Microscope Objectives and Finance
The objectives we would all love to throw away are the 'achromats'. These are corrected for two colours (red and blue) and are calculated for maximum sharpness in the Green. I am not so sure I understand that either. The theory thus goes that if you can put up with a monochrome image from an achromat and want maximum sharpness, do not use a blue filter, as you would expect for the shortest wavelength, but a green filter, where the lens will perform best. The 4X objective is normally a simple two or three element cemented achromat, since at this power nothing better is needed. A 10X objective needs a few more elements and an air space, but a good quality achromat will still suffice, both for colour correction and field size. We have a superb old Zeiss Winkel 10X, that is neither Plan nor fluorite, and performance is adequate, to say the least.
The dearer lenses start to shine at 20X, and are close to obligatory at 40X. For photomicrography, anything higher power is not needed, and too close to the specimen. Fluorite lenses are corrected for three colours, and although they should be specialised lenses for UV work, are such a jump ahead of the achromats that they represent the highest quality affordable objectives. Zeiss make fluorite objectives as normal (neofluar) or wide angle (plan neofluar). I have had a good look through a Zeiss 40X plan neofluar 160mm, and the image looked as sharp as a more normal 25X lens. No, the owner is not selling. Recently low dispersion glasses that rival fluorite, without the engineering problems, have become available, but on 'infinity optic' objectives only.
The manufacturers, normally eager to sell highest cost items, advise the use of apochromats only for photography, rather than visual work. You can correct a lens for as many colours as you like, by raising the number of curved glass surfaces. The apochromats are corrected for four colours, and represent the giddy limit. Not only do you have to find 3000 US dollars and upwards, you also have to find a 160mm example. Think of smashing that through a slide by mistake. The Russian optical industry makes cheaper apochromats. If you know of a good sharp Russian objective, please let me know.
The wider the aperture of an objective, other things being equal, the higher is its theoretical resolution, according to the sacred equation I have forgotten already. The aperture ratio is stamped on the barrel of most objectives too, and should be as low as possible for a high NA. Most real-world specimens are not two-dimensional, and of course the wider the aperture of a lens, the shorter its depth of field, so sharpness gains are made, but less is in focus. When you stop down a correctly-set condenser, you are of course stopping down the objective. Unless you are a big-budget, status-chasing Academic, in my opinion forget high NA. If it is a sharp lens for its aperture, that is enough to ask for.
In conclusion, if you are offered a demonstrably sharp 160mm Plan objective between 20X and 40X at a ludicrously high price, pawn the Silver and buy it.
Note with all these fine objectives that the edge distortion as shown in the chlorella photo is still present. The current 6.4mm x 4.8mm sensor area is really pushing all these objectives. If the frame is not 'overfilled', planapo definition is superb. Powers of 40X, 25X, 16X and 10X are all essential so the frame is not overfilled before changing down to the next lower power.