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DIA refers to the diameter or size of the lens. BC stands for Base Curve, which shows how much the lens is curved. This diameter and the base curve have the opposite effect. For example, a smaller diameter causes the lens to loosen, and a larger diameter causes the lens to stick tightly to the eye. On the contrary, the base curve becomes tight when it is small and loose when it is big.
As you said, I used 8.5mm curves, and when I used 8.6mm curves, I was walking around. This is why. In this case, if the diameter is the same size, the base curve should be a little smaller, or 8.5mm should be used as it is. If you use a base curve of 8.7mm with a diameter of 14.5mm, you are more likely to move around your eyes.
Bass curve 8.4mm is a curve turned to a compass with radius 8.4mm, and 8.7mm is a curve turned to a compass with radius 8.7mm.
8.7mm is a flatter lens and 8.4mm is a little more convex. The base curve is not a choice of the customer, but an expert's recommendation.
There are a lot of 8.4mm in the eyes of Asians, but nowadays, there are a lot more from 8.6mm to 8.7mm. There are many places where 8.7 is preferred if the lens is not mushy.
Less pressure, less strain on the eyes. However, if the lens is removed, the corneal curve may be less floating or folds in the eyes.