A telecentric lens is a type of camera lens that was designed with unique properties that make it useful for certain automation applications.

In a telecentric lens, the chief rays of light (the rays that pass through the center of the lens) are parallel to the optical axis of the lens. This means that the angle of incidence of light on the image sensor does not change with the distance from the lens. This property is called telecentricity.

One of the main advantages of telecentric lenses is that they can produce images with uniform magnification. Uniform magnification means an object will appear the same size regardless of the lens focus. This makes them useful for applications such as metrology or machine vision, where precise size measurement is required even when the object being imaged is not flat, is at an angle to the lens, or is positioned at various distances from the lens. 

In contrast, a telephoto lens is a type of camera lens that you may be familiar with from photography. This type of lens is designed to magnify distant objects, making them appear closer and larger in the image. Telephoto lenses are often used in photography and videography to help isolate the subject and create a shallow depth of field. This can be useful for creating images which can help to draw attention to the subject and create a sense of depth.

When you focus a telephoto lens on a subject, the magnification of the image will change depending on the distance between the lens and the subject. As you move the lens closer to or farther away from the subject, the image will become either larger or smaller on the camera sensor plane, respectively. Another important feature of telephoto lenses is their ability to compress the perspective of the scene, making distant objects appear closer together than they actually are. This can be useful for creating dramatic and dynamic images, such as landscapes or cityscapes.