What is Spherical Aberration?
Spherical
Aberration is an optical problem that occurs when all incoming light
rays end up focusing at different points after passing through a
spherical surface. Light rays passing through a lens near its horizontal
axis are refracted less than rays closer to the edge or “periphery” of
the lens and as a result, end up in different spots across the optical
axis. In other words, the parallel light rays of incoming light do not
converge at the same point after passing through the lens. Because of
this, Spherical Aberration can affect resolution and clarity, making it
hard to obtain sharp images. Here is an illustration that shows
Spherical Aberration:
As
shown above, light rays refract or change their angle when passing
through the lens. The ones closer to the top and the bottom of the
illustration end up converging at a shorter distance along the optical
axis (black/red dotted line), while the ones closer to the optical axis
converge at a longer distance, creating different focal points along the
same axis. The point of best focus with the “circle of least confusion”
is illustrated as the thick green line. Spherical Aberration is not
just caused by lens design, but also by the quality of the lens
material. Lenses made of poor quality material and large bubbles can
drastically impact light refraction.
A perfect lens would have all light rays converge in a single focal point, as illustrated below:
The
best focus point with the circle of least confusion is therefore
located right on this focal point. A normal spherical lens design would
not allow the above to happen though, so specialized precise methods by
manufacturers have been developed over the years to reduce the effect of
spherical aberration.
Spherical
aberration is most pronounced when the diaphragm of the lens is wide
open (maximum aperture). Stopping down the lens even by a single stop
dramatically reduces spherical aberration, because aperture blades block
the outer edges of spherical lenses. A clear example of this can be
found in the focus shift article.
A perfect lens would have all light rays converge in a single focal point, as illustrated below:
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