What is focal length of a lens is a question asked by several new photographers or someone who just bought his or her first DSLR. You hear about the 18-55 mm lens, the 70-300 mm lens but ever wondered what these measurements actual mean? These numbers represent the focal length of a lens.
Focal length has nothing to do with the physical length of the lens. It stands for the distance between the optical center of the lens and image sensor of the camera. Take a look at the picture below for better understanding.
For example, lets assume that you have a 18-55 mm lens mounted on your camera. If you take a picture completely zoomed out, the focal length would be 18mm meaning that the optical center of the lens is at a distance of 18mm from the image sensor. Likewise, If you zoom in completely, the focal length would increase to 55mm. The red dots are the optical center of the lens. In a 18mm lens, this center is just at the beginning of the lens, right where you attach it to the camera body. When you zoom in on a subject, this center gets farther away from the sensor and the focal length increases. On telephoto lenses, a 70-300mm lens for example, the minimum and starting focal length is 70mm so this center is farther away, even when when completely zoomed out.
How does it effect your pictures? Well closer the optical center of your lens is to the imaging sensor, wider your angle of view would be, meaning that you will be able to capture a larger area in the same picture. Farther the optical center gets from your image sensor, narrower your angle of view will be, meaning that you are focusing on a particular subject and cropping rest of the areas out.