements in 14 groups. One extreme aspherical (XA) element, pictured in orange, helps to control variations in distance-related aberrations, while an extra-low dispersion (ED) aspherical element, pictured centrally in dark blue, suppresses both chromatic and spherical aberration.

Controlling spherical aberration (SA) ensures that marginal light rays entering at the edge of the lens focus at the same plane as paraxial light rays entering centrally. And to state the (perhaps) obvious: if light rays emanating from an object focus at different planes behind the lens (instead of all at the sensor plane), it risks appearing blurry. See this slide for ray diagrams of a lens with and without SA, and note how the ray bundle at the point of 'Best Focus' is tighter for the 'perfect' lens with no SA. The end result is a sharper image.

There's another aspheric, pictured in purple, and these three more exotic elements we've discussed in this slide work together to suppress aberrations and achieve high resolving power across the frame wide open, particularly on the long end where, arguably, it matters more for a lens of this type.