MicrobeHunter.com Microscopy Forum

My Optiphot has a phase contrast condenser with a dark field stop. The regular plan objectives give decent dark field through 20x with that single DF setting. Today I tried the setup with a 20x plan apo objective and discovered that it would not give DF. Instead it gave oblique illumination that looked more like DIC. Around the edges the image looked somewhat more like DF. Moving the condenser up and down did not remedy the situation, nor did cranking up the light intensity.

I assume plan apo objectives can give decent DF, so maybe the plan apo objective needs a different size DF stop than supplied in the phase condenser?

I assume plan apo objectives can give decent DF, so maybe the plan apo objective needs a different size DF stop than supplied in the phase condenser?

One needs to match the darkfield stop to the NA of the objective rather than the magnification or the type. Larger stop should do it.

You may need to DIY or obtain a dedicated darkfield condenser

I also encountered this with my 25x 0.65 NA objective. As 75RR states you'll need a larger custom patch. On my microscope I noticed a difference of just 1mm in radius had a large effect, and the margin of error for good contrast and dark background was narrow. Careful condenser height adjustment was needed to get even illumination, and can be a fiddly to get right.

I would suggest you try oiling the condenser to the slide (although the objective is used dry) and see if this improves the DF for NA's of 0.4-0.7.

Thanks, all. I'll try a larger stop, probably in a dedicated DF condenser.

Oil would be a good idea too, but due to the clean up required, I've shied away from using it much.