照相机镜头有两项一般属性：f制光圈和焦距。
Camera lenses have two general attributes: f-stop and focal length.
f制光圈(或称光圈)决定相机的景深(对焦区域)以及有多少光能通过镜头到达传感器。
The f-stop, otherwise known as the aperture, determines your depth of field (the area in focus) and how much light can pass through the lens to reach the sensor.
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