Example
1) Dynamic Range
Dynamic range or light sensitivity range of a sensor indicates the ratio of light exposure between the highest brightness a camera can capture (saturation) and the lowest brightness it can effectively capture (typically when noise becomes more important than signal, i.e., SNR < 0 dB). This range indicates the maximum contrast that can be effectively captured by the sensor. The larger the dynamic range, the better, as the sensor is able to capture higher-contrast scenes. Note that dynamic range is expressed on a logarithmic scale in EV (same as f-stop), thus an increase of 1EV corresponds to a doubling of dynamic range.
2) 感光度(ISO sensitivity)
ISO Sensitivity indicates how sensitive the camera sensor is with respect to light. In terms of exposure, the higher the ISO sensitivity, the better, as less light is needed to make an exposure. However, increasing the light sensitivity (from analog or digital gain amplification) also increases the image noise. On this graph we compare manufacturer-provided ISO number (indicated in the image's Exif), versus the DxO Labs measured ISO (as defined in the ISO #12232 standard). ISO Sensitivity is also referred to as ISO Speed.
3) 信噪比(SnR)
The signal-to-noise ratio (SNR) indicates how much noise is present in an image, compared to the actual information (signal). The higher the SNR, the better, as less noise will be present in the image. The scale is logarithmic (in dB): + 6dB means two times less noise (as a reminder 3dB corresponds to 1 f-stop or 1EV).The SNR is here measured at the image mid-dynamic (which corresponds to a gray with 18% reflectance, commonly accepted as the average photographic scene reflectance).
4) Tonal range
Tonal range indicates how many gray levels are distinguishable up to noise in an image. The tonal range corresponds to an integration of the signal-to-noise ratio (SNR) over the dynamic range of the sensor, and is therefore representative of the average noise level. The larger the tonal range, the better the image: noise is lower, and subtle nuances can be distinguished. It is expressed on a logarithmic scale as the bit depth necessary to encode that number of gray levels: an increase of 1 bit of tonal range corresponds to doubling the number of distinguishable gray levels.
5) Colour Sensitivity
Color sensitivity is an extension of tonal range that takes into account the color rendering involved in raw conversion: it basically indicates how many colors are distinguishable in an image, up to noise. Typically, if the sensor is slightly color blind, color rendering implies a strong digital color amplification and therefore a strong increase of noise. The greater the color sensitivity, the better the image: noise will be lower and with lower coloration. Color sensitivity is expressed on a logarithmic scale as the bit depth needed to encode a given number of colors: an increase of 1 bit of color sensitivity corresponds to doubling the number of distinguishable colors.
鏡頭:
1)銳利度
2)色差 (Chromatic aberration)
3)微型對比度/發色 (Local/Micro contrast)
Example
4)散景
Example
GodzillaNeo wrote:
究竟什麼是"畫質"呢? ...(恕刪)
這真是大哉問啊！
要比較數位相機，大概就是比性能以及畫質這兩個部份吧！所以相機的畫質可是非常重要的部份，因此除了自我感覺良好之外，數位相機的畫質必須要被嚴格地以科學測量的方式來訂定，以便作客觀的評量，這樣才有辦法作客觀的比較。
畫素的數量不代表一切！為甚麼呢？ 答案其實很簡單，試問1200萬畫素的DC跟1200萬畫素的5D，在同一個環境下的照片，看起來會一樣嗎？，DC跟5D的畫質差異主要是來自於感光元件的差異，所以這邊我們必須要先把鏡頭的變數先排除。大多數的人都會同意，5D的照片比較好，那 “到底好在哪邊呢？”
高ISO雜訊較少? 對！ 動態範圍比較廣？ 對！ 顏色比較自然豐富？ 對！
我們來想想，如果5D是8000萬畫素，可是每個畫素的雜訊，動態範圍，及顏色的表現都跟DC一樣，那你願意用它嗎？
單位畫素的表現，其實就是數位單眼有別於輕便型相機(DC)的地方，為了測量這個表現，很多測試網站嘗試使用各種不同的方式來測量，我覺得最科學，最精確的就是由DXO實驗室所進行的測量。有興趣的攝友可以去
DxOMark的網站看看他們測量的標準流程（standard operation protocol）以及測量方式的理由（rationale）。
針對樓主的問題，我建議可以去DxOMark的網站看看，裡面有很多相機感光元件的排名，可以發現高畫素的感光元件表現不一定比較好，原因其實跟DC類似，就是畫素間距小的關係。有很多人認為使用高畫素的相機例如5D2，只要縮畫素就可以提高畫質，這是錯誤的說法，頂多就是可以讓高ISO雜訊好一些，對於動態範圍及顏色方面是毫無幫助的。這很容易理解，想想，用1200萬畫素DC拍的照片，就算縮圖到100萬畫素，也不會有5D的動態範圍及顏色表現的。
為了高畫素所得到的空間解析力，是必須在畫質上付出代價的，所以務必要針對最終影像要呈現的平台來選擇適當畫素的相機。如果將來是要印A2以上的大圖而且要求到藝術攝影(fine art photography)的等級，那高畫素就能帶來實實在在的幫助。對於A3+以下的應用，或是在螢幕上觀看，那就不建議使用太高的畫素。
註：說起來是想當然爾的事情，上面的說法只在同一個世代的感光元件間的比較才適用。不同世代的比較，像是D3X就是個例子，在畫素這麼高的情況下，D3X在低ISO時的動態範圍跟色彩深度的表現還是比D3要好上一級，當然畫質也就比較優越。相信未來1Ds4也一定也會挾帶著最新科技帶來讓人驚豔地表現。
Blog: http://www.drzyx.com
相簿: http://www.flickr.com/photos/zhiyixie
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