Think of a magnifying glass focusing sunlight onto a point; the distance from the glass to that tiny, bright point is essentially the focal length of the lens.

UVFFormula: $1/f = 1/v - 1/u$, remember that $u$ is almost always negative for real objects.

The lens formula establishes the relationship between the distance of an object from the optical center, the distance of the image, and the focal length of the lens. According to the Cartesian sign convention, light travels from left to right; distances measured in the direction of incident light are positive, while those against it are negative. For a convex lens, the focal length is positive, representing convergence of light rays.

Always assign the sign to $u$ and $v$ before plugging them into the formula; forgetting the negative sign for the object distance is the most frequent error in lens problems.