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The distance reference circle is a small, semi-visible marking found on the surface of a progressive lens. It is usually a faint circle that is permanently etched into the glass or plastic. This mark tells the optician exactly where to measure the lens to verify that the distance prescription is correct.
read more about distance reference circle ...Progressive lenses are complex and have different powers in different zones. To manage this, manufacturers use a standard layout. The distance reference circle is always located in the upper section of the lens. This is the area designed for viewing objects that are far away, like a road sign or a movie screen.
An optician cannot just check a progressive lens anywhere. If they measure the power too low or too high, the reading will be wrong. The distance reference circle highlights the exact "sweet spot" where the distance power is stable and accurate. An optician places this circle in a machine called a lensometer to confirm the prescription matches the doctor's orders.
These markings are designed to be almost invisible so they do not distract you. They are usually laser-etched. To see them, you often need to hold the glasses up to a bright light or a dark background. Sometimes, gently breathing on the lens to fog it up will make the circle appear for a few seconds.
This marking is a critical tool for quality control. Before you receive your new glasses, the optical shop checks them. The distance reference circle ensures they are measuring the right spot. Without this guide, it would be very difficult to know if a complex progressive lens was made correctly or if it is defective.
The distance reference circle is also used to check for a manufacturing error called "unwanted prism." In a correctly made lens, the optical center for distance should be exactly between the two distance reference circles (or determined by them). If the optician finds that the optical center is shifted away from this point, it means the lens will bend light incorrectly, causing eye strain or double vision even if the prescription power itself is perfect.
No. It is placed high enough on the lens that it is usually out of your direct line of sight. The etching is also so faint that your eye focuses past it, so it does not affect your vision.
No. Single vision lenses have the same power everywhere, so they do not need a specific reference circle for verification. This feature is unique to progressive (no-line bifocal) lenses.
This is a sign of a bad fit. If you can see the etching when you look straight ahead, the glasses may be sitting too high or the lenses may be cut incorrectly for the frame.
You do not need to use these markings yourself. They are for the optician. However, if you look through the top part of your glasses and your distance vision is blurry, take them back to your optician. They will use the distance reference circle to re-check the power and make sure the lenses were made properly.