oil condenser questions

[Leitzcycler]

Oil condenser is completely new thing to me. During my university years I never saw one or even knew such a thing exists. I think most people didn’t either.

High aperture condensers for Leitz Orthoplan are rare and expensive. There seems to be oil condenser of 1.25 NA as shown e.g. here viewtopic.php?t=7775
I have only the standard condenser of 0.9 NA. I assume I would get more out of my Apo 100 1.32 objective with oil condenser. However, I have a Lomo condenser of 1.4 NA exactly like this one https://www.ebay.com/itm/LOMO-condenser ... Sw3AFcp4IJ
Well, would it be possible to use the Lomo condenser in Orthoplan? I did a preliminary experiment by holding the condenser in my hand and using a PL Fluotar 40 0.7 objective the sample looked quite as good as with the standard Orthoplan condenser. It would be possible to make an adapter to fix the Lomo condenser into the condenser holder. However, as dovetail and centering system must be made and this needs some hours work, I would like first ask it this idea would really work. I would highly appreciate if someone have time to aswer a couple of stupid questions as follows:

1. Why Leitz produced oil condenser with 1.25 NA while many other microscopes use 1.4 NA condenser?
2. How much more I can actually see in practice with 1.25 or 1.4 NA oil condenser compared to standard condenser with Apo 100 1.32 objective? Is the difference worth of investing an original oil condenser or making a compensatory part from some other condenser?
3. The Lomo condenser does not state if it is actually ment to be used with or without oil. There should be letter M (=масло) as there are in Lomo oil objectives. However, no M in the condenser. Despite of this, is it safe to use oil or does the oil and cleaning fluid ruin the lens? This condenser seem not to have an oil cavity around the front lens.
4. If the Lomo condenser must not be used with oil, would it still give some advantage over the standard one (at least it has an oblique system)? Are dry 1.4 NA condensers common?
5. The front lens diameter is rather big in Lomo condensers while it is rather small in Leitz condensers. Why? Could this be explained in optical terms?

[BramHuntingNematodes]

There's any number of mechanical contrivances one could think of. In any case, rest assured that any condenser with an na higher than 1 is an oil condenser unless specifically stated otherwise.

[mrsonchus]

Leitz did also produce a 1.4 (oil) condenser for the Orthoplan - I had one with my Orthoplan.

[ImperatorRex]

For brightfield observation it is often common practice to close the condensor aperture anyway to something like 2/3 rd.
So full condensor aperture of 1.4 would anyway be reduced. The n.A. of a dry condensor, 0.9 n.A. is therefore very often quite adequate.

Calculation for a 1.3 n.A Objective with dry and immergated condensor aperture: you will get something like 400nm better resolution (16%)

[apochronaut]

Rayleigh's Criterion is used to calculate the effect of a dry condenser on an oil immersion objective. For a .90 condenser and a 1.25 objective , the effective N.A. of the objective works out to be around 1.14 or so.
The principal negative effects a standard condenser has on high N.A. objectives are off axis aberrations. For this reason the default condenser most microscope companies began offering was a high dry achromat/aplanat, which despite lowering the N.A. of an oil immersion objective , corrected much better off axis than an oiled 1.25 abbe. It was also becoming apparent that many users were not taking the time to oil the 1.25 abbes, so a .90 achromat/aplanat was a solution.
If someone did want to oil the condenser, a 1.25 was still available and some companies upgraded the abbe to an abbe aspheric, which is about as good as an achromat/aplanat.

Condensers of a higher N.A. than a 1.25 abbe aspheric can provide are definitely valuable with objectives over 1.25 N.A. I use an oiled 1.4 N.A. achromat/aplanat with 1.30, 1.32 and 1.40 oil objectives and notice a dropoff in resolution when I use either a .90 achromat/aplanat or a 1.25 abbe aspheric. With a 1.25 planachro, I do not notice any advantage to using a 1.4 achromat/aplanat over the abbe aspheric or dry achromat/aplanat.

One thing to be aware of is, even if a condenser has an N.A. of 1.4, if it does not indicate whether it is an achromat or an aplanat; it probably isn't. It is probably a high N.A. abbe type, which will still render it susceptible to off axis aberrations. It is likely then, that although it may be a slightly better condenser than a 1.25 abbe, it may only outperform a .90 achromat in the central part of the image. High N.A. abbe types were used with high N.A. non-plan objectives. Their performance usually falls short of expectations for high N.A. plan objectives , where an achromat/aplanat or aspheric might give a somewhat reduced N.A. but more highly corrected illumination field.

[Leitzcycler]

Thanks for valuable information!

Now another question: What is this and for what purpose?
https://www.ebay.de/itm/Leitz-Special-K ... Sw8X9cm4w1

It looks like you could put an objective here upside down to function as a condenser. How about inserting a high aperture oil objective here? Would it work as a subtitute for oil condenser?