Focal length
![](/Images/godic/202501/23/Focal-length.svg4944.png")
![Thick lens diagram](/Images/godic/202501/23/Thick_Lens_Diagram.svg4945.png")
![Images of black letters in a thin convex lens of focal length f are shown in red. Selected rays are shown for letters E, I and K in blue, green and orange, respectively. Note that E (at 2f) has an equal-size, real and inverted image; I (at f) has its image at infinity; and K (at f/2) has a double-size, virtual and upright image.](/Images/godic/202501/23/Thin_lens_images.svg4945.png")
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them to a focus in a shorter distance.