什么是孔径

攝影筆記 2007/10/04 15:20
Quoted from 美國紐約攝影學院攝影教材

　　正如我们已经知道的，镜头的速度是指镜头传送光线的能力。如果我们不希望镜头接纳最大的光量,就需要一种减少通过镜头光量的方法。我们是利用改变镜头孔径大小的方法达到这一目的的。孔径就是由可变光圈（叶片组）在镜头中央产生的圆孔如下图所示

光圈好比是水龙头。如果把它开大，就能有大量的光线进入；如果把它关小，就只会进入较少的光线。

什么是f制光圈
　　镜头孔径的大小可以用一个诸如f/1.2、f/8、f/16…的数字来表示，称之为f值。f值越小，镜头的圆孔越大。因此，假设某只镜头设置为f/2时，看上去可能如图2.23x所示，而同一只镜头设置为f/16时，则可能会如右图所示。

　　观察你的照相机。在镜头上会找到一系列f值设定值，称作f制光圈。如果你的镜头上具有这些f值光圈数字，请按从小到大的顺序将它们写在下面的横线上。（如果你的镜头上面没有这些f制光圈数字，也没必要烦恼，我们不过是在了解和掌握这一概念）。
　　f /-f /-f /-f /-f /-f /-f /-f /-

f制光圈之间的关系是什么
　　这些特定的f值光圈数字具有什么意义呢？这是一组"不可思义"的数字，认识和运用它们可以更容易地控制曝光，其意义如下：开大一挡光圈，进入照相机的光量会加倍；缩小一挡光圈，光量将减半。
　　这个概念就这么简单，但却非常重要。
　　f /4孔径所接纳的光线是f/5.6的两倍，f/5.6接纳的光线是f/8的两倍，f/8接纳的光线又是f/11的两倍，依此类推。
　　f值的完整序列如下：
　　f/1，f/1.4， f/2， f/ 2.8，f/ 4，f/ 5.6，f/8，f/ 11，f/16， f/22，f/32， f/ 44，f/64。
　　与你刚才写下的序列对照一下，或许只是上述序列当中的一部分，并可能与下面的序列非常相像，例如：f/ 2.8，f/ 4，f/ 5.6，f/8，f/ 11，f/16，在这个例子中，这只镜头应该叫做f/ 2.8，镜头因为这是它的最大孔径。
　　摄影者都应该熟知自己镜头的这些光圈。不知道你现在是否领会了为什么f/8所接纳的光量是f/16的4倍？或者为什么f/ 11接纳的光量是f/ 4的f/8？如果还未领会的话，请在继续向下阅读之前把这五节再读一遍。
　　认识并理解从f/1至f/64的序列会成为今后工作中颇有价值的一件工具。如果你现在还不是非常熟悉的话，请立即仔细研究并牢牢记住它。了解这些光圈数字之间的变化规律有一个小诀窍：每个数字都是向前数两级所对应的那个数字的两倍，即
　　1… 2… 4… 8… 16… 32… 64… 1.4… 2.8… 5.6… 11… 22… 44
　　考察上页的一系列照片，就会知道改变孔径是怎样改变进入照相机的光量，以及是怎样影响最终照片的。所有这些照片都是在相同的条件下使用相同的胶片拍摄的，即照明条件保持不变，快门速度保持不变，唯一变化的就是孔径。
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