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.
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.