噪点 · 信号的地板Noise · The signal floor

难度LEVELL2 · 进阶L2 · Intermediate 领域TOPIC调色

先学：Prerequisites: 动态范围与亮度 色深 · 色彩精度

学习目标LEARNING OBJECTIVES

学完你能说清噪点从哪来、ISO 到底在放大什么，以及拍摄和后期各该怎么让画面离地板远一点。

概念说明CONCEPTS

噪点是信号的地板：光线太弱时，传感器收到的光子本身就忽多忽少（散粒噪声），电路读数再叠一层误差（读出噪声），两层合成画面上的脏颗粒。

  • ISO 是放大器不是灯——开高 ISO 不制造光，只把已有信号连同噪点一起放大
  • 欠曝是噪点的放大器——暗部本来就贴着地板，后期一拉亮，地板整个翻上来
  • 彩噪比灰噪难看——暗部的彩色杂点最显眼，因为三个通道的噪声互不相关
  • 向右曝光（ETTR）——不过曝的前提下多给光，信号站得离地板越远，画面越干净
欠曝与向右曝光：信号和噪点地板的距离

配套看：动态范围 · 色深 · Log

操作步骤STEPS

  1. 拍摄端：不过曝的前提下向右曝光，别指望后期救暗部
  2. 后期端：降噪（NR）放节点树最前面——先清地板，再谈调色
  3. 时域降噪管运动画面，空域降噪管静止细节，先用时域
  4. 锐化永远放最后：锐化会把噪点一起锐化

练习PRACTICE

动手任务

找一条欠曝素材，先把暗部拉亮看彩噪炸出来；再上时域降噪对比；最后故意先锐化再降噪，看顺序的代价。

自检清单

做完任务后逐条自查，全部能打勾才算过。

常见问题FAQ

降噪能把噪点去干净吗？

不能。降噪是权衡：抹掉噪点的同时也在抹细节。地板能压低一点，但没拍到的信息救不回来。

胶片颗粒和数码噪点是一回事吗？

不是。胶片颗粒是成像介质本身的纹理，数码噪点是信号不足的缺陷。一个加分一个减分，别混为一谈。

Learning Objectives学习目标

After this card you can say where noise comes from, what ISO actually amplifies, and how to keep your picture away from the floor — on set and in post.

Concepts概念说明

Noise is the floor of the signal: in low light the photons themselves arrive unevenly (shot noise), and the sensor circuitry adds its own error on top (read noise). Two layers become the dirty grain you see.

  • ISO is an amplifier, not a lamp — higher ISO creates no light; it amplifies the signal you have, noise included
  • Underexposure is a noise amplifier — shadows already sit near the floor; lift them in post and the whole floor comes up
  • Chroma noise looks worse than luma noise — the coloured speckles in shadows stand out because the channels' noise is uncorrelated
  • Expose to the right (ETTR) — give the sensor more light short of clipping; the further the signal stands from the floor, the cleaner the image
Underexposed vs ETTR: distance between signal and noise floor

See also: Dynamic range · Bit depth · Log

Steps操作步骤

  1. On set: expose to the right short of clipping — do not count on post to save the shadows
  2. In post: noise reduction (NR) goes first in the node tree — clean the floor before grading
  3. Temporal NR for moving images, spatial NR for static detail; start with temporal
  4. Sharpening always comes last — it sharpens the noise too

Practice练习

Hands-on task

Take an underexposed clip: lift the shadows and watch the chroma noise bloom; compare with temporal NR applied; finally sharpen before denoising on purpose and see what the wrong order costs.

Self-check

Work through the task, then tick every line; all checked means you own this card.

FAQ常见问题

Can NR remove noise completely?

No. NR is a trade: wiping noise wipes detail with it. The floor can be pushed down a little, but information never captured cannot be recovered.

Is film grain the same as digital noise?

No. Grain is the texture of the medium itself; digital noise is a defect of insufficient signal. One adds character, the other subtracts quality — do not confuse them.

本卡术语Terms in this card

降噪Noise Reduction Lift（暗部）Lift 调色Grade 合成Compositing 节点Node 位深Bit Depth 动态范围Dynamic Range