Glasses and Coatings
In addition to differences in
magnification and brightness, competing lenses can differ in such
optical qualities as color fidelity,
sharpness, and contrast. The tricks used by lens
designers to manipulate these qualities are worth briefly surveying,
since they give rise to a number of technical terms which have been
appropriated by various marketing strategists hoping to liberate you of
your well-earned cash. (In other words: these things are good to
know when you’re shopping for a birding lens). These design
techniques involve, among other things, the use of higher quality
glass, the optimal placement of those glass elements within the optical
system, and the application of transparent coatings to the lens
surfaces to improve their receptivity to light.
A typical telephoto lens will have anywhere between
10 and 20 glass elements of various shapes and sizes, all arranged in a
precise layout so as to accomplish the lens designer’s goals of
providing a particular combination of focal length, maximum aperture,
and field of view.
Precisely how this all works is complex, and
largely irrelevant for our purposes. A simplified block diagram
of a hypothetical telephoto lens is shown in the figure below.
Fig. 3.3.1: A
hypothetical block diagram for a telephoto lens.
Each oval or polygon represents a glass element.
Combinations of elements help to negate the color
dispersion introduced by individual elements.
The important thing to understand is that each lens element bends (or refracts) the incoming light in a
way, and that combinations of these lens elements work in tandem so as
to focus all those light rays into the same plane—namely, the plane of the imaging
sensor (represented by the focal point of the eyeball in the figure
The amount of bending that each element applies to
the light depends on the so-called index
of refraction of that element, which is determined by the
optical properties of the type of glass used to make that
element. The problem is that different wavelengths of light
(i.e., different colors) refract at a slightly different angle,
resulting in what is known as dispersion.
Thus, in a single-element lens system like the one shown below, the
different colors (and shades of color) of a bird would be focused onto
planes. Since the imaging sensor occupies only one plane, only
one color would be in focus, and everything else would be
blurred. We say that the resulting image suffers from severe chromatic aberration.
Color dispersion in a single-element system.
Different wavelengths of light refract at slightly
different angles, resulting in chromatic aberration.
This is one of the reasons why
camera lenses invariably contain multiple optical elements: so that
downstream elements can act to mitigate, or “clean up” the dispersion problems created
by more upstream elements. Intuitively, you can think of
different elements with opposite dispersion propensities as “cancelling each other out”, so that all the wavelengths are
channeled back into a consistent light path, resulting in a properly
focused image in all color channels. That’s the ideal case.
In reality, most optical systems exhibit at least a small amount of
chromatic aberration. If you’ve ever zoomed way in on an image
(on the computer) and noticed some off-color halos (“color fringing”) around objects, that’s likely
caused by chromatic aberration (though it could also be caused by lens flare or by artifacts in the
digital sensing process).
The problem is that the dispersion properties of “normal” glass are such that crafting a
system of optical elements that collectively result in a sharply
focused image is extremely difficult in practice. One way to make
this easier is to use glass with different dispersive properties than “normal” glass. The Abbe number of a given type of
glass describes these dispersive properties, with higher Abbe numbers
corresponding to lower propensities for dispersion. Using glass
with a high Abbe number will therefore simplify the task of a lens
designer in counterbalancing the various sources of
dispersion, since the dispersion introduced by any single element will
be smaller and therfore easier to correct. Since no lens design
is ever 100% perfect, this means
that lens designs incorporating low-dispersion glass elements should,
as a general rule, suffer less from chromatic aberration. And
that’s a good thing.
There are two popular classes of low-dispersion
glass now in popular use. The first, developed separately by
researchers at Canon and Zeiss in the 1970’s, is known as fluorite and is formed from calcium flouride crystals grown
synthetically in a laboratory. Fluorite elements have excellent
dispersion properties, as well as higher light transmission than
current alternatives (resulting in slightly brighter images).
Unfortunately, fluorite lenses are expensive to produce, for various
The other popular class of low-dispersion materials
is known as ED, or extra-low dispersion glass.
The term is a very general one, admitting any manufacturing process
that results in substantially lower dispersion (i.e., a higher Abbe number) than normal optical
glass (excluding true fluorite elements). Thus, different
manufacturers routinely tout their latest low-dispersion glass as ED, UD (“ultra-low dispersion”), SLD (“super-low dispersion”), HD (“high-definition”), or some other
acronym-of-the-week. The manufacturing processes used for these
various materials are poorly documented and often proprietary, though
some at least involve the doping of normal optical glass with fluoride
What all of these technologies have in common is
that they aim to produce optics which are apochromatic—meaning that they lack (or
minimize) chromatic aberration. A common designation for
apochromatic optics is APO.
Thus, APO, ED, UD, CaF2 (for calcium
fluoride), and many other similar designations all attest to
degree of apochromatism
(meaning that they should suffer very little from chromatic
aberration). In practice, this means that these optics should
produce very little color fringing,
or unnatural color separation in images. Knowing how these
various technologies stack up against each other (i.e., which ones are
better than others)
would, unfortunately, require detailed comparisons which in most cases
don’t seem to
have been performed as of yet.
The only really sure bet would be that an
optical system incorporating one or more low-dispersion elements should
(if designed properly) produce better image quality than a comparable
only standard optical glass. Beyond that, the only way to know
which of several optical systems all featuring different varieties of
low-dispersion glass is the best would be to compare the images taken
through those lenses.
