The distance between the focal point and optical centre is called?

The distance between the focal point and optical centre is called?

فوکل پوائنٹ اور آپٹیکل سینٹر کے درمیان فاصلے کو کیا کہتے ہیں؟
Explanation

The distance between the optical center and the principal focus is called the focal length of the lens.

A positive focal length indicates that a system converges light,

while

negative focal length indicates that the system diverges light.