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Microscope Optics

Live cladoceran crustacean with two eggs. Zeiss Winkel 10X achromat. The ledlyt has been raised to the highest position giving a very flat illumination. Definition is good, but shadow detail inside the body is lost.

An old Physics textbook will fill you in on the basic compound microscope. The Web pages of the major microscope makers, especially Olympus, have much useful information, although the 'infinity optics' system (see below) is promoted to the exclusion of anything else.

A basic monocular compound microscope has an objective and an eyepiece, as the Physics textbook will explain. A binocular (not a stereo) microscope has a prismatic beam-splitter mounted above the objective, that divides the image out to two eyepieces, allowing both eyes to look at the single image. If the collimation of the prisms is good, the eyepieces are at the right separation for your eyes, and you know what you are doing, both eyes can relax, and if neither eye is perfect, you will see more with both.

Any real-world objective is designed after great calculation to deliver an optimised image at just one distance behind the objective. The most common distance, engraved on the objective barrel along with the power, is 160 mm, and the rest of the microscope is designed so the eyepiece or eyepieces examine the image formed at that point. An ideal image is as flat as the subject, and in practice none are. 'Plan' or wide-field objectives are designed for a flat wide field, and cost a great deal more. As always, the cost, if affordable, is worth it. Plan is a luxury on a 20X objective, and almost a necessity on a 40X if you want to fill the field.

Older good microscopes often compensated for objective field curvature by designing a reverse-curve into the eyepiece so the result appeared flat to the eye. If the objective forms an image directly on a semiconductor chip, the second correction is lost. Fortunately as time went by, objective fields got flatter, and eyepiece correction became superfluous.

About ten years ago, the Big Four microscope makers launched 'infinity optics'. The objectives were calculated to form an optimal image at infinite distance, that is the rays coming out of the objective were parallel. A second lens in the microscope tube then focused the parallel rays into an image for eyepiece examination. The second lens could then be fine-tuned to deal with some of the distortions remaining in the objective, and a good sharp picture results. Few other microscope makers have followed the Big Four. For an explanation of the advantages of the 'infinity optics' system, see the Web pages of the Big Four. The ledlyt has been demonstrated to Olympus, and runs perfectly on 'infinity optics' models, for eyepiece use.

The disadvantages of 'infinity optics' are as follows:-

Since the second lens is a computed part of the system, you are tied to the original maker's objectives, and the objective standard thread has been discontinued by some, to emphasize this. The second lens is likely to defeat attempts to mount a microscope camera in the right place, so again you are tied to buying a camera system from the original maker of the microscope. The click-stop objective changer delivers nowhere near perfect optical centring of the objective, so concentricity with the second lens is compromised. I would of course take an 'infinity optics' microscope if you paid me, but would sell it the same day and get an older classical machine.

If you want to do both visual work and photomicrography, you will need an older 160 mm type microscope, and if you want really good optics, that will now limit you to secondhand gear. You can then join the club, desperately hunting for a really good secondhand 40X 160 mm objective that will not require a bank loan.