When is an image produced by a lens sharp? (optics) Very basic question about optics: suppose I have a thin lens with a given focal length $f$, a screen and an object at distance $a$ from the screen. What is the mathematical relation that must be fulfilled so that the image of the object is sharp on the screen? Thank you very much in advance for your answers. Julien.

Use $${1\over x}+{1\over a-x}={1\over f}$$