For anyone who needs a different prescription power for distance and reading – which is almost everyone as we age – there are two main options for glasses bifocal lenses or progressive lenses. Bifocal lenses have been around for years, the concept is quite simple, the top of the lens is the prescription to see well in the distance, and the bottom of the lens is the prescription to see well to read. This allows both of the prescription powers to be in the same pair of glasses and reduce the need for a second pair of reading glasses. Progressive lenses, or progressive addition lenses, are similar to bifocal lenses but have a smooth gradient from the top of the lens to the bottom of the lens. Instead of a line demarcating the different regions of the lens, there is a progression of powers that help with the intermediate distance vision between the distance and reading zones.
Why are Bifocals or Progressives Needed?
As we age, the condition called presbyopia develops and alters the ability to focus on things up close – particularly when reading.
The natural crystalline lens in the eye gets more rigid with age and is unable to flex to focus on these near items as well. These changes will often be noticeable around the age of forty to forty-five.
Since the eyes are unable to focus up close as well as when the lens was younger, glasses are needed for reading and other near tasks.
While these reading glasses could provide excellent vision within an arm’s reach, anything beyond would be slightly or very blurred.
Thus, the two different power prescriptions, one for distance and one for reading must be found within the same pair of glasses or two pairs would be needed.
How Do Bifocal Lenses Work?
Bifocal glasses are designed for clear vision at two distances. This is where the name bifocal is derived – there are two (bi-) focal points (focal) in the lenses.
Most of the time, bifocals are created to have clear vision in the distance for driving, watching TV, and other distant tasks and clear vision at about two feet for reading, sewing, or looking at a phone screen.
There will be things that are slightly blurry and unable to be seen perfectly clearly with bifocals if the object is between the two focal points and neither the top nor the bottom of the lens is able to provide the appropriate prescription.
How Do Progressives Work?
Designed like bifocals, progressives should give both clear distance vision through the top of the glasses and clear reading vision through the middle of the glasses.
However, the middle of the lenses has a slow transition from the distance prescription to the near prescription which can be used to see things at an intermediate distance ranging from two to ten feet.
These progressive lenses will not have a visible line and allow a seamless transition from seeing far away to seeing up close.
