過去實務界改變影像階調變化的工具是伽瑪值的調整，而且認爲人類視覺感受有固定偏好的伽瑪值範圍，這個範圍大約是1.1。所以，數位相機或攝影機在攝取影像的伽瑪值是0.4-0.5的範圍，顯示影像的螢幕則設定在2.0-2.5的範圍內，儼然成爲工業界標準(Stokes, 2001)。過去極少研究對於這個伽瑪值的偏好範圍進行實徵研究(Empirical Research)，本研究即透過兩兩配對比較方法，分別比較七種不同灰階影像內容的伽瑪偏好值，結果發現人類視覺偏好的伽瑪值並不固定，基本上是因影像內容不同而有不同的伽瑪偏好值。進一步由原始影像的色階分佈圖預測該影像最受喜好的伽瑪值，其線性回歸可解釋的變異量僅達55%。但是，預測最受偏好的影像平均照度(illuminance)值則回歸可解釋的變異高達98%。這個結果可能意味著，工業界以固定範圍之伽瑪變化企圖使影像階調表現達到最佳化的觀念值得深思，文末也進一步探討此結果的可應用性。
Gamma correction has been a common method for practitioner of computer graphic to adjust image's tone level. In previous works, researchers always set the net gamma value as 1.1 to fit the uniform model of visual perception (Stokes, 2001). There were very few works on searching visual preference for gamma. This study used pair comparison method to investigate whether human does prefer specific gamma value for any image content or not. We made 13 gamma transformations for 7 photos respectively, and asked observers to choose the preferable one from two transforms. Results showed that the most preferable gamma of 7 photos were not constant. It could not give a clear indication for the relation between the optimal gamma value and the visual preference among our test images. A series of analyses were further performed on the image properties to explain the visual preference. It turned out that the resulting brightness of the photos after each gamma adjustment showed a strong correlation to the visual preference.
