1　测光原理测光原理其实就是假设所测光区域的反光率均为 18%来给出光圈快门组合参数。“18%”这个数值来源是根据自然景物中中间调 (灰色调 )的反光表现而定 ,一般白色表面可以反射近90 %的光线。标准灰卡是一张 (8× 10英寸 )的卡片 ,将这张灰卡放置于主景同一测光处 ,则所 A photometry principle Photometry principle is actually assumed that the metering area of ​​light reflectance are 18% to give the aperture shutter combination parameters. The source for “18%” is based on the reflectivity of the midtones (gray tones) of the natural scenery, and generally a white surface can reflect nearly 90% of the light. The standard gray card is a (8 × 10 inch) card, this gray card placed in the same scene at the same photometry Department, the