#8
Post
by apochronaut » Thu Apr 29, 2021 3:00 pm
Colour correction and resolution are quite independent factors in an objective but reduced levels of either or both interacts with the other, potentially overlapping and obscuring one another.
An acceptable but not perfect analogy is the effect on the image of a coin in the bottom of a pool. The airy disc separation mentioned above is like dropping two stones in the water creating interacting ripples, representing diffraction. The chromatic aberration and spherical aberration are represented by the displacement of the image of the coin, relative to it's actual position as you view it from above. The ripples( diffraction) can obscure the image of the coin. The displacement will distort it's image, even if the stones were not dropped. The two together is a disaster.
If we were to replace the water with some sort of clear syrup, then the ripples would be much less or non existent and they would not obscure the image of the coin as much. This represents an increase in N.A. However , the dispersion of the light distorting and displacing the position of the image of the coin, would only be reduced if the refractive index of the syrup was closer to that of air, than water is.
In lens systems that critical component , the magic syrup in the above analogy, has traditionally been low dispersion fluorite glass. More recently, some companies are using proprietary synthetic fluorite and probably/possibly others . Fluorite, being low dispersion, causes less splitting of the component wavelengths of the light and less spherical aberration. This allows for superior colour correction with fewer lens elements than would be required in an equivalently corrected objective using conventional glasses . For even better colour correction, add those lens elements back in with some more fluorite and you have an apochromat
What this means is that in a lens system, where a glass lens of standard dispersion of a particular magnification would need such and such a correction after the light passes through it, a low dispersion lens would need less correction. This gives the lens designer more flexibility in design possibilities with fewer elements to produce a compound lens with lower chromatic aberration. An offshoot of that is that because increased angles of incidence to a lens( higher N.A.) produce greater dispersion, a higher N.A. lens can be made with lower ca, when low dispersion glass is employed. The situation is more severe the higher in N.A. one goes and the higher in magnification too, the latter because the lens curvatures are more severe.o
So, essentially a higher N.A. and better colour correction go together because of the use of low dispersion glass but they are solutions to somewhat different technical problems created as the light passes through the glass.
Fluorite glass can hide in objective lenses and not be revealed by the manufacturer. There seems to be a certain design ethic or maybe also performance level that an objective that qualifies as fluorite must have. Apochromatic objectives utilize many design tools, including low dispersion glass, so are not usually referenced as fluorite. Terms such as Plan Fl.apo, Plan semi-Apo, Super Apo or Advanced Achromat might belie the fact that the mfg. has varied somewhat from their previous practices in the lens formulas and included more or less low dispersion glass in the formula.
There is nothing to stop a mfg. from making a higher N.A. than normal achromat or a lower than normal N.A. fluorite or apochromat and examples of such exist. There is a 40X .65 Lomo apochromat out there, which would be a joy to use with variable thickness cover slip samples. I own and sometimes use a very odd 60X 1.25 oil immersion Spencer objective from the 1940's. There is nothing to indicate that it is anything but an achromat but in performance it does lean slightly towards the fluorite class. Objectives that are not described as fluorite but have a higher than normal N.A. , usually have one or more elements of low dispersion glass in order to assist in the design goal of a higher N.A. , which will almost always additionally allow for better colour correction, a performance characteristic that higher N.A. objectives almost always possess.