Transcript Aspheric Lenses

Light refracts colors according to
their wavelength. White light has
a tendency to break into a
spectrum as it’s medium changes.
This divergence is called
dispersion.
We want glass that has a low
tendency to do this. Hence, glass
makers have invented lowdispersion (LD) glass, extra-low
dispersion (ED) glass,
Extraordinary-low dispersion
(ELD) glass and ultra-low
dispersion (ULD) glass.
The low dispersion glass types
are often found in telephoto
lenses where fast f/stops are
desired.
The effects of dispersion are
called chromatic aberration.
Early solutions for chromatic
aberrations included incorporation
of radioactive materials into
optical glass.
EF 14mm
f/2.8 L USM
EF 17-40mm
f/4 L USM
EF 70-200 f/2.8
L IS USM
“L” Series Lenses
About their “L” lenses, Canon
says "these lenses use special
optical technologies [such as]
Ultra-low Dispersion UD glass,
Super Low Dispersion glass,
Fluorite elements, and Aspherical
elements to truly push the optical
envelope."
The truth is long lenses may use
ULD and Fluorite glass, but wide
angles and lenses of shorter than
200mm do not.
Wide angles may use aspheric
lens elements but not LD glass
types. Long lenses do not use
aspheric lenses.
Aspheric Lenses
Spherical Lens Element
Aspherical Lens Element
Most lens elements are
ground using a single focal
point.
An aspheric lens is ground
around multiple focal points.
Typically, you find greater or even
negative curvature on the
periphery of the lens.
The reason for incorporating this
level of technology is to produce
smaller and lighter lenses.