While the use of low-dispersion glass aims to reduce
chromatic aberration, the use of transparent coatings on lens surfaces
aims to improve brightness and contrast, by reducing reflection.
Recall that as a ray of light enters a glass element, some of the light
is refracted (bent inward,
while still passing through the glass), while some of it is instead reflected (redirected backward, so
it doesn’t pass through the glass). The portion of incoming light
that is reflected is effectively lost, thereby reducing light
transmission through the lens, and therefore reducing overall
brightness of the image. But because reflection can also happen
on internal surfaces in a
camera lens (i.e., after passing through
a glass element to the far side), reflected rays can also decrease
image contrast, by
contributing to the overall ambient light level (in
a non-informative way) via random scatter. Thus, reducing the
amount of reflection that occurs at glass boundaries is an important
goal in lens design.
This goal is most often addressed by applying one or
more coatings of a transparent solvent to the individual elements in a
camera lens. The precise chemical formulation of these solvents
is often kept secret by the companies that develop them, who give
them names such as “Special T* Coating” and the like, but the basic
principle behind their effectiveness is as follows. Consider the
figure shown below.
In part A of this figure (left),
we have a single glass element with a single light ray (drawn in dark
red) entering from the right. Upon striking the outer surface of
the glass element, most of the light ray is bent (refracted) toward the
interior of the glass element (i.e., leftward), while some of the light
is reflected back toward the right. Later, when the light ray is
about to leave the glass element, it again splits into a refracted
portion that continues traveling leftward and a reflected portion that
bounces off toward the right. That reflected portion eventually
reaches the far (right) side of the glass element, where it too is
subject to a splitting event: most of the light ray passes
through to the air bordering the glass on the right, while some of it
reflects back toward the left again.
Fig. 3.3.3: The
effect of anti-reflective coating.
(A) At each surface, some of the light is refracted (passes through)
and some is reflected, resulting in loss of brightness and contrast.
(B) With an appropriate coating (shown in pink), a secondary
reflection is made, which cancels the first reflection and causes
the canceled energy to be transmitted back through the lens,
improving brightness and contrast.
In a properly focused optical system, many such
light rays (like the dark red one in the image above) originating at
our subject (a bird) would conspire to form an image on the camera’s
sensor plane. In a manner of speaking, these rays carry
information about the image to be formed. Meanwhile, the various
random reflections of these
information-bearing rays would contribute to an overall reduction in
the image contrast, thereby degrading the image quality by making the
bird look less distinct in the image (because the overall information
content has been diluted due to scattering of reflected rays).
Those reflected rays that
escape back out through the lens barrel also contribute to an overall
loss of brightness in the image, and (remembering the material covered
in section 2.5) therefore to an increase in
photon sampling error and
thus pixel noise.
In part B of the above figure, we’ve applied one or
more layers of antireflective coating
to the lens (shown in pink in the figure). Precisely how this
particular medium interacts with incident light at the photon level is
beyond the scope of this discussion, but the overall behavior can be
decomposed into two primary effects, as follows. First, the
coating layer now imposes two surfaces through which the light must
pass: the outer, air-to-coating surface, and the inner,
coating-to-glass surface. At each of these the incident light
again splits into a refracted component and a reflected
component. For a well-chosen coating (i.e., one having an index of refraction intermediate
between that of air and that of the glass), the sum of these two
reflected components will be less than the reflected component that
would result from bare glass alone, so that the overall loss of light
be reduced. Second, if the layer of optical coating is of an
appropriate thickness (one quarter the average wavelength of incident
light), the twin reflected rays will be out of phase with one another, and
will therefore cancel each
other out. Because of the (sometimes
counterintuitive) way that light waves work, this cancelation not only
decreases the total amount of reflected light, but actually increases
the amount of light transmitted through the lens.
The result is
an increase in both contrast (due to a reduction in stray light from
internal reflections) and brightness (due to increased light
transmission through the glass surface).
Today, most optics from both brand-name and
third-party manufacturers have some sort of antireflection
coating. Many products boast “full multicoating”, meaning that all surfaces of all
glass elements are coated. Products described as “fully multicoated” have mutiple coats applied to
every glass surface in the lens, with different layers formulated so as
to target different wavelengths of light. Keep in mind that while
the improvement in light transmission due to coating on a single lens
surface may be only a few percent, the total improvement resulting from
coating all lens surfaces in a design with 10 or 20 glass elements can
be many times higher, so that today’s high-end optics boast
transmission rates as high as 99.9%.
In summary, special glasses such as fluorite (CaF2)
ED/UD/SLD/etc. are important for reducing chromatic aberration and
retaining sharpness, with true fluorite elements being the best (and
most expensive) and non-fluorite or semi-fluorite ED variants being
(generally) only slightly less effective (but much cheaper) than
fluorite. Lens coatings are also very important in improving
brightness and decreasing light scatter due to reflection, resulting in
higher contrast images in which the bird and all its details are more
References for this Section
1. Zeiss Inc.
of FL’s. Zeiss Press Release #003/05.
2. J. Petykiewics
(1992) Wave Optics.
Klewer Academic Publishers.