What is this double-concave lens element from a microscope objective?
Asked 11/14/2017
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I disassembled a microscope objective to clean haze and found that the first two elements had originally been cemented together as a doublet. One of those elements is difficult for me to name: both optical faces are concave, but one side is much more strongly curved and smaller in diameter than the other. It also has a 45° bevel around the edge on one side.
Does this shape have a specific name, or is it still just considered a biconcave lens element? Also, does the beveled edge change how the element is classified?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Short answer is that it is indeed a bi-concave lens element.
When two lenses are attached, they are called a doublet. The most common type of doublet you'll find is used to reduce chromatic aberration (and frequently spherical aberration as well) by using two elements made of materials with different refractive indexes.
The beveling of the lens does not affect the lens classification. In this case, it is probably beveled to both reduce weight and prevent light entering the outside edges of the concave lens from bouncing around the inside of the lens and reducing the image quality.
Originally by user68136. Source · Licensed CC BY-SA 4.0
user68136
8y ago
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It would still be classified as a biconcave lens element. The two concave surfaces do not need to be identical for the lens to be called biconcave; one side can be more strongly curved than the other.
The fact that it was cemented to another element means the pair formed a doublet. In microscope objectives and other optics, doublets are commonly used to reduce aberrations, especially chromatic aberration and often spherical aberration, by combining glasses with different refractive properties.
The beveled edge does not change the lens classification. That bevel is typically a mechanical/optical detail rather than a different lens type. It can help protect the edge, reduce weight, and reduce stray internal reflections that would otherwise lower image quality.
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