The eyepiece (or ocular) is designed to project either a real or virtual image, depending upon the relationship between the intermediate image plane and the internal eyepiece field diaphragm. Explore how eyepieces can be coupled to the human eye or a camera system to produce images generated by the microscope objective.

The tutorial initializes with the eyepiece in Viewing mode, and the intermediate image plane (I(3)) located in the center of eyepiece fixed diaphragm. This diaphragm is positioned a small distance to the right of the front focal point (or plane) of the eyepiece (F'(e)). Use the Diaphragm Diameter slider to adjust the opening size of the eyepiece diaphragm in order to modify the light rays and size of the image produced on retina image plane (I(4)). A pair of radio buttons in the lower left corner of the tutorial window can be utilized to toggle between Viewing and Projection modes. In the Projection mode, translation of the Diaphragm Diameter slider will adjust the size of the real image that is projected onto a camera detector or conventional film emulsion.

The diameter of the fixed eyepiece diaphragm determines the field size observed by the microscopist. Image planes of the eyepiece, when utilized in projection mode, are presented in the tutorial window when it initializes. The principal focal points are F'(e) and F(e), the front and rear focal points, respectively. The intermediate image plane (Image Plane (3)) is located in the center of the fixed eyepiece field diaphragm, which is placed either before or after the eyepiece field lens, depending upon the design. This image plane is conjugate to Image Plane (4). When the eyepiece is utilized in projection mode, the length a represents the distance from the eyepiece fixed diaphragm to the principal plane of the eye lens, while b is the distance from the eye lens to Image Plane (4). Because a is greater than the front focal length of the eye lens (f'), the image formed at Image Plane (4) is a real (not virtual) image. The distance f denotes the rear focal length of the eye lens.

When images are examined in the microscope, an intermediate image (see Image Plane (3) in the tutorial window) is formed by the objective at a distance a, which is slightly closer to the eyepiece than its front focal length, F'(e). This prevents the formation of a real image after the ocular lens, as is illustrated in the case for the eyepiece operating in projection mode. Together, the eye and eyepiece form an image on the retina (Image Plane (4)) as though the eye were seeing the virtual image.

In situations where distance a is less than the focal length, then the reciprocal equation relating focal length to a and b reveals that b must be less than zero. Therefore, a real image is not formed to the right of the eyepiece in the absence of the eye or a camera. Instead, a virtual image (Image Plane (3')) appears at a distance corresponding to a' or -b to the left of the eyepiece (or b to the right). When observing the image through the eyepiece, the image-forming beam diverging out through the eye lens appears to originate from a virtual source (located at Image Plane (3') in the tutorial window).

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