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2.1 Lens Equation
The basic lens equation is illustrated in Fig. 2.1 and can be stated as 1f=(1lâ²)â(1l), where f is the effective focal length, l is the object distance, and lâ² is the image distance. In Fig. 2.1, l is negative and lâ² is positive, in accordance with the sign convention. The focal length f = 1âP, where P is the power of the lens. For example, if l = â1 and lâ² = +1, then P = 2 and f = 0.5.
An important special case of the lens equation is illustrated in Fig. 2.2. This is when the object is assumed to be at infinity, as is the case in most infrared zoom lens applications. In accordance with Eq. (2.1), 1âl = 0, f = lâ², and the image plane lies in the focal plane of the lens. In the above example, f = lâ² = 0.5.
2.2 Stops and Pupils
The aperture stop is the limiting aperture of the optical system. The aperture stop for a simple lens is shown in Fig. 2.3. The entrance pupil is the image of the aperture stop in object space and is coincident with it. The exit pupil is the image of the aperture stop in image space and is also coincident with the aperture stop in the figure. The field stop limits the size of the detector at the image plane.
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