#5
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by apochronaut » Mon Jun 14, 2021 1:18 pm
Not exactly. Curvature of field can be corrected for with little or no effect on other aberrations or distortions, so plan achro objectives are still achromats and therefore not necessarily do they benefit from any compensation carried out in the eyepiece in a fixed tube system. In fact, the opposite might be true, unless their planarity was specifically dependent on a complementary eyepiece, usually referred to as plan compensating, a somewhat different thing than compensating.
The Olympus example above is kind of an advertising page for Olympus and specifically references infinity systems and wierdly a laser guidance infinity system to boot. I don't know about you but I am not in the habit of turning my microscope into a laser gun : I am more interested in having just a microscope, and there is no convincing argument that having an objective that is internally compensating is better. Corrections and compensations still have to be applied and as long as the image arrives at my cornea with as complete corrections and or compensations as possible, it is all good. If that weren't the case , then Zeiss and Leica and Meiji and the emerging companies from China, might as well just give up. It is the CHOICE of Olympus designers to proceed in that fashion, to develop an integrated objective system. They can also appeal to macro photographers, an emerging market, and go head to head with Nikon and Mitutoyo in that market.
There was a bit of a revolution in the relationship between achromat and apochromat at the time that infinity optics arrived.
Up until that time, which actually took place back in the 30's but from a practical market perspective around the early 60's: achromat objectives used eyepieces that could apply corrections to remove some residual undesirable optical artifacts. The huygens design had been found many years ago to apply some corrections for c.a. , Kellners as well but with the addition of some extra field. Ramsden provided an extra f.o.v. but at the expense of poor ca control. In all cases though, field curvature was a problem. Eyepieces that provided a degree of correction as well as flatter fields slowly became available, with names such as Planoscopic, Periplan . They were expensive.
However, for apochromat designs, all of those eyepieces were woefully inadequate although the proprietary designs in some cases were o.k. Apochromat objectives required integrated overcorrections and then subsequent undercorrection( compensation). Complementary proprietary eyepieces were designed, usually called compensating or Comp. or C or in some cases K but with the tradeoff that in order to have complete corrections and a flat field , the f.o.v. would be small ; smaller than an achromat f.o.v. but much cleaner across a flatter field. Apochromat designs with compensating eyepieces provided narrow fields of view until the 1960's.
Compensating eyepieces were recommended for photography with both achromat and apochromat systems. Why were compensating eyepieces not recommended for visual viewing with achromats? It is because, on or near to the axis, there is little or no correction or compensation required with either type of objective. Most of the nastiness takes place at the periphery, where the eye would see severe inverted ca when using compensating eyepieces with achromats but the camera field could be adjusted to exclude the periphery, while taking advantage of the compensating eyepiece's flatter field and usually slightly sharper resolution near to the center of the field.
When infinity optics came along, it was at a time when new glass formulas allowed for improvements in planarity and achromat corrections. It also allowed for the possibility to include overcorrection in an achromat, as grappling with wide field planarity became popular. The tube or telan lens, then the eyepiece, could provide additionsl corrections and compensations if necessary, thus achromat and apochromat could be designed to take advantage of complementary lens designs downstream. From that point on, both achromats and apochromats could use common eyepieces because they could be tuned to accept the same downstream optics. All eyepieces were part of a compensating system. Even eyepieces that are neutral are part of a compensating system because they need to preserve that system. They need to be neither positive or negative. Just magnifiers and flat.
Last edited by
apochronaut on Tue Jun 15, 2021 12:31 am, edited 1 time in total.