The 4 Types Prescription Lenses
There are 4 basic types of vision correction prescription lenses:
Each prescription lens corrects a specific defect in your vision.
The correction required determines how thick the lens will be. The more correction required the thicker the lens.
This is perhaps the most important issue determining whether you will choose CR-39SRC plastic, polycarbonate, Trivex or glass material for the lenses.
Let's discuss vision correction.
Single Vision Lenses
How Single Vision Lenses Work
Single vision prescription lenses have one correction for either near or distant vision.
Unless you tell your eye examiner anything different, they will measure your distant vision for a focus at "infinity" of about 10 feet or more.
For near vision they will measure your near vision for as many inches or feet away you need to focus at.
How Bifocal Lenses Work
Bifocal lenses have two corrections, the upper part of the lens is set for "infinity", or distant use, the lower part of the lens (the segment) is set to focus close up for reading.
With the normal "flat top" bifocal lens when you move your eye from the top part of the lens (the distance correction) to the lower part (bifocal segment correction) you change the focal point of your eyes from "infinity" down to about 15 to 18 inches.
Your prescription will have the bifocal portion listed on the form as "add".
For example +1.50.
When you order bifocal sunglass lenses your prescription should include the segment height for the lens.
Normally it would line up with your lower eyelid.
How Trifocal Lenses Work
Trifocal lenses have three corrections where the upper part of the lens is distance correction, the middle part intermediate distance and the lower part near vision.
The smaller 7mm to 12mm segment is the intermediate correction that focuses at about 24 to 36 inches.
The lower bifocal segment of the lenses focuses at about 12 to 18 inches, just like a regular bifocal.
A note about trifocal lenses: there is no choice of power on the reading segment.
If the doctor prescribes lets say a +2.00 for the bifocal part, which is the lower part, the upper segment will always be 50% of the bifocal.
In this example the +2.00 bifocal will have a +1.00 intermediate power. The +2.00 will focus 12 to 18 inches and the +1.00 intermediate part will focus at 24 to 36 inches.
How Progressive Lenses Work
Progressive lenses are the "state of the art" for people who need more than two fixed focal lengths.
This is the most sophisticated multi-focal lens available, and has the greatest focal range, most nearly approaching natural vision.
The invisible reading segment is a narrow verticle channel that starts out at the middle of the lens with a distance focal point, and gradually focuses up closer and closer until at the bottom of the lens it will focus down to about 10 to 12 inches.
The best part of this lens is that it does not have fixed focus steps like flat top trifocals and it will provide a continuous focal point all the way down from infinity to about 10 inches.
So no matter what you look at you can find a spot in the lens that will get you in focus.
You just have to tilt your head up and down until you find the part of the lens that produces the best focus for what you are looking at.
Prescription Aviator SunglassesWe provide prescription aviator sunglasses with your prescription. To get your very own prescription aviator sunglasses is a 3 step process:
- Get your prescription from your optometrist and email it to us;
- Purchase new Randolph Aviator Prescription frames, or provide your own frame for us to use;
- Purchase the aviator sunglasses frame you want;
- Purchase the CR-39 prescription sunglass lenses you want.
We will do the rest. The whole process will take 2-3 weeks. Have a prosperous day!