Thanks zzffnn, and again, thanks all!
Since I am not yet clear about the resistance of black paint to immersion oil, as first aid, I covered the frustum with the "infamous black insulation tape" (for certain applications, it is fine). Light leakage is mostly blocked. I centered the condenser carefully (I believe I did that OK before, but who knows).
The specimen, as before, was a strew slide of diatoms. I had prepared the slide and mounted it inverted, namely, the diatoms are very near the coverslip.
The medium is NOA61 (it has a high nD of 1.55-1.60). I also have many mounted in Pleurax but did not inspect them yet.Result: very good darkfield with the 40X(0.6-1.0) iris oil planapo objective! at fully open iris ! for NA=1.0 ! not NA=1.1, but my objective only reaches 1.0. The illumination was sufficiently bright.
I like to think that the improvement relative to the preliminary experiment is because I blocked stray light.
Here is a quick crude comparison. Single images, of a strew slide, where not all diatoms are in focus. Resized, but otherwise untouched.
I wanted to demonstrate a small difference between the condensers in the darkness of the background.
I found it easier to create a uniform darkfield across the FOV, by centration, with the Ultracondenser, than with the phase contrast condenser.
On the other hand , the phase contrast can create DF when used dry, with low power objectives.
Ultracondenser. 40X1.0 oil planapo..jpg [ 176.86 KiB | Viewed 2429 times ]
Phase contrast condenser position -D -, 40X1.0 oil planapo.jpg [ 188.11 KiB | Viewed 2429 times ]
DF is not as exquisite as DIC, but is funny and aesthetic.What still did not work was oblique at higher NAs. Failed to observe it with both objectives objective,
the 100X1.3 oil Planapo (Ph3) and the 63X1.25 oil Neofluar (Ph3) yielded a very bright image with poor contrast and resolution. This can be attributed to any of the factors listed above by zzffnn, thickness of slide or others, or perhaps because those objectives
are phase contrast.