The lens is located just behind the iris (the colored portion of the eye) and can be seen through the pupil. When we change focus from distant to near objects, it is the lens that changes to create the change in focus.
Unlike the cornea which stays pretty consistent throughout our adult lives, the lens continues to grow in size. A cataract is a loss of clarity in the lens and happens to all of us that live long enough.
As we age, our ability to change the shape and focal power of the lens diminishes (the exact cause of this is still somewhat debated, but believe this 45 year old optometrist, it happens!).
As the lens grows the focal power of the lens will change slightly, but it takes a much larger change in the lens size to create a noticeable change in prescription. These type of changes don’t usually happen until later in life (after 60 year or age or older) and they typically cause a change in prescription toward nearsightedness or a reduction in farsightedness. This type of change typically accompanies the development of cataracts which can ultimately result in poor vision and require a surgical removal of the eyes natural lens and replacing its optical power with an ocular implant.
Another cause of a change in the lenses optical power is Diabetes. In Diabetes, the body’s fluid levels can become quite out of balance, resulting in swelling or shrinking of the lens. This can result in a rather large swing in refractive power, up to 5.00 Diopters of change in a period of a few days. Once the blood sugar levels are normalized, vision will usually stabilize, but the final prescription may end up being quite a bit different than before the blood sugar levels were altered.
So answer #2 the “Big Question” is changes in the optical power of the eyes natural lens.