Magnification of a lens

Lenses give an image of the object. The size of the image compared with the object depends on the magnification of the lens.

Magnification = image height (II')/object height (OO')

The magnification is also equal to image distance/object distance or v/u.
Magnifications don't always have to be bigger than one. For example in a camera and in your eyes the image is smaller than the object.

For a convex lens with an object distance of 15 cm and an image distance of 7.5 cm the magnification would be 7.5/15 or +0.5, the image is real and half the size of the object.