Why do some objectives have an iris?
In order to preserve darkness of the background for
darkfield microscopy, the objective cannot have a
numerical aperture (N.A.) higher than the lowest N.A. marked on the darkfield condenser. An iris that can reduce an objective's N.A., can allow you to use higher N.A. objectives for darkfield work. Objectives with an N.A. above 1.2 require an iris for darkfield. For ordinary brightfield observation, the iris can simply stay wide open.
Shown at left is a
Plan Semi APO 50x oil objective with an iris. This objective lens has a N.A. of 0.87 and is typically used with the
Meiji biological microscopes.
Do I need special objectives for darkfield microscopy?
In most cases, from a transmitted light observation, you will only need a darkfield stop in the condenser. At higher magnifications, you will need an objective with an iris as well as a darkfield condenser.
