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当奥斯卡·伯纳克为其在35mm胶卷上拍摄2倍于18×24mm电影幅面的实验相机寻找一只镜头时,他最初使用的是手边的一只徕兹显微物镜。要在43mm对角线的画幅上得到理想的影像圈,需要使用2英寸焦距的镜头。那时的标准是不列颠皇家显微镜协会应用的英制标准。当徕卡拥有了自己的为摄影目的设计的镜头时,2英寸焦距转换为52mm焦距,在镜头上标志为50mm。从这时起,大多数50mm镜头的实际焦长为51到52mm。
When Oscar Bernack looked for a lens for his experimental camera that shot twice the size of an 18x24mm movie on a 35mm film, he originally used a Leica microscope at his hand. To get a perfect image circle on a 43 mm diagonal frame, a 2-inch focal length lens is required. The standard at the time was the imperial standard applied by the Royal British Microscopy Society. When Leica has its own lens designed for photographic purposes, the 2-inch focal length is converted to a focal length of 52mm and the lens is marked 50mm. Since then, most 50mm lenses have focal lengths of 51 to 52mm.