There are two types of condensers. The ones that needs oil immersion and the ones that is “dry” or without immersion oil, to be able to attain maximum resolution.

A darkfield condenser makes sure that no direct light can enter the objective. This is obtained by creating a very wide cone of light. In addition, because darkfield condenser creates such a wide and narrow light-cone, immersion oil must be applied between the condenser and the slide.

To convert a brightfield condenser to a darkfield condenser for low power objectives and for some high power objectives, a patch stop is added as close as possible on the front focal plane of the condenser. Then the patch stop will block all light entering the objective without being diffracted and/or refracted by the specimen.