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The inset is the specific placement of the reading portion of a bifocal or progressive lens. The reading segment is not placed directly underneath the distance portion; it is always shifted slightly inward, toward your nose. This small but important detail is necessary to match the natural way your eyes work when you read.
read more about inset in a bifocal lens ...When you look at something far away, your eyes look straight ahead. But when you look at something up close, like a book, your eyes naturally turn inward to focus on the same point. This is called convergence. The inset of the reading segment is designed to follow this inward movement, placing the reading power directly in front of your pupils when you look down to read.
In traditional bifocals, a standard inset amount is often used based on the average person's eyes. In modern, high-definition progressive lenses, a more precise approach is taken. An optician will measure both your distance pupillary distance (PD) and your near pupillary distance. The difference between these two numbers gives the lab the exact custom inset needed for your eyes.
If the inset of a reading segment is incorrect, your eyes will not be looking through the "sweet spot" of the reading zone. This can cause the reading area to feel very small or hard to find. It can also lead to blurry or double vision up close, which often results in eye strain and headaches.
The amount of inset required changes based on how close you hold your reading material. If you hold a book very close to your face, your eyes must turn inward much more sharply than if you hold it at arm's length. High-definition lenses take this "working distance" into account. A person who does fine needlepoint work will need a larger inset than a person whose primary near task is reading a computer monitor.
No. The inset is a feature only for multifocal lenses that have a dedicated reading zone, like bifocals and progressives. A single vision lens does not have a reading segment, so it does not have an inset.
No. Decentration is the process of moving the entire lens to align the distance optical center with your pupil. Inset is the additional inward shift of just the reading segment from that distance center.
It is usually very similar for both eyes, but in a custom progressive lens, the inset can be calculated individually for your right and left eye to provide the most precise alignment.
The inset is a technical measurement that your optician and the lab will handle for you. However, you should talk to your optician if you get new bifocals or progressives and find that reading is uncomfortable. If you feel like you have to turn your head to read or are experiencing double vision up close, it could be a sign that the inset was not made correctly.
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Inset & Corridor of Progressive Lenses. Horizons Optical. https://horizonsoptical.com/blog/inset-corridor-progressive-lenses. Accessed April 3, 2026.
Quick Reference Guide ANSI Z80.1-2015. The Vision Council. https://thevisioncouncil.org/sites/default/files/ANSI%20Z80%201-2015_Quick%20Reference%20v2.pdf. Accessed April 3, 2026.
Determining the Vertical and Horizontal Positioning of Multifocal Lenses. Survey of Ophthalmology. https://www.sciencedirect.com/science/article/abs/pii/S0039625798000137. Accessed April 3, 2026.
What Is Pupillary Distance (PD) & How To Measure It. All About Vision. https://www.allaboutvision.com/eye-care/eye-exam/types/measure-pupillary-distance/. Accessed April 3, 2026.