高階數位機背RAW檔銳利度之研究

在21世紀高畫質時代，高解析度數位攝影是提高大圖輸出與數位典藏影像品質的首要條件，但如何獲得最佳的影像解析度，過去在出版印刷業界缺乏科學化的探討，因此經常被詬病擁有一流設備，但製作不出一流作品。本研究以數位機背做為影像擷取媒介，從光學的角度，對數位攝影系統中影像RAW檔銳利度擷取採取科學量測，探討數位影像RAW檔銳利度參數設定對影像解析度的影響，藉此研究出最佳調整參數。本研究針對數位攝影系統六大模組中，鏡頭模組對數位機背影像RAW檔解像力所造成的影響進行研究。根據光學傳遞函數理論之調制傳遞函數(Modulation Transfer Function, MTF)檢測出各鏡頭的最佳光圈值，再以最佳光圈值結合數位機背的銳利度自訂參數獲得最理想，不至於銳化過度的高解析度影像。研究結果顯示，鏡頭最佳光圈未必越小解像力越高，不同鏡頭的最佳光圈各不相同。數位機背的銳利度參數中，「強度」與MTF值以及「銳化過度」比率正相關，但50%MTF數值太高，會導致「銳化過度」。各鏡頭的最佳「強度」「半徑」「臨界色階」銳利度參數組合略有不同，但大約落在9-0.8-3左右。要獲得絕佳組合還是得為鏡頭與數位機背量身訂作。

關鍵字

數法院位機背； RAW檔；銳利度；影像品質

並列摘要

In this high definition image era, extreme high resolution becomes the first priority for digital photography to improve the image quality of large format printing and digital archiving. How to achieve the optimal sharpness for reproductions have not yet been well-studied in publishing and printing industries. Many photographers have good cameras and lens but still cannot obtain high quality photos. Therefore, the present study focuses on the sharpness optimization of digital camera back for RAW image files in a scientific way. The first step was to test the Modulation Transfer Function (MTF) of different camera A Study of RAW File Sharpness Captured by High-end Digital Cameras lens using Imatest software with ISO 12233 test target. The optimal aperture (f-stop) values for each lens were determined based on the 50% MTF values. The next step was to fine-tune the parameters of sharpness enhancement for the digital camera back using the optimal aperture values. It came up with the best parameters for sharpness enhancement based on an index of oversharpening. The experimental results suggested that the brand and focal length of the lens have no direct link to 50% MTF values. All camera lens without using sharpness enhancement in the digital camera back would reduce the 50% MTF values to about 30%. However, enhancing the MTF values by means of heightening the amount of sharpness enhancement would result in oversharpening. It would be unacceptable for large format images. The optimal parameters are camera lens dependent. The optimal values for ”amount”, ”radius” and ”threshold” are about 9, 0.8 and 3. The best sharpness of RAW image files can be achieved by doing the test and then finding the best parameters.