So say I was an adventurer with poor eyesight and wanted to see. Should I consult a cleric to heal my eyes or should I go to a wizard and get glasses with some kind of illusion spell effect that corrects my vision?
Mundane spectacles are your best bet, magical glasses are uncommon, and clerical healing of eyesight doesn't canonically appear in the rules
Within the rulebooks, the following exist to improve eyesight:
- Non-magical eyeglasses exist within D&D, at least in the Forgotten Realms. A bent pair of gold spectacles worth 25gp appears in Tomb of Annihilation p.156. An old man in the Red Library wears a pair of horn-rimmed spectacles. We can presume that, as in our world, spectacles are worn to improve eyesight, and this inexpensive item would be an optimal choice.
- Eyes of Minute Seeing, an uncommon magic item in the Dungeon Master's Guide p.168, allow a character to "see much better than normal out to a range of 1 foot".
- Eyes of the Eagle, also DMG p.168, give advantage on Wisdom (Perception) checks that rely on sight.
However, it's heavily up to the DM whether you can find a magic item for sale, whether a wizard NPC can craft one, and whether an NPC can invent new items:
- The rules for crafting a magic item appear on DMG p.128-129. It's heavily up to the DM, and it's described as a long and laborious process requiring the work of at least one highly trained individual, which is therefore likely to be expensive.
- The guidelines for inventing a new magic item appear on DMG p.284-285. The rules for creating a new spell appear on DMG p.283.
- Rules for buying items appear in Xanathar's Guide to Everything, as do further rules for crafting items.
In terms of clerical abilities, no actual cleric spell specifically cures naturally poor vision (although the DM can of course declare otherwise, or state that healers of eyesight do exist regardless). Even with spells like cure wounds, greater restoration, regenerate and heal, you can canonically only cure hit point damage, status conditions (of which blindness is one, but not merely poor vision), diseases (specifically described in the DMG as the communicable sort rather than generic maladies), temporary exhaustion, and curses.
However, none of these cure naturally being short-sighted. Your best bet is to simply ask your DM whether healers of eyesight or crafters of spectacles are more readily available in your world (unless you are DM, then naturally it's up to you).
Traditional healing magic of 6th level or higher works under RAW for certain types of lost eyesight, and Illusion magic works within the standard game lore, but you'll have to homebrew it.
Using Healing Magic
DMG chapter 9 has optional rules for "Lingering Injuries". These range from broken ribs to losing a limb. There is no description for near sightedness, but there is an entry of "Lose an Eye". As with other lingering injuries that involve loss of a body part, reversing this effect requires the Regenerate spell; Cure Wounds doesn't cut it when it comes to missing pieces.
But you're eyes aren't gone, they're just broken. Most of the lingering injuries which describe damaged but not missing body parts are cured by "magical healing", which could be a Cure Wounds, or even just a Goodberry. For example, Internal Injury, Broken Ribs, Limp, and Festering Wound all go away with any form of "magical healing".
But there are exceptions: Horrible Scar and Minor Scar require "(m)agical healing of 6th level or higher, such as heal and regenerate,". Also note that Heal reverses blindness. Lesser Restoration also reverses blindness, but not scars. From this, I think we can infer the following:
Below 6th level magic can only speed up the body's natural healing process; cuts (low hp) and broken bones (limp) that the body could eventually heal itself can be cured instantly with Cure Wounds or Healing Word. It cannot heal typically permanent afflictions like scars and missing body parts. Lesser Restoration can reverse the "blindness" condition, but in context of everything else covered here, I don't think this would include someone who was born blind. Poor eyesight of the kind you describe is a typically permanent affliction, so I do not believe it can be healing with these spells of lower that 6th level.
But can 6+ level spells do the trick? Eyesight can be damaged for a number of reasons. The most solid argument for 6+ level spells curing eyesight might be for corneal scaring. We already know that these spells can cure Horrible Scars, so why would this be any different for eyeballs? Other causes of eyesight are also defensible. In general, I think anything that can restore a missing body part, for example Regeneration, should be able to fix eyes that have become damaged: macular degeneration, cataracts, and damage from viewing the solar eclipse in the manner of the current US president are all fair game. But we might want to exclude damage from aging, because that would imply that this spell can stall death and potentially allow for immortality - I think we can all agree this is a reach.
But what about someone born with bad eyesight? Regenerate only restores "severed" body parts, so perhaps someone who is born without legs could never grow a pair this way. This "severed" language to me suggests the this spell cures injury, not conditions you are born with. At that point, you would need Reincarnation, Wish, or maybe Illusion...
Using Illusion Magic
There's no RAW spell or ability for this, just flavor. The PBH and DMG both describe illusion as being about "deception", but I think we can infer a bit more from the illusion spell list, in particular Invisibility, Mirror Image, Color Spray, and all of the spells that include the word "Illusory" or "Illusion". In contrast to the school of Enchantment, which is about deception of the mind, the Illusion schools seems to be largely about control over light.
The lens of an eyeball or pair of glasses serves the function of manipulating light. In that sense, lenses are a form of illusion magic, except without the magic. So yes, I think it would be entirely appropriate for the school of illusion to provide some sort of magical remedy for bad vision. Real life holographs are achieved with optical technology far more advanced than mere bifocals. Therefor, if an illusionist can manipulate light to create holograms, it stands to reason that they could cast a spell on a creature's eyes, or on a sheet of glass, to improve vision.