| 摘要 |
A three-group zoom lens is provided of negative, positive, and positive refractive power, respectively, in the order from the object side. The first lens group is formed of a negative lens element and a positive lens element, the second lens group includes a stop for controlling the brightness of an image, two positive lens elements, and a negative lens element, and each lens group includes at least one aspherical surface. When zooming from the wide-angle end to the telephoto end, the first lens group is moved nearer the second lens group, and said second and third lens groups are moved continuously toward the object side. When focusing from an object at infinity to an object nearby, the third lens group is moved toward the object side. Specified conditions are satisfied in order to ensure that the exit pupil is an appropriate distance from the image plane for efficient light detection, while providing a zoom lens having an FNO of approximately 2.8 or less and a zoom ratio of 2.5 or more in a compact design that provides high-speed focusing and favorably corrects for aberrations, including chromatic aberrations.
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