I am learning about eyepieces, and "eye relief" and what sorts of eyepieces an amateur may want for their purposes, and was wondering if there are eyepieces with diopter adjustments, or if, due to the optical systems in reflecting and refracting telescopes, diopter adjustment like those in a DSLR viewfinder are never found in the designs of eyepieces.
Are there eyepieces with diopter adjustments? The answer is both yes and no.
Diopters are not needed on telescopes for simple spherical myopia (near-sighted or far-sighted vision issues) but may be needed if a person suffers from cylindrical myopia (astigmatism) and wants to be able to use the telescope without wearing prescription eyeglasses.
For near-sighted & far-sighted vision issues there is generally no need of a diopter adjustment on a telescope as their is on an SLR or DSLR camera for spherical correction. Also, cameras generally don't a way to compensate for astigmatism ... but it turns out you can get eyepiece adapters that compensate when using a telescope (more on that later).
Why? To answer the question, it's important to first recognize what the diopter adjustment does on a DSLR camera.
On a DSLR, the "SLR" stands for "Single Lens Reflex" because the camera is designed to allow the photographer to look through the same lens that will be used to capture the photo (as opposed to viewfinder or rangefinder cameras... or even twin-lens reflex cameras where the photographer looks through a separate lens used to "approximate" the image that will be captured.
If you remove the lens and inspect the top of the mirror chamber, you'll notice that the mirror is shining light up onto a piece of frosted glass. The glass will often have etched marks to serve as aids when taking the photograph.
The distance that light must travel through the lens, into the camera body, onto the reflex mirror and up onto the frosted glass (focusing screen) is actually the same as the distance through the camera and onto the imaging sensor (when the mirror swings clear to capture a photograph).
This means that when you look through the viewfinder, you're technically not looking "through the lens" ... you're actually looking at a back-lit projection screen (the frosted glass).
This means you could have an image which is precisely focused on the focusing screen (and also precisely focused for the imaging sensor) but appears out of focus due to eyesight issues.
The Diopter Adjustment allows you to adjust for near-sighted/far-sighted issues (spherical myopia). To adjust, manually put the camera out of focus and point at something with no contrast (such as a blue sky outdoors or a plain white wall if indoors) and adjust the diopter until the etched markings on the focus screen are as sharp as possible. Once you've done this, you can be confident that if the image projected onto the focus screen is well-focused, your eyes will also perceive it as being well-focused. It also means you may not need to wear your prescription eyeglasses when using the camera.
With a telescope, you aren't looking at an image on a focusing screen (such as frosted glass)... you really are looking through the telescope. When doing visual observing, there is no camera imaging sensor involved.
This means if you are near-sighted or far-sighted (both of which are spherical corrections) you can just adjust the telescope's focus to compensate. There is no need for a diopter adjustment.
Spherical myopia means the curvature of your eye is spherical... but is either slightly too flat ... or slightly too round to focus accurately at certain distances (resulting in a person being near-sighted or far-sighted).
But there is also cylindrical myopia (aka astigmatism). This means that instead of having uniform curvature (spherical curvature) on your eye, your eye's curvature is a bit more barrel-shaped where the curvature in one direction is stronger than the orthogonal direction.
TeleVue (and so far as I know ... only TeleVue offers such a thing) makes a product called Dioptrx. This is an adapter that fits onto many TeleVue eyepieces (not all TeleVue eyepieces are compatible with Dioptrx). It is designed for those who suffer from astigmatism.
You would need to know your corrective lens prescription to get the correct Dioptrx adapter (and since your right vs. left eyes will each have their own prescription, you need to decide which Dioptrx to order based on the eye you prefer to use when viewing through the telescope.
If you follow the link to the TeleVue Dioptrx page, you'll notice that the instructions offer helpful information on how to read your eyeglass prescription and how to identify the cylindrical correction needed for your eye. But you'll also notice it doesn't matter if the correction needed is positive or negative and also the axis can be ignored. This is because you can't just rotate the lenses in your eyeglasses ... they have to mounted in your eyeglasses frames in the correct orientation to work properly. But you can rotate the telescope eyepiece. This is why the positve/negative and axis values can all be ignored (TeleVue marks the mounting ring of the Dioptrx with letters ... so once you decide which axis offers the sharpest view, you note the letter at the top and just make sure to always rotate the eyepiece to the same orientation relative to your eyes when using it.
The Dioptrx astigmatism corrector allows you to use your telescope without needing to wear prescription eyeglasses. If your astigmatism if very weak (the cylindrical correction is a low value ... such as 0.50 ... then Dioptrx may not be needed. But if your correction is very high (e.g. 2.00 or more) then Dioptrx will probably be a big help.
Some additional thoughts after @TimCampbell's answer, there could be some cases where a diopter offset setting eyepiece on a telescope would be invaluable!
"Diopter" adjustments separate from "focus" adjustments are only needed if there are two image planes that need to be in focus at about the same time.
- DLSR: One image on the film or sensor, and another ultimately on the user's retina
- Microscopes: Two eyepieces
- Binoculars: Two eyepieces
- Telescopes: Two people, only one who adjusts focus
On telescopes, there's only one focal plane that the eyepiece views, so a diopter control would be redundant with the focus control.
However if there are two people using the telescope at the same time, one person could with an eyepiece set at 0 diopters and then offset to the other persons diopter offset before they looked, making it more convenient for them.
Maybe their hands are shaky and they don't want to touch the focus or have mobility issues, in which case the known diopter offset would be invaluable!