This tutorial explores how conjugate planes are in common focus when a microscope of finite tube length is adjusted for proper Köhler illumination.

	Interactive Java Tutorial
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To operate this tutorial, use the sliders to shift the focus of the conjugate planes in the illumination light rays (left slider) and/or the image-forming light rays (right slider). You will notice that as the sliders are moved, all images (either the filament or the specimen) drift in and out of focus together.