Darkfield condenser

[Scoper]

I note that some of the Chinese scope retailers offer two types of darkfield condensers…one that covers a higher range of n.a. of 1.36-1.25 while the other covers the n.a. lower range of 0.7-0.9.

While I recognize the higher range n.a. condenser is used with higher mag objectives, is the lower range n.a. condenser worth having?

Thks

[zzffnn]

The dry (air spaced, without needing to oil condenser top lens) 0.7-0.9 condenser is for convenience and cannot be used for high NA objectives. To me, they are not that useful. I cannot speak for other people.

[apochronaut]

With very few exceptions and the most common one is the Toric DF condenser supplied from about 1970 through to the early 2000's by AO and Reichert , DF condensers break down into those two types. The only reason for this is due to the illuminated field each can provide. Theoretically, an oil DF condenser could be used for all objectives but typically high N.A. oil condensers can only usually illuminate a maximum field in the range of 750 microns. For any field greater than that you need a dry , lower N.A. condenser which have a wider field of illumination.
What this means is that you can generally use an oil D.F. condenser with a 25X objective and up, if your f.o.v. is 20mm or less ; depending on the individual characteristics of the specific condenser. Since most mfg. don't make a 25X anymore and in the past, DF performance with an oil condenser was one of the reasons for the existence of 25s instead of 20s, in the modern world, an oil DF condenser is therefore limited to use with 40X objectives and up. Dry DF condensers can be used up to 20X and if it's lower N.A. limit is high enough, also with a 40X but not always. Although the field of a dry DF condenser should cover that of a 40X objective, the lower N.A. of the condenser can be too low for a higher N.A. 40X. Some 40X have an iris, to accomodate the requirements of a DF condenser.

[zzffnn]

For wider field coverage, DIY darkfield will work: get a typical Abbe condenser, remove its top lens element, keep its lower lens element, and put a coin at back focal plane of its lower lens. If I remember correctly, this will cover view field of 4x objective.

For even wider field, such as 2x objective, remove condenser, shine light from side and / or above, then put a dark background far away (where condenser was).

[apochronaut]

You can also use a phase condenser usually. One or more of the larger phase condenser diaphragms will work as a DF condenser with certain lower magnification objectives, 10X and lower. I think with one AO system I could get DF with a 20X as well by using the 20X phase objective with the 97X phase diaphragm. The DF is of a better quality than using an abbe condenser because phase condensers are usually more highly corrected and you get the benefit of being able to precisely center the condenser with the phase alignment mechanism.