Question 1

What determines lens thickness?

Accepted Answer

Four things: the strength of your prescription, the refractive index of the lens, the size of your frame, and how well the lens is centred on your pupils. Even with a fixed prescription, changing the other three can reduce thickness substantially. Thickness grows with the square of the distance from the lens centre, which makes frame size far more influential than you would expect.

Question 2

Do I have to buy the thinnest lens?

Accepted Answer

Usually not. 1.74 is the thinnest, but it is also the densest material, so it is not always the lightest. Its optical clarity is lower too. At moderate powers, 1.60 or polycarbonate often strikes a better balance. The table above shows the actual difference in millimetres for your own prescription; if it is under 1 mm, moving up a tier buys you nothing you can see.

Question 3

Why did my optician's lens come out different?

Accepted Answer

This tool uses the industry-standard Vogel base curve rule and the exact sagitta formula. Real lenses use manufacturer-specific base curve sets and aspheric surfaces. Read the result as an estimate with a tolerance of roughly ±0.4 mm. The ranking and the ratios between lenses are far more reliable.

Question 4

Why does PD matter?

Accepted Answer

The optical centre of the lens must sit directly in front of your pupil. If your frame is wider than your PD, the lens centre is shifted, a larger blank has to be cut, and the edge gets thicker. Choosing a frame close to your own PD buys you thinner lenses for free.

Question 5

Is this medical advice?

Accepted Answer

No. This tool does not diagnose, does not measure your prescription, and does not issue one. It performs a geometric thickness calculation from prescription values you already have. Only an eye doctor can determine your prescription.

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