qualitative difference between infinity corrected objectives

[microb]

So Olympus is 180mm, Mitutoyo is 200mm, and Edmund Optics/ThorLabs has 150mm. Somewhere I'm told that the large that number the more stable the image is for putting a sensor somewhere along there. But I never really understood that.

[PeteM]

Nikon makes the case that longer reference tube lengths have fewer off-axis aberrations and that the ideal would be between 200-250mm: https://www.microscopyu.com/microscopy- ... al-systems.

Olympus claims 180mm and keeping the DIN 45mm parfocal is the best compromise, all things considered.

I haven't seen a rationale or defense of Zeiss' 165mm reference tube length, but researchers seem to like their infinity scopes - especially the inverted ones.

What I can say is that comparing plan apos and plan fluorites head to head on Nikon Eclipse CFI, Leica DM "HC," and Olympus UPlanFl and UPlanApo - they all look good. Both Nikon and Olympus went through two major infinity objective generations, with better coatings and deeper UV transmission. For visible light, I don't see much difference in the few cases where I have a generation 1 and generation 2 example. My somewhat older Leica HC and HCX Plan Fluotars and one (60x) Plan Apo hold their own - perhaps with even better contrast than Olympus. I don't have Zeiss infinity to compare.

Two hundred mm does mean a bit more length and bulk in the head. Nikon's 60mm parfocal distance also adds height. To preserve low and ergonomic controls, the stands of many makers were redesigned, with both Nikon and Olympus claiming gains in ease of use.

How much difference this makes - I don't know. Nikon, Leica, and Meiji went with 200mm reference tube lengths. Olympus, Motic, and most of the generics are happy with 180mm. Zeiss' is mostly 165mm, except the Motic-made "Primo" line at 180mm.

[Scarodactyl]

Thorlabs' imaging objectives are 200mm (though many of the specs like parfocal length are not standardized).
I agree that it doesn't seem to be a major quality concern.

[apochronaut]

I think a lot of the choice of length was partially historically driven, becoming less so over time.

It is important to remember that the infinity reference length is in almost all cases entirely in the head, affecting the head size and physical projection. The parfocal length and length of the infinity space is only relevant in so much as how much extra length can reasonably be added in there while still maintaining a manageable architecture. The main reasons any specific company settled on a given reference tube length are a combination of design/ergonomics of the viewing section while keeping in mind any potential extension caused by infinity space accessories. in some cases also, balancing an attempt at allowing some degree of reverse compatability with later finite objectives might also have been a factor, while tweaking certain of those designs into an infinity format without ripping off the patents of others, something the Japanese struggle with culturally.
I wouldn't be surprised if certain later generation Zeiss 160mm objectives work in the 164.5mm system. When a company makes a wholesale system change as all of the major companies that persisted until about 2000 did, they are inclined to maintain an ergonomic profile fairly consistent with their most recent designs , so a compact reference length would be preferable stylistically in certain cases. AO for instance, already had a 160mm finite microscope that had a negative tube lens, allowing for an physical extension of the tube and head. Pegging the new infinity system around 180mm ( 180mm + focal plane distance) to some degree allowed for an consistent design profile Then they stuck with it.

There does seem to be a consensus that getting out to 200mm or so is somewhat optimal, so it was a balancing act. No doubt such considerations played a role in Leica tweaking the AO system that they inherited, out a little longer but obviously 182mm worked well. Try out any AO or Reichert 45mm parfocal planapo or planfluor objectives even from the 1970's and you will know. It seems that companies that came to the concept later and later models were less conservative in the choice of reference length, building the microscope around the reference length, rather than incorporating the refrrence length into a design that had 160mm tube aspects to it. The Chinese mostly followed what Olympus or Nikon wanted out of them. I have seen some Chinese and possibly Indian stuff that used the AO/Reichert 182mm length but very little. It seems that system might have been purchased from Leica.

Interestingly , the first widely sold infinity systems used Jentsch heads, which alter the tube length when the interpupillary distance is adjusted, so are not a first choice for an infinity system.. AO used a Jentsch head in the series 10/20 ( 1962-1980) which incorporated a travelling tube lens following the expansion and contraction of the eyepiece width. From 1980 to 2002, AO/ Reichert used Seidentopf heads.
The B & L Balplan/Flat Field Dynazoom group of microscopes don't get referenced much as an infinity system but it was, starting around 1965- 1992 or so. The tube lens in the Balplan head is buried up inside the head and travels with the eyetube travel as well, maintaining a precise reference length.
I have never measured the Balplan reference length but it is probably shortish too. I will someday, since I have never seen it published, not to mention the fact that B & L in the 1970's wanted to distance themselves from AO, the infinity correction company in their back yard , at the time.

edit. Bausch & Lomb used 156mm. The tube lens is located between the deviating prism and the beamsplitting prism and thus, does not act on the photo tube at all.

[Hobbyst46]

I haven't seen a rationale or defense of Zeiss' 165mm reference tube length, but researchers seem to like their infinity scopes - especially the inverted ones.

If "research" in this context means biology, biomedical studies, biophysics, clinical research etc, then IMHO researchers in these fields are much less interested in the mechanics and optical details of the microscope, especially since most top-level microscopes do not have very prompt unique hardware advantages (my personal feeling). Differences in software (Leica vs Zeiss, etc) do exist and are even more important than hardware differences - except for price, obviously. Daresay that fundamentals of microscopy in the sense that are discussed here are not really the concern of top edge researchers in the above-mentioned fields.
Just my opinion.

[apochronaut]

They sometimes have brand adherence based on previous experience and might search out ,required options on that basis.
I agree though, software plays an ever increasing role in system choices, less so any perceived optical superiority.
The success and continuation of companies like Thorlabs , Seiwa and Navitar are indicative of this. They offer selected solutions mostly, not often systems.