Читать книгу Optical Engineering Science - Stephen Rolt - Страница 18

The reader might be used to ascribing a single focal length to an optical system, such as for a magnifying lens or a camera lens. However, in this general description, the system has two focal lengths. The first focal length, f₁, is the distance from the first focal plane (or point) to the first principal plane (or point) and the second focal length, f₂, is the distance from the second principal plane to the second focal plane. In many cases, f₁ and f₂ are identical. In fact, the ratio f₁/f₂ is equal to n₁/n₂, the ratio of the refractive indices of the media associated with the object and image spaces. However, this need not concern us at this stage, as the treatment presented here is entirely general and independent of the specific attributes of components or media.

In classical geometrical optics, the object location is denoted by the object distance, u, and the image location by the image distance, v. In the context of this general description, the object distance is simply the distance from the object to the first principal plane. Correspondingly, the image distance, v, is the distance from the second principal plane to the image. In addition, the object location can be described by the distance, x₁, separating the object from the corresponding focal plane. Similarly, x₂ represents the distance from the image to the second focal plane. This is illustrated in Figure 1.8.