Театр теней

Achromat, Apochromat, and Fluorite.

(Больше для себя)


For the canonical achromat with the common C/F focus and the shortest e-line focus (so called C-e-F correction), using BK7/F3 glasses for the front and rear lens, respectively, this gives longitudinal chromatic error (as the separation of the green focus from the common blue/red focus, a standard measure of secondary spectrum) as:

Δf = - (3.348732 - 4.5799n1 + 2.230453n2)f (46.1)

with n1 and n2 being the indici of refraction for the chosen wavelength for the front and rear lens, respectively, and ƒ the system focal length.

Substituting indici for the e-line (n1=1.51872, n2=1.61685 for BK7/F3 doublet) in the above relation gives Δƒ=-0.000546ƒ, or -ƒ/1832 as the separation between the green (e-line) and red/blue focus (the minus sign indicating that the latter is farther away from the objective).

In apochromatic objectives, due to their more strongly curved surfaces (apochromats usually combine glasses with more closely matched dispersive powers than achromats; as a consequence, lens surfaces (typically the inner two radii) have to be significantly more curved in order to bring color foci together), spherochromatism is often significant, commonly involving both, lower- and higher order spherical aberration. The main difference versus primary spherical is that secondary spherical cannot be fully corrected (without putting a Schmidt-type aspheric on surface), only minimized to nearly 1/6 of its magnitude, by offsetting it with the primary form.
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в краткой форме: самый оптимальнейший дизайн, основанный на классических флинт/крон стеклах, имеет теоретический предел в коррекции хроматических абераций порядка F * 0.001

это хорошо на фокусном 50мм, приемлимо на 100мм, и начинает серьезно гадить от 150мм и дальше. попросту говоря, телевики от 150мм без применения низкодисперсного стекла (ED, ULD, SLD), которое является подражанием флюориту (этот по рефракции без дисперсии вне конкуренции) - говно. вот почему так много замечательных старых объективов 50-105мм, и вовсе нет замечательных старых телевиков - Canon FD 200/2.8 стоит меньше $200, и это еще дорого за такое "чудо".