Reference Guide

page 6
FLOATING SYSTEM (CLOSE FOCUSING ABERRATION COMPENSATION MECHANISM)
When a lens is designed, the first step is to determine the focal
length and speed. Next comes the closest focusing distance. In
most 35mm still camera lenses, performance is optimized for
intermediate to long distance, with somewhat poorer performance
at close range.
The most common problem in close focusing is called "curvature of
field." This means that the image projected on the flat focal
plane is not flat, but instead bowl-shaped, or "concave." The
result is loss of sharpness in the corners of the picture.
Curvature of field is hardest to correct with large aperture and
short focal length designs.
The Canon Floating System was developed to hold curvature of
field to a constant minimum regardless of the focusing distance.
It moves, or "floats" one of the elements independently as the
rest of the lens moves during the focusing process. Another very
important benefit is further reduction of spherical aberration in
wide aperture formulas.
There are 2 variations on the Canon Floating System. With wide-
angle lenses, the floating element is internal. The EF 24mm f/2.8
is an example of this type. With other lenses such as the close
focusing 50mm f/2.5 Compact Macro and extremely large aperture
formulas such as the 50mm f/l.OL and the 85mm f/l.2L, the rear
element or group is fixed while the rest of the lens moves.
One other form of a "floating" element is used in the EF l35mm
f/2.8 Soft Focus lens. However, in this case, the photographer
controls the moving element to cause spherical aberration,
resulting in a Soft Focus effect. It's important to note that
without making this adjustment, the EF 135mm f/2.8 is extremely
sharp.
FLUORITE & ULTRA-LOW DISPERSION (UD) GLASS
Chromatic aberration increases with focal length because of the
dispersion of white light into its component wavelengths.
Standard optical glasses can be combined to correct this problem
for about 2/3 of the visible spectrum, resulting in what has
become known as an achromatic formula.
However, fluorite crystal can correct chromatic aberration over
virtually the entire spectrum, thus enabling the production of
apochromatic lenses. "Apo" lenses produce spectacularly sharp
images, making them highly desirable for color photography.
Canon leads the photographic industry in the use of calcium
fluorite lens elements. In the EF line, they can be found in the
300mm f/2.8L, 600mm f/4.0L, 50-200mm f/3.5-4.5L, and 100-300mm
f/5.6L. All of these lenses are fully corrected apochromatic
formulas.
Another way of reducing chromatic aberration is by mixing
fluorides with standard optical glasses, resulting in ultra low-