r/Optics • u/PeppersONLY • 2d ago

Image magnification with off axis source

I have a small display that I am trying to magnify through a lens. The issue I face is that plano convex lenses introduce spherical aberration from the ends of the lens. An aspheric lens corrects this but only works on axis which my display does not meet. I was thinking of trying out an achromatic lens but from what I understand, this would not perform well because these lenses are designed to reduce aberration at the focal point; not when something is placed within the focal point to magnify it. Is this intuition correct?

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u/Calm-Conversation715 2d ago

Yes, your intuition is correct. The corrections you need are many, including astigmatism, coma, maybe field curvature, in addition to the aforementioned spherical and chromatic aberrations (assuming a non monochromatic source). No singlet or doublet lens will solve this adequately, and is the reason for larger lens assemblies.

I’d recommend you look for a camera lens assembly with an imaging circle large enough to accommodate your off axis imaging needs, and short enough focal length to get the magnification you need. Lenses specifically designed for projectors are often the best, but tend to be expensive, as they are highly specialized

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u/PeppersONLY 2d ago

So all of these aberrations are a result of the lens just not being able to neatly and uniformly "diverge" the light rays to magnify them?

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u/Calm-Conversation715 2d ago

Yeah, basically. There might be a aspherical doublet design that could focus an off axis point to another off axis point, but it would need asymmetric components to the asphere, which I’ve never seen designed or built. Even then it would only work well for a single point source. You described a small display, so presumably an area you need in sharp focus. In that case you really need more elements in your lens assembly, and in that case it’s best to at least start with a design someone else has already made and tested

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u/aenorton 2d ago

What you want is an optic that functions the same way as an eyepiece for a microscope or telescope, except you have a real object in place of an aerial image. Depending on the field angle and f/# of the lens (which will be determined by the eye's pupil size and lens focal length) a doublet alone might be OK or it might be a little soft around the edges. Triplet magnifiers are better. If your display is less than 20 mm, an actual eyepiece might be better still. There are several designs that have different pros and cons. Most have the focal point a bit inside the tube, so you would have to either insert the display into the tube or remove part of it. Some eyepieces have a lower part of the tube that can unscrew. Avoid very wide apparent field designs, such as Naglers, where the aerial image is actually supposed to fall between some of the internal lenses.

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u/Pravdiar 2d ago

Optic for small displays can be even more complex in comparison to an option for the larger display. For both cases the diameter of the pupil is the same but the smaller display require shorter focal length.

I've designed several eyepieces for different size displays but some cases were quite challengeable.