Objective lens image quality

[AntoniScott]

I have several microscopes, more for the fact that I have a microscope fetish and also love the superb optical quality. The popular standard objective magnifications are 4x, 10x, 40x and 100x (oil immersion). One of my microscopes (Olympus BH2) has a five objective capability so I removed the 100x and added a 20x and a 60X objective.

I obtained an Olympus CH2 (an entry level scope with more economical objectives) that had the 4x,10x 40x and 100x oil. The objectives differ in appearance and desognation to those of the higher quality BH2.

I also have an older microscope (a Bristoline) with the short objectives that was standard with all manufactureres until the recent introduction of the very large and longer objectives that are standard on all brands today. It has the 4x,10x, 40x and 100x oil. Normally I would not have purchased a microscope with an unknown and commonplace name like Bristoline but the seller reassured me it was of high optical quality despite its age (probably 1970's, 1980's vinatge).

Lastly I have a vintage microscope (a Seibert Wetzlar) that is probably 130 years old and is optically in superb condition. It is a non-working scope because it was dropped during shipment causing bending and permanent damage. The broken cast iron base was replaced with a non-orininal piece and machined to fit the scope. However the objectives are clean, the glass is scratch-free and clear, a remarkable feature considering its age. Interestingly, the objective thread is the same as today, despite the 130 year age.

I have also had experience with modern microscopes (OMAX, AmScope, Nikon,etc), so the question is begged to be asked. Is there a better optical quality objective ?

Because the objective thread is the same in all my scopes, I was able to install similar magnification objectives on the same turret so that an accurate comparison coud be made using the same pathway between original substage light source, objective, binocular prism and eyepieces. A true comparison coud be made.

I have to say that I found NO difference in optical quality between any of the manufacturers objectives of the same magnification. There may have been a slight difference in contrast but I can't be sure. It could have been more likely to be my bias or that I was determined to see a difference, that I did not.

Lastly, the only difference that I did see was with the 40x (I think it was 40x) of the 130 year old Seibert Wetzlar). The color was superb (when viewing cell nucleii using H&E stain) and as good as any of the modern lenses. The exception was with edge abberation or focus. The cells would appear out of focus around the periphery of the field-of-view. Generally speaking, the optical quality was stunning especially considering the age of the glass and the lack of modern computer technology.

[apochronaut]

You are mostly correct in your assessment because objective lenses are made to a specification on axis and that specification is stamped on the barrel. Differences in resolution, colour correction , planarity and contrast as examples, occur outside the axis to a more or less degree and often other optical components in the system are designed to correct for some or most of the aberrations and distortions that occur in the objectives.

Your older Seibert microscope would not have had as much effort put into off axis corrections. In it's day, Huygens type eyepieces had been found to offer fairly good correction and for most average lab microscopes where cost effectiveness was important and where the user was most often observing for detail, a decently corrected narrower field was adequate. It's on or near axis performance was very close to a modern instrument. At the research level, a higher N.A. fluorite or apochromat compound system could be employed on the same stand and although considerably more expensive, it could be relied on to yield even finer detail with a slightly better f.o.v. That appears to be more in line with what you have.
It was largely due to the demands of modern pathology that the need for wider, better corrected microscope fields increased. Histopathology, Cytopathology, Neuropathology , Forensic pathology, in fact all branches of pathology require broad microscope fields , so improvements in microscope optics since the second world war have been largely, f.o.v. driven. That is where the more modern microscopes perform above the level of the old standbys. To some degree, optical coatings and the refined analysis of the physical structure and ray tracing have improved contrast in modern microscope optics so the apparent resolution is slightly better for a modern .65 N.A. objective than a pre war version usually but that is for fine optics. With cheaper microscopes that has been negated somewhat by sloppier quality control procedures. The N.A. of a 50.00 objective would be determined mathematically and then tested under a best case scenario. Many examples of the objective would leave the factory floor with underwhelming performance however, and still get sold. Add to that the potential for mechanical misalignment upon assembly and one can easily see that outside of the wider f.o.v., cheap modern microscopes might be a speculative purchase in terms of image quality.

[BramHuntingNematodes]

To paraphrase Tolstoy, every good objective is good in the same way, and every bad objective is bad in its own way.