I pulled apart a microscope objective lens to clean it. I was using it to take photos and noticed they were coming out blurry/cloudy. Turns out the first two lens elements were "cemented" together. With what, I don't know, but the (formerly) optically invisible adhesive had gone bad and was starting to show. All the other lens elements were in pristine condition.

Anyway, that's just background information. Trying to discuss the the first element is difficult because I don't know what to call it. Maybe you can help me identify it.

  1. It's concave on both sides, so I wouldn't call it plano-concave.
  2. The external (input) concave is much more pronounced (deeper), and also has a significantly smaller diameter than the internal (output) concave, so I probably wouldn't call it bi-concave, either.
  3. The most unusual thing about it; the edge around the first concave is bevelled at a 45° angle. The other is not. I thought this might make it an asphere, but I'm not sure because the surfaces are still somewhat spherical, and I've only ever seen convex-like aspheres.

It's hard to tell just by looking at it (you can feel the wider concave by running your thumb across it), so I made a 3D model. The arcs are only approximations. The wider concave is probably shallower than I've drawn it in relation to the other.

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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.

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