Leitz Dialux questions: Mixing 160/170 objectives and best objectives for water organisms

[ProtistAdmirer]

Hello! I am a relatively new microscope user and have just acquired an early '70s Leitz Dialux, which I'm excited to use to advance my learning. I have a couple of questions I'm hoping to get help with:

1. Mixing 170 and 160 objectives: My scope has the 170mm tube length and I understand that Leitz issued a technical memo stating that 160 objectives may be used with no loss of image quality as long as they are at least 16x magnification. But would mixing 160 and 170 objectives result in a loss of parfocality so that I have to constantly refocus when switching objectives and may smash one of them on the slide if I'm not careful? Do I need to stick with either all 160s or all 170s for best results?
2. Best objective for water organisms: I mainly look at pond water and want a very good quality objective for viewing and photographing protists. That would point me towards trying to get my hands on Plan Apos (or NPL Fluotar?). But these will have a high NA and I've read that a lower NA objective will have a greater depth of field, which may be more useful for this purpose, since a lot of these organisms aren't very flat. What then would be the ideal objective? Are good quality lower NA objectives even available for higher magnifications? What would you guys recommend?

Thank you in advance!

[Leitzcycler]

To question 1: I have used both in Orthoplan with no problems. In priciple, you are right with the parfoaclity. The most difficult objective to focus is 100x oil immersion. So it is most convenient if you have that objective and the one with the second highest magnification (e.g. 40x) of the same tube length.

[ProtistAdmirer]

Thanks Leitzcycler. So if I used the 170 length for the 3.5x and 10x objectives, and then 160s for the 45x and 100x objectives, I wouldn't smash the lens on the slide no matter what? But I would have to make a step change in my focusing adjustment when changing from the 170 10x to the 160 45x? A few extra turns of the fine focus?

[PeteM]

There were also short barrel Leitz objectives. These require a "Plezy" adapter to be parfocal with the 45mm parfocal ones.

You might consider phase contrast objectives and a phase condenser for pond critters.

[Leitzcycler]

I checked this morning: Actually the difference in focus between 100x oil objectives of 160 and 170 is very small. So you shouldn't smash anything.

[MicroBob]

With Leitz 160 and 170mm there are 2mm tube length difference. This leads to a little less parfocality for the weaker, not the stronger objectives.
For a newbie I would recommend to keep using ordinary achromats until a bit of experience in slide making and microscopy setup are gained.

Bob

[deBult]

My most used objective for observing pond samples is by far the 20*

And yes I have that one in Dplan, SPlan and SPLAN-Apo (Olympus user) versions and going higher up the ladder is a joy to use, but even the simple 20* is worth having for exploring water samples.

[ProtistAdmirer]

Thank you all for your thoughts so far. I’ve gotten more knowledgeable even since posting this and am taking all your comments into consideration!

[ProtistAdmirer]

I checked this morning: Actually the difference in focus between 100x oil objectives of 160 and 170 is very small. So you shouldn't smash anything.

Very helpful, thanks for checking!

[ProtistAdmirer]

There were also short barrel Leitz objectives. These require a "Plezy" adapter to be parfocal with the 45mm parfocal ones.

You might consider phase contrast objectives and a phase condenser for pond critters.

Coincidentally, 2 of my current objectives have those Plezy adapters. I plan to sell those objectives.

I hadn't even considered phase contrast until you said this, and in another coincidence, I just bought a "parts scope" (mostly for the other objectives) which happens to have a phase condenser and 40x phase objective! I wouldn't have sought that out, but now that you've said this I'm very excited to try it out. I can do darkfield with it too, which is another bonus. Thank you.

[PeteM]

Darkfield (DF) would be best with a regular brightfield objective, not phase.

As you know, there will likely be a DF stop in your phase condenser. It should probably work pretty well at 10x and 20x. You might get satisfactory results at 40 or by offsetting the stop or a phase ring.

If you have a really good objective (Plan Fluorite or Apo) on a Plezy adapter, you might consider keeping it.

[ProtistAdmirer]

Good to know. Thanks PeteM!