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.



 
The important thing to understand is that each lens element bends (or refracts) the incoming light in a prescribed 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 above). 
    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 different 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. 




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 technical reaasons. 
    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 ions^[1].
    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 some 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 product incorporating 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. 
    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 would 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^[2].  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%^[3].
    In summary, special glasses such as fluorite (CaF2) and 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 visually distinct.

References for this Section

1. Zeiss Inc. (2005) Definition of FL’s.  Zeiss Press Release #003/05.
2. J. Petykiewics (1992) Wave Optics.  Klewer Academic Publishers.
3. http://www.canon.com/technology/canon_tech/explanation/thin_film.html