Hi,
Having an interest in optics as well as electronics I feel I can add to this conversation about spectacles to correct presbyopia, which is a condition that comes to us all around the age of forty years old and is simply the inability to no longer focus on close objects due to the loss of elasticity in the ciliary muscle which allows us to focus on close objects when younger.
Before any of the short sighted folk come along and say "I just remove my specs to see close up" all you are doing is exploiting your inherent short sightedness to allow you to do so.
Anyway, as some of you may already know there are a number of options to allow you to see close up as age progresses so I have broken it down with some other (hopefully) useful information for anyone interested.
If you have distance sight that doesn't need correction all you require are weak magnifying glasses commonly known as reading glasses
Anyone who has worn spectacles to correct a distance vision error whether it be short sight long sight or astigmatism, once they get to their early forties and begin to struggle with close up tasks will have a number of options which are as follows -
Two pairs of specs one for distance and another for close up,
Bifocals which have a segment in the bottom of the lens for close up - these give two distinct fixed focal points one of distance and the other for close up,
Trifocals which are similar to bifocals but have a third focal point for middle distance - these aren't particularly common however,
Varifocals (not to be confused with multifocals which is the term used for "varifocal" contact lenses where the wearer is looking through the distance and close up parts of the lens simultaneously)
The issue with varifocal lenses is that there are a number of different designs, the cheaper ones having a large degree of inherent astigmatism at the edges which cause a blur that the wearer has to learn to ignore.
If you can afford to it's better to go with at the very least mid range or the more expensive "free form" varifocals which are fitted to quite exacting measurements for your own eyes.
Another thing to remember when ordering varifocal lenses is to request a "short corridor" this means that there is less travel between the distance and close up parts of the lens and subsequently less head movement when switching from distance to close up.
Admittedly whilst being practically a full time contact lens wearer there are occasions when I require to see something small in a piece of vintage equipment I will remove my varifocal specs and use a cheap pair of over the counter "ready readers" with an LED headlight for extra help.
Whilst a little dated, a good book to read about optical errors and correction is "The Eye Book" By John Eden
Regards
Andrew