A lens is any medium that transmits light and has an Index of Refraction and shape designed to bend light. The strength of lenses are measured in diopters. Lenses come in many forms but there's a few critical ones that relate to EM.
This is the lens inside your eyeballs. It is not actually the strongest part of your internal refractive state but it is the part that is quickly adjustable. The ciliary muscle can flex this lens and change it's power. The ability to change the power of this lens to focus on near work is called accommodation. As you age, the crystalline lens thickens and is prone to presbyopia and cataracts.
If the refractive state of your eye causes myopia or hyperopia then you probably wear contacts or glasses. This "corrects" the light coming into your eye to compensate for the refractive state in your eye.
Glasses are lenses worn in front of the face, typically at a vertex distance of 10 to 13 cm. Glasses are easily changed and have lower risks than contact lenses, but at higher powers may have some trade-offs.
Glasses are only perfectly tuned through their optical centers. If your Pupillary Distance, Pupillary height, or lens adjustment are off, you might not be looking through the optical center of the lens, and you are certainly not looking through the optical center when you look around instead of straight ahead. The power of the lens will vary, and unintentional cylinder can be introduced. You may also not be wearing the lens at the intended vertex distance, affecting the effective power of the lens. The stronger the lens the more of a difference this makes.
Contact lenses are in physical contact with your eye, and have a vertex distance of 0. They carry high risks of infection, dry eye and other eye injuries, and require extensive care, and are hard to swap out multiple times a day, but have fewer distortion effects than glasses at higher powers.
How to use Contacts with EM
Choose a power for your contacts to match, your prescription, normalized, or differentials, and then use glasses over the contacts to get to the other focal planes you need. This ruins the cosmetic benefits of wearing contacts, but may make the process easier for people with strong lenses or who need to not wear glasses for sports or other reasons.
- Since it's easiest to get contacts at full prescription, and many people want contacts for outdoor activity, most people use prescription contacts and plus lenses over. This may put you in the range to use cheap drug store readers, but it's better to get good ones from a custom lens seller.
- The next most common strategy is to use differential contacts with minus lenses over for normalized. This has the advantage of less total lens over the eye, and a thinner contact may be more comfortable and allow more oxygen through to the surface of the eye. (And in specific cases may affect the cost of contact lenses, when stepping down from the extended range brand to the normal range brand in the same material/design.) This has the disadvantage of requiring glasses for outdoor activity.
Cautions about 3rd party sources
If you buy glasses online, you can pretty much get optically clear glass and an anti reflective coating in any of the cheap brands. When wearing contacts, oxygen permeability and the fit of the lens to your eye surface are important to eyeball health. Whenever possible, get contacts fitted by a professional optometrist. If you get any contacts other than those you are fitted for, you introduce significant additional risks on top of the expected contacts risks. You can get contacts in the same brand, material, diameter, and base curve as your professionally fitted ones, but event the difference in thickness could affect fit. A bad fit may not be obvious to the wearer, but can cause damage to the cornea over time.