Kohler question: Zeiss Standard

[littlejackal]

Hi everyone,

I've finally cobbled together enough of a Zeiss Standard scope to start some functional viewing. I'm trying to set up the scope for kohler illumination but I'm seeing a few oddities that I wasn't expecting to encounter.

I've been following the following video:

https://www.youtube.com/watch?v=n00nXlGg2fc

No problems at the beginning -- stage to highest level, condenser to highest level, pop slide in and move stage down until focus is achieved. Stop the field stop diaphragm down to smallest aperture. This is where the questions start. The video mentions that the field stop diaphragm would likely be out of focus and the condenser should be moved down until I see crisp blades. My condenser is at its highest point and I'm already seeing a fairly sharp image of the diaphragm. If I move the condenser down everything defocuses. If anything I feel the condenser should be moved UP a little to achieve a little MORE focus but that's not an option.

That aside, let's assume that the diaphragm is crisp enough as is at the very top. I move the condenser around until it's fairly centered and then expand the field stop diaphragm to its widest, however it doesn't even come close to touching the edges of my field of view at its widest aperture. I feel there's probably a good 10-20% of view that isn't lit.

This is kind of where I stopped because I'm concerned that I either missed something, or perhaps I have cobbled the scope together from incompatible parts etc.

I'm happy to describe the parts used -- I'm working on finally taking some nice photos so I can post a "my scope" thread to point to in the other subforum but I'm not at that stage yet. Just curious if anything here pops out as suspect.

[Hobbyst46]

I did not see the clip you linked, just the starting frames, but here is:
If you are using the 10x objective, some condenser settings do not provide illumination of the complete FOV, coast to coast. Perhaps that affects your impression that the field aperture is not open.
1. Turn on the lamp and look down at the field aperture (not through the eyepieces - just look straight at the aperture). Try to open it completely then close it completely. If it is not faulty, it should be possible to achieve a fully open status; when closed, a narrow opening remains in the center - about 2mm diameter. If these requirements are met, the field aperture itself is OK.
2. Supposing you use the 10x objective. And the basic condenser with a flip-out top lens. Look thorugh the eyepieces and see if switching the top lens in and out changes the illuminated fraction of the FOV (it should). It should be switched in for high magnifications.
3. If there is an auxiliary lens under the condenser, move it in before you set up Kohler. It should yield near-complete illumination of the FOV. Again, this lens is useful for the low-mag objectives (because of their low NA).
4. The field aperture blades can be in focus even if the condenser is not at its top position. BTW, the top position of the condenser is usually limited by a tiny stop screw, barely visible under the stage. When I set Kohler with my 10X0.30 objective, the condenser is slightly below maximum height.

[littlejackal]

This is good info, thank you. I'm going to take another crack at this when I'm not as preoccupied with the day job. I think this answered some of my questions, the remainder I'll try to illustrate with photos as well.

[75RR]

.
Something to practice on:

https://www.microscopyu.com/tutorials/kohler

[MicroBob]

Basic condensers don't give the sharpest and most contrasty image of the field aperture, this might lead to the conclusion that something is wrong. It isn't and you will get a good if not perfect image of your specimen.