Looking for opinions

[Harold]

Now that I've has some time to experiment with Saul's dark field adapter for my Olympus BH2, I'd like to get a dedicated dark field condenser. There are 2 such condensers made by Amscope that will fit nicely in the condenser holder of the BH2, one is dry, NA 0.9, and the other is oil, NA 1.35 - 1.25. My most often used objectives are an SPlan 20x, NA 0.46 and an SPlan 40x, NA 0.7.

The question is which one would serve my purposes best. I'll be doing some photography with which ever one I ultimately chose. I'm open to suggestions. Thanks.

[PeteM]

If those are the objectives you'll be using, the dry one with the 0.9na would be my choice.

[Harold]

Thanks Pete. Your input is appreciated. I know the dry would be easier to use and the numerical apertures add up, but I wondered if the oil version would offer any useful increase in resolution even with the comparatively low NA of my objectives.

And of course there is the lower cost

[LouiseScot]

Thanks Pete. Your input is appreciated. I know the dry would be easier to use and the numerical apertures add up, but I wondered if the oil version would offer any useful increase in resolution even with the comparatively low NA of my objectives.

And of course there is the lower cost

Hi

I think the NA of the objectives you are using will limit the theoretical resolution. With an air coupled condenser the overall max resolution will always be limited to the max NA of 1 (air n =1). Resolution depends on wavelength and on overall NA which, in turn, depends on refractive index: NA = nSinθ where n = refractive index and θ is the half angle of the light cone presented to the lens in question. So you need a medium of refractive index > 1 between objective and slide, and between condenser and slide, to get the benefit of overall higher NA, and hence theoretical higher resolution (water n = 1.33, immersion oil = 1.52, glycerol = 1.47).
I bought an AmScope DF air condenser some time ago and it worked fine. However, I did wonder whether the oil DF condenser might be a better choice. However I'm not 100% certain how well it works if just used as an air DF condenser? I can't see any obvious reason why it couldn't be used for both, if needed. Perhaps extraneous light could be a problem though it would seem a simple matter to add a suitable aperture stop? Maybe if someone on here who has an oil DF condenser can enlighten? Of course, AmScope also flog an expensive 100x objective with an onboard iris for DF. I wouldn't buy one of those myself... Both the DF condensers themselves seem overpriced for what they are. It's worth bearing in mind that you may need a brighter light source to get the best DF results, whichever condenser you have.

Louise