First off, there are two types of lenses: convex lens and concave lens.
Convex lenses cause parallel light rays to converge at a single point known as the focal point. These types of lenses are characterized by having a thicker center. When light rays pass through the lens, they don’t travel in straight lines; instead, they bend slightly inward. This event is called refraction. Light rays pass through the lens and refract, then meet up in a single point to produce an image.
Because the image passes through the lens to the opposite side, it is projected upside-down. This process, however, isn’t sufficient to give a sharp image. The reason why is that a spherical convex lens by itself cannot cause light to intersect to one point. The focal point where light rays passing through the center of the lens converge slightly differs from the focal point where light rays passing through the edge of the lens converge. This is called a spherical aberration.
Chromatic aberration may also occur. This refers to the fringing of colors. When light passes within a convex lens, the focal point for red light, which has a long wavelength, is farther from the lens than the focal point for blue light because the latter has a short wavelength. This is why the colors shift, resulting in colors “bleeding.”
To make light connect in a single point, one will need technology because the direction in which spherical aberration occurs in a convex lens is contrary to a concave lens. By combining two or more lenses, light rays can be caused to converge to one point. This process is called aberration correction. To correct chromatic aberration, one should join two lenses.
In case you didn’t know yet, convex lenses are used in refracting telescopes, spotting scopes, and in the camera you’re using for digiscoping. Two convex lenses, with one placed in front of the other, are even used in binoculars.
To obtain the most impressive images possible, camera lenses use multiple individual lenses which correct focus and color bleeding. Lenses for single lens reflex (SLR) cameras consist of a large number of lenses. Each lens is in charge of adjusting focus, distortion, colors, even changing the focal distance.
What about concave lenses? These lenses, which are thinner in the middle, are the exact opposite of convex lenses. Instead of converging the light rays that pass through them, concave lenses diverge or spread the rays of light.
When parallel light rays go through a concave lens, the refracted rays diverge, making them appear to come from a single point called the principal focus. The distance between the principal focus and the center of the lens is known as the focal length. If you look through a concave lens, the image won’t be upside-down, but it’ll be smaller. This type of lenses is used in movie projectors.