Iris Diaphragm

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In this series of images, the condenser housing has been removed from the microscope (not possible on all microscopes). This particular microscope has a frosted filter, which diffuses the light coming from the halogen bulb in the base of the microscope. The filter has been rotated carefully to one side so that the iris diaphragm is visible. Note the position of the silver lever and the corresponding diameter of the opening in each of the photographs. While the opening at the far right will allow more light to enter the condenser and be passed through the specimen slide, the depth of field will also be reduced. To obtain more depth of field, close the iris diaphragm. The position at the far left will give maximum depth of field.

Depth of field is the amount of space between those images which are too far from the lens to be in proper focus and images which are too close to the lens to be in proper focus. Maximum depth of field is important when using depression slides, demo-tube type slides, or any thick mount. For thin mounts, or if you want to focus on a particular layer in a thicker mount, it may be more effective to open the iris diaphragm and admit more light.