為什麼 用DC 拍照的 noise 比數碼單鏡反光相機多?

Friend,

I introduce you the measurable term called SNR = Signal-to-Noise Ratios, that is how sensor engineer/designer evaluate the QUALITY/EFFICIENCY of a sensor.
The references and detail complicated equations and formulas are left to the end.
為什麼 用DC 拍照的 noise 比數碼單鏡反光相機多?
DC = P&S camera, the sensor size is very small 1/1.6-1/1.8 inches, meaning the sensor manufacturer need to pack N megapixels in to a tiny area. each tiny cell need to process the signal received from more photons, the energy comsumption per cell is increased, and the temperature increased in proportion in that tiny area.
DSLR has a bigger sensor, more than 1-2 inch in size ..., therefore, DSLR allow longer exposure/BULB SHUTTER and higher ISO sensitivity, because of the bigger sensor, and the more advanced signal processing.

圖片參考：http://www.dpreview.com/news/0209/canon/eos1ds/canonsensors-001.jpg

CMOS Sensor by Canon

圖片參考：http://www.robgalbraith.com/data/1/rec_imgs/135_nikon_d2h_LBCAST.jpg

CCD sensor by Nikon

圖片參考：http://learn.hamamatsu.com/articles/images/signaltonoisefigure1.jpg

SNR = Signal-to-Noise Ratios, the BIGGER THE NUMBER, the better sensor performance, or LESS NOISE.
Sensor noise
Noise is an unwanted signal, either contained in the relevant light
signal or added to it by the imaging process.
Noise prevents accurate measurement and
evaluation of the light signal distribution (image).
For any electronic measuring system, the signal-to-noise ratio (SNR) characterizes the quality of a measurement and determines the ultimate performance of the system.
The three(3) primary sources of noise in a CCD imaging system are photon noise, dark noise, and read noise, all of which must be considered in the SNR calculation.
1. Photon or shot noise
2. CCD/CMOS image sensor noise
3. Read out or amplifier noise

1. photon or shot noise
As a natural part of light, the number of photons impinging on a CCD/CMOS image
sensor that generates charge carriers is not exact, and can only be described
using probability. Photon or shot noise is a characteristic of light, which is best
described by a statistical distribution with an uncertainty. This noise is directly
proportional to the magnitude of the light signal itself.
(related to sensor size, the more photons collected in a fixed area, the more
photo noise.)

2. CCD/CMOS image sensor noise
Noise influences that may occur on a CCD/CMOS image sensor include:
- "dark charge" or "dark current" (e.g. thermally induced charge carriers)
Dark noise arises from statistical variation in the number of electrons thermally
generated within the silicon structure of the CCD,
which is independent of photon-induced signal, but highly dependent on device temperature.
The generation rate of thermal electrons at a given CCD temperature
is referred to as dark current.
Cooling the CCD reduces the dark current dramatically.
(related to the TEMPERATURE, the lower the power consumption, the less processing of photons in an area, the lower the temperature, the less noise.)

- transfer noise (additional or lost charge carriers due to shift of charge carriers
between the registers of a CCD/CMOS image sensor)
- fixed pattern noise (spatially fixed differences in noise behavior or sensitivity)
FIXED PATTERN (SPATIAL) NOISE
FPN refers to a non-temporal spatial noise and is due to device
mismatches in the pixels & color filters, variations in column
amplifiers, and mismatches between multiple PGAs and ADCs.
FPN can be either coherent or non-coherent.

- noise that is dependent on the setpoint of the CCD/CMOS image sensor, etc..

3. read out noise
A noise signal can be added during the conversion of the charge carriers into
a voltage signal and the subsequent processing and analog-to-digital conversion.
This is caused by the amplifier processing chain and the actual analog-to-digital
conversion process. This noise is uniformly added to the image.

睇完真是學到野. 謝謝.

大多數輕便數碼相機（DC）都是使用CCD來拍攝影像。而CCD在運作時所產生的熱量都會令影像產生雜訊。感光度越高，雜訊越多。DC的機身都較為細小，散熱方面並不理想，加上運算影像的晶片及其它元件都會產生雜訊影響整體的雜訊量。而且所有元件都緊密地收藏於機身內（特別是那些卡片型DC），雜訊較半專業的DC為多。DSLR的體積較大，而且是針對較資深及專業的攝影師而設，控制雜訊方面都較為嚴謹。所以較普通DC的雜訊少。CCD的面積越大，所產生的雜訊亦會越多。較大的機身便有較大的空間來安裝其它元件以避免雜訊的干擾。

個人覺得是,因為一個像素大小的關係,現今一般dc 的感光元件大小都是1/2.5 吋-1/1.8 吋.但都有500萬像素,有一些更可以去到1千多萬像素,反看dslr 的感光元件:由17.3 x 13.0mm-36 x 24 mm ,但為為何36 x 24 mm 感光元件大小的Canon EOS 1Ds Mark II 都只可以做到有效像素：1670 萬像素呢?因為這樣,我敘覺得是否和感光元件和像素空間大小有關呢?其實感光元件在曝光時是會產生熱力,http://www.dcfever.com/reviews/fujifilm/s5pro/page2.php
這個是 Fujifilm FinePix S5 Pro的測試報告,看來好像和雜訊沒有關係,但看看最下面,(同時可能為避免 CCD 出現過熱，功能每次最多只可以維持 30 秒。),這便可以說明,曝光時感光元件是會產生熱力的.
大家可以想像一下,Canon PowerShot G7在1/1.8 吋的大小下,有1000 萬個像素在工作,和Canon EOS 400D在22.2 x 14.8mm 的大小下1010 萬個像素在工作中,假設兩機曝光時間相同,好明顯400d 每一個像素都有比較大的空間,曝光時產生的熱可以更少,這是個人的推理

因為DC的感光完件比較DSLR的細小很多.對光的感應能力弱,於是要把訊號放大很多倍才夠光,這過程把原本不明顯的雜訊也變得明顯起來.