Difference between compensating and correcting optics

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
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pippo1234
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Difference between compensating and correcting optics

#1 Post by pippo1234 » Sat Jun 12, 2021 3:28 pm

Note that the field stop shows a yellow diffraction band, the hallmark of fine compensating optics. Don't let the advertising of the pushy companies fool you. ALL microscope optical systems are compensated. Some claim in the objective, some claim elsewhere. It doesn't actually matter where, as long as the optics are fully compensated. If your microscope shows a blue band, then it has correcting optics and you never know how good those corrections are!
The above is by Apo in a thread on the buy/sell forum which I did not want to highjack.

Could anybody kindly provide a reference for the distinction between compensating and correcting optics? Are Nikon CF an example of the latter? In which case I get it.

TIA

apochronaut
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Re: Difference between compensating and correcting optics

#2 Post by apochronaut » Sun Jun 13, 2021 4:06 pm

Correcting means exactly what it says. Lens or lens system A distorts or aberrates a beam of white light during refraction. A second lens or lens system is employed that causes an opposite distortion or aberration, correcting the aberrations and or distortions without affecting the desired magnification or even increasing it.. A simple example is an achromat which has residual ca, being slightly naturally corrected by a huygens eyepiece.

Aberrations and distortions exist in many forms and when a lens is made to a certain magnification spec., the aberrations and distortions that co-exist along with that particular lens formula exist to varying degrees. A compound lens or group of compound lenses could be overcorrected for a certain defect within the system in order to pefectly correct for another. A further optic in the system can be then made to be neutral for the one defect and undercorrect for the other, or compensate for the overcorrection upstream in the system.
Compensating optics are usually more complicated than correcting optics. A simple example is an apochromat objective that perfectly corrects for spherical aberration but in doing so overcorrects for lateral ca. A compensating eyepiece can be made to be spherical aberration neutral yet provide a negative ca correction or compensate for the overcorrection.


One is negative and the other positive. Undercorrected optics will produce yellow banding along the outside edges of the object. Undercompensated optics will produce a blue band along the outside of the object. Correcting optics will have a blue fringe at the field stop. Compensating optics, yellow.

Hobbyst46
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Re: Difference between compensating and correcting optics

#3 Post by Hobbyst46 » Sun Jun 13, 2021 4:31 pm

Two compensation methods for infinity corrected optics:
https://www.olympus-lifescience.com/en/ ... pe-design/

pippo1234
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Joined: Thu Dec 24, 2020 8:21 am

Re: Difference between compensating and correcting optics

#4 Post by pippo1234 » Mon Jun 14, 2021 9:07 am

Thank you both.

To summarise the distinction is mainly fluorite/apochromatic/plan achro(?) (compensating) vs run-of-the-mill achromatic (correcting). But even the latter benefit from compensating eyepieces if their magnification is higher than 40x. Am I close?

apochronaut
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Re: Difference between compensating and correcting optics

#5 Post 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.

pippo1234
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Re: Difference between compensating and correcting optics

#6 Post by pippo1234 » Mon Jun 14, 2021 10:06 pm

Wow, that was a tour de force! Thank you so much! I could never have figured out all that just by reading on or offline and, as you noted, some of the articles on the issue have, understandably, got a bit of an advertising bias.

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