胶片颗粒和数码噪点

两者都是“颗粒状”的，但成因、分布和观感都不同。

放大看，胶片照片和高感光度的数码照片都会出现细小的不均匀。它们常被混为一谈，其实是两种东西。

胶片颗粒从哪里来

胶片的感光层里有大量卤化银晶体。显影后，曝光过的晶体变成银粒（黑白片），或在彩色片里留下染料云。它们大小不一、位置随机，在照片上就是颗粒。感光度越高的胶片，晶体通常越大，颗粒越明显。曝光不足再增感冲洗，颗粒也会变粗。

数码噪点从哪里来

数码噪点主要来自两部分：光子本身到达的随机性（散粒噪声），和传感器读出电路的噪声。光越少，随机性越显眼，所以噪点集中在暗部，提亮阴影时会一起被放大。彩色噪点表现为红绿色的小斑块，这在胶片上不会以同样的方式出现。

分布不同

胶片颗粒在中间调最容易看到，在极亮和极暗处较弱。
数码噪点在暗部最重，在亮部几乎看不到。
降噪会把噪点抹平，同时抹掉细节，留下塑料般的平滑表面。颗粒保留下来的那种“有东西”的质感，降噪后的照片通常没有。

后期加颗粒时要注意

先控制噪点，再加颗粒。颗粒盖不住彩色噪点，只会让照片更脏。
颗粒的大小要和照片的尺寸相称。手机屏幕上看，过大的颗粒像压缩失真。
让颗粒在亮部弱一些。很多编辑软件的颗粒是均匀叠加的，可以配合蒙版减弱高光里的颗粒。
颗粒本身不会让照片像胶片。影调和颜色不对，颗粒只是一层纹理。

FilmLook 看到的是长边 2048 像素的副本，最细的颗粒和噪点在缩小时会丢失。它对质感的判断偏向整体观感，而不是像素级的分析。

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All guides

Film grain vs digital noise

Both look granular, but they form differently, sit in different tones and read differently.

Look closely at a film photo and at a high-ISO digital photo and both show a fine unevenness. They are often treated as the same thing. They are not.

Where film grain comes from

Film's emulsion holds a great many silver halide crystals. After development the exposed crystals become grains of silver (in black and white) or leave clouds of dye (in colour). They vary in size and sit at random, and that is the grain you see. Faster films usually have larger crystals and more visible grain. Underexposing and push-processing coarsens it further.

Where digital noise comes from

Digital noise has two main sources: the randomness of photons arriving (shot noise) and the noise of the sensor's read-out electronics. The less light, the more the randomness shows, so noise gathers in the shadows and grows when you lift them. Colour noise appears as small red and green blotches, which film does not produce in the same way.

They sit in different tones

Film grain is most visible in the midtones and weaker in the brightest and darkest areas.
Digital noise is heaviest in the shadows and hardly visible in the highlights.
Noise reduction smooths noise away and detail with it, leaving a plastic surface. The sense of texture grain keeps is usually gone afterwards.

If you add grain in editing

Deal with noise first, then add grain. Grain does not hide colour noise; it only makes the picture dirtier.
Match the grain size to how the photo will be seen. On a phone screen, oversized grain looks like compression damage.
Keep grain lighter in the highlights. Many editors lay grain evenly over everything; a mask can reduce it in bright areas.
Grain alone does not make a photo look like film. If tone and colour are wrong, grain is just a texture on top.

FilmLook sees a copy 2048 pixels on the long edge, so the finest grain and noise are lost in the reduction. Its reading of texture is about the overall impression, not a pixel-level analysis.

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