Street Photography Magazine

Chromatic aberrations in photographic

systems are caused by the refraction of light

within the elements of the camera’s lens.

Various types of specialized elements

provide effective ways to combat their

effects.

The ‘Right’ Glass

Optical glass is available in a wide range

of qualities. Nikon uses its patented ED

(Extra-low Dispersion) and Super ED glass,

while Canon uses its own fluorite, UD

(Ultra-low Dispersion) and Super UD glass.

There is not enough natural fluorite in the

world to cater for all lens manufacturers’

needs, so Canon began producing artificial

crystal fluorite in the 1960s. Third-party lens

manufacturers all produce various types of

lenses with low-dispersion elements, such as

Tamron’s LD (Low Dispersion), XLD (Extra Low

Dispersion) and AD (Anomalous Dispersion),

Sigma’s FLD (F Low Dispersion), ELD (Extra

Low Dispersion) and SLD (Special Low

Dispersion), as well as Tokina’s SD (Super Low

Dispersion) and HLD (High Refraction, Low

Disper sion) models, to name just a few.

Low-refraction glass is most often used in

telephoto lenses due to the particular

sensitivity of long focal lengths to chromatic

aberrations. However, some wide-angle

lenses are also built using this type of element.

Photographic lenses are never made

completely out of elements with low

refractive indexes, but instead use just one or

two within a system of conventional glass

elements. This helps to keep manufacturing

processes simple and the cost of production

down to an acceptable level.

As well as increasing costs, ED glass is also

said to scratch more easily than conventional

optical glass, although this is not generally a

problem because ED glass is not usually used

for front or rear lens elements. Fluorite is said

to be more sensitive to impact than normal

glass and its refractive index actually

fluctuates with changes in temperature.

Low refractive index elements are

nowadays a standard component in most

high-end photographic lenses throughout the

range of available focal lengths.

Spherical vs. Aspheric

To keep costs down, most optical systems use

spherical lenses. With their spherical surfaces,

these are cheap to produce but are inherently

prone to significant chromatic aberration.

Chromatic Aberrations

in Camera Lenses

Chromatic Aberrations | Lenses

Every lens-based optical system has to deal with chromatic aberrations

in one form or another. It is impossible to eliminate them completely,

but specially constructed lenses can help to lessen their effects at source.

The following sections explain what to look out for when purchasing

cameras, lenses and optical accessories.

Aspheric lens

R

Spherical lens

R

The surface of a spherical lens is shaped like

part of the surface of a sphere. Aspheric lenses

have a more complex shape that is more

difficult to manufacture.

Conventional glass

ED glass

Secondary spectrum

Secondary spectrum

The low refractive

index of ED glass

reduces the degree of

dispersion of incident

light, thus

reducing (but not

completely

eliminating) the

resulting aberrations