A condenser is a sub stage lens that focuses light on the specimen. The Abbe condenser is the most widely used condenser system in use on microscopes today. It is typically used with an adjustable iris, in effect allowing the operator to change the diameter and focal point of the light entering the slide. By moving the condenser up and down and changing the opening on the iris, the contrast and detail in the specimen can be precisely set for best image quality. All microscopes at magnifications over 400x will require an Abbe condenser or the equivalent.
The potential N.A, (numerical aperture) of a system is only as good as its weakest link. Any objective with an N.A. of 0.4 or more will require a condenser and oil and water objectives will require a condenser in keeping with their higher N.A. The typical Abbe condenser has a numerical aperture (N.A.) of 1.25 and will thus allow the use of objectives with an N.A. up to 1.25. Higher N.A. objectives will require a higher N.A. condenser system.