50mm定焦镜头在较大价格范围内的平均光圈是F1.4，便宜的（镜头）光圈为F1.8，较贵的（镜头）光圈为F1.2。
The 50mm prime on average allows an aperture around F1.4 on a mid ranged price lens, F1.8 on the cheaper ones and F1.2 on the pro series.
就像镜头有一个光圈范围，你的相机也有一个快门速度的范围。
Just like a lens has a range of apertures, your camera has a range of shutter speeds.
有些镜头不能完全覆盖这个光圈范围。
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