wickerprints
Offline
• • • •
Upload & Sell: Off
|
 p.2 #7 · What does fluorite actually do? | |
Fred Lindsey wrote:
Am I right in assuming that fluorite elements are not pure crystalline, or are they?
No, they are crystalline.
So a fluorite element does not have to be aspherical?
As I mentioned in my previous post, the use of fluorite and the use of aspherical elements relate to the correction of two different types of aberrations (chromatic and spherical, respectively). The former is a function of wavelength; the latter is not. That is to say, if one had a lens that was composed only of spherical surfaces, and the incoming light was of a single wavelength, you still would see spherical aberration manifest as out-of-focus areas of the image.
Actually, looking at the element combinations in Canon's lenses, would having an aspherical lens in a Super-telephoto be so difficult to manufacture because of the fact that it would require such small differences from a spherical element that they don't use it, because the current manufacture processes would bring adverse effects to the lens' performance?
Large aspherical elements are difficult to manufacture because the process by which they are made does not scale well with size. Canon employs two methods for aspherical element production--grinding and casting. In grinding, a rough blank is precision ground to the desired shape, which has been modeled by computer. In casting, soft glass is pressed into a pre-fabricated mold to give the desired shape. Both methods then require subsequent polishing. The precision required for aspherical elements makes both methods difficult to scale.
But the real reason why the super-telephoto lenses don't use aspherics is not due to difficulty of manufacturing. It is because from a optical standpoint, they are not needed. Aspherics are used in wide-angle primes and zooms, in which the angle of view is large enough that spherical aberration becomes pronounced. By the nature of telephoto lenses--i.e., their small angle of view--spherical aberration is naturally low.
I'm assuming then that fluorite is separate to Aspherical elements, but the combination could, in theory, produce the better results than just fluorite alone.
Because chromatic and spherical aberrations do not necessarily occur to the same extent in a given optical design, it is not generally the case that both technologies need to be used to correct for them. There are other aberrations to consider as well (coma, astigmatism).
I think I understand extraordinary partial dispersion now - is this just the reduction of the refractive index at the outer points of the element to reduce the difference between the effective focal lengths of the red, green and blue light rays, and thus reduce chromatic aberration especially at the corners, and induce a higher degree of visual sharpness?
No. The refractive index is a function of wavelength, not material thickness or geometry. See Snell's Law. Extraordinary partial dispersion, as mentioned in my previous post, refers to atypical differences in refractive index as a function of wavelength, compared to other types of glass. Most glasses share similar dispersion curves--if you plot wavelength on the x-axis, and refractive index on the y-axis, then you will see similarities in the shape of these curves for a variety of silicate glass. Fluorite, in addition to having lower dispersion than most glass, has a different shape to its curve. The different shape means it is possible to selectively correct portions of the visual spectrum with a combination of elements made with different dispersion characteristics.
So, if I research and write about the structure of CaF2, am I also then talking about Fluorite, as they are the same thing.
For the purposes of discussing fluorite in optical lens design, yes.
Although not as important, I would still like to know if DO elements are also fluorite.
No. Although DO and fluorite elements are both used to correct chromatic aberration, they do so using different physical properties. The DO lens uses diffraction, not extraordinary dispersion.
Edited on Nov 08, 2009 at 03:31 PM · View previous versions
|
his further research.