Oblique Beams
An oblique beam (also called a pencil) of rays from an extraaxial (or off-axis or nonaxial) object point contains meridional rays that can be traced by the ordinary computing procedures already described, and also a large number of skew rays that do not lie in the meridional plane. Each skew ray intersects the meridional plane at the object point and again at a “diapoint” in the image space, and nowhere else. Skew rays require special ray-tracing procedures, which will be discussed in Section 8.3. These are much more complex than for a meridional ray, and it is observed that skew rays were seldom used before the advent of electronic computers; now they are routinely traced by all lens designers since the available computing ...
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