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Some objectives come with a built-in iris diaphragm. What is the use of this diaphragm?

April 26, 2017built-in diaphragm IRIS

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Some objectives come with a built-in iris diaphragm. What is the use of this diaphragm?

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The diaphragm is partially closed down during darkfield microscopy in order to reduce the numerical aperture of the objective below the lower of the two numerical apertures inscribed on a darkfield condenser. This action preserves the darkness of the background in darkfield observation. The iris diaphragm is absolutely necessity for high numerical aperture oil immersion objectives (above N.A. 1.2) when using an oil immersion darkfield condenser. For ordinary brightfield observation, this iris diaphragm is left wide open.