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Segment Height (SEG Height)The location of the "line" - which determines the amount of lens that will be used for Distance and Reading – is called the SEG Height. In a bifocal lens it is where you see the "line". In a progressive lens, it is the place where the lens turns to reading at the bottom. It is determined to be about 60% Distance and 40% for Reading. It can be changed if your doctor recommends it.
Technically, if your Optometrist is asking, we put the SEG at "the D line divided by two - minus 2 mm".
Basically, if you take the middle of the lens and draw a line across it, then measure 2mm below the line, that is where the change will occur. In a frame with a lens that is not very tall (32mm and below) we will make the line closer to the middle to leave a little more room for reading.
The SEG is really affected by where you like your glasses to rest on your nose. Imagine if you put a line on your glasses (using tape or a grease pencil) and then put your glasses high up on your nose and then slide them down. You will notice that the line moves down. But this also changes the distance between your eye and the lens.
This change in distance can make a difference in the clarity of your vision at either distance or reading. (if one is good, the other may suffer etc) The strength of your prescription can also affect how well you see both far and near.
Sound complicated?? Well, scientifically, yes – it is complicated. BUT.......we make thousands of these types of glasses with no problem whatsoever as our SEG Height calculation works very well for most people.
If you get a pair of progressive glasses and the SEG doesn't seem correct, take them to your eye-doctor and have the SEG line measured as though they had no lenses in them. When you get that number, let us know and we will remake them for you. We will only do this remake once, so make sure you get the correct number that will work for you.
Some doctors may not be too happy about you asking them for help with glasses that you did not purchase from them. But if you paid them for your prescription, and they want to keep you as a patient, I am sure they will be more concerned about your eye-health and assist you.