Street Photography Magazine

Aspheric lenses have better reproduction
characteristics. Because they have
asymmetrically curved surfaces, they used to
be extremely expensive to produce and were
only built into extreme high-end lenses when
the technology was introduced in the 1970s.
New production techniques have reduced the
cost of aspheric elements to a point at which
some manufacturers even build them into
mid-range and budget lenses, such as the
current US$100 Nikkor 18-55mm basic zoom.
The other 10 elements in this particular lens
are spherical.
Most lenses are built using a combination
of spherical and aspheric elements. The more
aspheric elements a lens contains, the lighter
and more compact the overall design can be.
Aspheric elements can also help to reduce
distortion and increase the optical
performance of wide-aperture lenses.
Aspheric elements are divided into
different grades according to their shape.
The ones used in cheaper lenses (like the
Nikkor mentioned above) are usually hybrid
models constructed from spherical glass
lenses glued to additional plastic elements
that give them their aspheric shape. The
intended aspherical effect is somewhat
reduced by the fact that plastic disperses
light more than glass. Further up the price
scale are pressed glass aspheric lenses, and
most expensive of all are ground glass

aspheric elements, which also have the best

reproduction characteristics.

Whether high-end or budget, most lens

elements are spherical, and individual

aspheric elements are added to enhance

optical performance.

Diffractive Elements

Canon introduced the then revolutionary EF

400mm f/4 DO IS USM lens at Photokina 2000.

The new Diffractive Optical (DO) elements in

the lens made it extremely compact and

reduced chromatic aberration significantly.

The technology hasn’t been developed much

further, and the lens since added to the range

is the EF 70-300mm f/4.5-5.6 DO IS USM. This

amazing lens weighs just 720 grams (1.58 lb)

and is only 10 cm (3.9 inches) long. A direct

comparison with the identically priced EF

70-300mm f/4-5.6L IS USM (1,050 grams,

14.3 cm) demonstrates the real advantage of

DO elements, in that they are significantly

smaller and lighter than their conventional

glass counterparts.

DO elements are built using a unique

two-piece construction that consists of two

diffraction gratings mounted between two

glass elements. DO elements designed for

photographic purposes have to include at

least two diffraction gratings in order to cover

the entire spectrum of visible light.

Diffraction gratings are difficult to

manufacture due to the variable distances

between the openings in the grating and their

extreme thinness of just a few micrometers.

You can see the pattern of the grating if you

hold the front element of a DO lens up to the

light. A DO element on its own has similar

dispersion characteristics to those of a normal

glass element and produces its own chromatic

aberrations. The all-important difference is

that a DO element disperses the component

colors of the spectrum in the opposite

sequence to that of a normal lens element.

Combining a conventional element and a DO

element thus cancels out color deviations and

eliminates chromatic aberrations.

The disadvantages of DO elements include

their tendency to produce bokeh that mirrors

the structure of the grating rather than the

unobtrusive circles produced by conventional

lenses. These DO circles of confusion tend to

look like a sliced onion and are similar to the

ring-shaped ones produced by mirror

telephoto lenses.

The precise shape of bokeh depends not

only on the lens but also on the nature of the

subject, and whether you find it attractive

when it does occur is a matter of personal

taste. With just two lenses being introduced

in 11 years, DO lens technology remains a

niche product and has yet to break into the

mass market.

Chromatic Aberrations | Lenses

Diffraction grating

Glass elements

DO elements use custom diffractive gratings positioned

between two lens elements to alter the path of the light rays

entering the lens. This structure is visible to the naked eye if

you look at a DO element head-on (shown on the left).

Aspheric element

UD element

This Canon EF 24mm f/1.4 lens has one aspheric and one UD
element. Aspheric elements are used to improve the quality
of high-end lenses and to make value lenses less expensive.