Lenses are engineered to have specific focal lengths besides the curvature of two sides of a lens what determines the focal length

I believe the correct answer is: material that the lens is made from.

Lenses are engineered to have specific focal lengths, besides the curvature of the two sides of a lens. The focal range, the distance between your optic nerve and the retins in lens, of the lenses is determined by the material that lenses are made from.

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