Mt Spokane Photography said:
Pitbullo said:
It may be a stupid question, but what is DO design compared to non-DO?
Its pretty easy to google this, the subject is pretty difficult to explain in detail. Canon's recent DO patent used in the new 400mm DO uses two DO surfaces which are bonded to regular lens elements and placed very close together. It works better than the traditional DO method of designing a lens.
Basically, Diffractive Optical lenses can successfully bend light more sharply than glass lenses. This means a lens can be made shorter. A shorter lens weighs less. Diameter is not going to become smaller, it might even get larger. This is a over simplification, but you get the general idea.
Something worth noting is that the thing about diffractive optics that makes it especially appealing to use at long focal lengths has as much to do with its inverted dispersion characteristics, and not merely its refractive power.
What I mean by this is that yes, a diffractive lens element can achieve greater weight reduction for a given refractive power, much in the way that a Fresnel lens element is less massive than an equivalent smooth convex lens of the same material. This facilitates the placement of a diffractive element with higher net refractive power in the optical path without requiring a lens of higher curvature.
But if this were the only part of the story, then why not simply use a material with higher refractive index? The answer is because of the adverse effect on dispersion, which typically increases with refractive index, and becomes increasingly hard to control as a function of focal length.
Dispersion, as one may recall, is the variation of refractive index as a function of wavelength. A lens with high dispersion will exhibit a greater difference in the angle of refraction of red light compared to violet light; whereas a lens with low dispersion will show a smaller difference. The creation of a rainbow by refracting white light through a prism is the dispersion phenomenon in effect.
Dispersion obviously creates problems for panchromatic lens design because the difference in refractive indices will result in chromatic aberration, which can be broadly classified into two coexisting types:
Axial: the plane of sharpest focus varies with wavelength.
Lateral: the focal length (image magnification) varies with wavelength.
In practice, every properly designed lens exhibits some combination of the two simultaneously. In lenses with particularly fast apertures, axial color predominates because the depth of field is so small, allowing the observer to see the effect (which is why it disappears when stopped down). But in lenses with long focal lengths, the latter predominates, and it is independent of f-number.
The correction of dispersion is accomplished by using glass of different refractive indices in different power combinations, thus allowing the partial compensation at the extremes of the spectrum. Further correction (apochromatic design) is accomplished by introducing materials with anomalous dispersion--in particular, fluorite, which not only has low dispersion, but its distribution of dispersion is different than other glasses (a detailed explanation is available for those who are interested).
However, the problem with fluorite is that the refractive power is low. So you'd have to use a lot of it--which is expensive--to achieve good correction for long focal lengths, which is why the EF 800/5.6L IS was designed with two large-diameter fluorite elements.
So back to DO. Why use DO? The reason is that while most glass (and fluorite) have dispersion characteristics such that index of refraction decreases with increasing wavelength (red light is bent less than violet), a DO lens is the reverse: red light is bent MORE than violet. This is a gift: because of this inversion, a DO lens can be simultaneously used to increase refractive power (shortening the barrel and reducing element mass), but it also facilitates this to the benefit of correcting CA. You end up using less fluorite (see the EF 400/4L DO IS design), and yet you get apochromatic performance. It's a win-win.
Well, not really. Nothing in lens design comes for free. The problem with DO is that manufacturing tolerances have to be extraordinarily tight, and even then, there is increased potential for reduced contrast and increased flare. Canon, to their credit, has done a remarkable job of mitigating these downsides through refinements in their manufacturing processes. But DO is always going to be a more expensive technology compared to traditional glass.
Anyway, that is the super-long, most-of-the-gory-details explanation. It's a combination of dispersion and refractive power that comprises the appeal of diffractive optics. It's almost uniquely suited to long focal length designs.
