About added achromatic lens optical theory

I built the excellent OpenFlexure microscope high resolution type, and its worked well.
However it’s difficult to understand the optical theory about the added achromatic lens. I’d like to know the theoretical relation added achromatic lens location, focal length and magnification, image plane location. Is there reference document anywhere?

Hi, I guess there is no specific document needed as this just how a basic microscope works. Let me try to explain in a few words: A microscope objective is an optical system consisting of a lenses, that is optimized to certain imaging conditions. Usually, the user wants a good image with minimal distortions, which may occur due to various reasons. One is the wavelength dispersion of the glass, i.e. different refractive index depending on the wavelength of light. Blue light has about 450 nm wavelength, red light 630 nm and a simple lens will have a slightly different focal point for each wavelength. In an objective, multiple lenses, made of different kinds of glass, are combined to compensate such effects. The same principle is used for an achromatic lens, usually made of two lenses glued to each other (“doublet”), to minimize “chromatic abberation”, what the common term for the above effect is. Making it simple, it just works as a regular lens but has less distortions when used for imaging, especially with white light.

A basic microscope consists of an objective lens and tube lens in front of the camera (let’s ignore illumination for now). The objective collects the light from the object and directs it to the camera. Modern objectives are “infinity corrected”, meaning that they are designed to collimate the light from the object plane (focal plane) and produce a parallel light beam. The tube lens does the exact opposite thing, it takes up the collimated beam and produces an image in its focal plane, on the camera chip.

The magnification now depends on the focal distance of the objective lens and tube lens. By convention, the nominal magnification factor of the objective lens (e.g. 40x) is specified for a 160 mm tube lens, but may depend on the manufacturer. Using a 50 mm tube lens instead, like in our case with the OFM, yields a magnification about 3 times less than the nominal magnification, but we end up with a more compact system. Here, magnification refers to the ratio of the image on the camera chip in respect to the real size of the object, not the apparent magnification for the human eye as in a traditional microscope with ocular(s).

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@j.stirling has added a knowledge base with a page explaining this to the instructions. It came after the current V7.0.0-Alpha2 release, but you can see it on a recent build here: Assembly Instructions (openflexure.gitlab.io)
That link will probably not work after a week or so, but this will all be in the V7.0.0-Beta release that we are hoping to get out soon.

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The knowledge base is still pretty sparse. My hope is that over time sections like the mechanics and electronics will also get information.

The optics section was written while I was (as a non microscopist) trying to understand the optics so that we could refactor the code to make it more understandable and more flexible for creating custom optics options for using different components. These started as my notes, which turned into a proposal for the improvements, which turned into this page so it was captured.

In the next version of the instructions there should be a little info icon next to the lens which links to this page.

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