Formula Lens Equation Focal length    Object distance    Image distance

Focal length

Unit
Focal length is the distance between the lens and the focal point of the lens. The focal point is the point into which the lens focuses all light rays. The focal length is a property of the lens and can be found out with the help of the object size $$g$$ and the image size $$b$$.

For example, if the object is 1 meter away from the lens, so $$g = 1 \, \text{m}$$ and the sharp image of the object is 2 centimeters away from the lens, so $$b = 0.02 \, \text{m}$$. Then the focal length of the lens is:\begin{align} f &~=~ \frac{ 1 \, \text{m} ~\cdot~ 0.02 \, \text{m}}{ 1 \, \text{m} ~+~ 0.02 \, \text{m} } \\\\ &~=~ 0.02 \, \text{m} \end{align}

Thus, the focal point of the lens used is at a distance of 2 centimeters from the lens.

Object distance

Unit
This is the distance between the lens and the object.

Image distance

Unit
This is the distance between the lens and the sharp image of the object through the lens. You can adjust the sharpness of the image by, for example, changing the distance between the lens and the object.