VIDEO
My youtube video “What’s Wrong with a Spherical Lens?” explains the
focus problems of both spherical and parabolic lenses and shows why an
ellipsoidal lens is better. The published video combines a silent screencast of
Optopus playing a script with separately recorded narration. At strategic
points the script pauses to afford the narrator (myself) sufficient time to
explain details. The script doesn’t try to predict the amount of time needed at
these points but simply resumes when the narrator presses the script/resume
button. The full-length video, posted on youtube, is relatively long. This
compressed silent version is the same
Optopus script with all pause commands
commented out.
SCRIPT OUTLINE
Optopus represents the three-dimensional lens by a two-dimensional slice
through its axis. Initially, a symmetrical spherical lens is shown. Equal
circles define the ingress and egress faces. The focus is imprecise. As the
lens thickness or width approaches zero the focus deviation approaches zero but
these are not practical lenses. The curve of a circle is too strong, reducing
the focal length from points further from the lens axis. This is evident in ray
trace crossover. A parabola has the reverse problem. The curve weakens too much
for points away from the axis. Varying the curve of the ingress face shows that
an ellipse curve can range between circle and parabola. The focus improves
further by making both faces ellipses and still further by changing the egress
face to bow inward instead of outward. The perfect single-face focus of an
ellipse at one of its foci is shown. Coincidental single- and two-face foci are
shown.