Exercise 5: Sensor Linear capture

The object of this exercise was to simulate a linear image by applying a curves layer. To do this I needed to open either a TIFF or JPEG file and convert it to 16bits, this is to prevent banding which may occur using 8 bits, Then a curves layer was used, creating a smooth a curve as possible, to make the image dark, this image was saved under a seperate file name. The original image and the darkened image were compared side by side paying particular attention to the histograms. After this comparison was made I then returned to the darker image, opened the Curves dialog, created a curve which made the image look as close as possible to the "normal" version.


Original Image Cloudia - Elephant Parade 2010





I chose an image which had an average tonal range as shown by the histogram below.



Original Histogram



A curves layer was then applied (Image>adjustment>curves) which replicates what an image looks like before being processed in-camera.




Curves layer applied to make a linear image.

Having applied the curves layer the original image now appears far too dark.



 



Image as captured and before processing within the camera.

  






On examining the histogram, as expected with an image this dark, most of the tonal values are squashed against the left-hand side.

Histogram of the image before being processed in camera



The next step was to create another curves layer which replicates the camera's processor applying a gamma correction curve.



Gamma correction curve.



  
















The gamma correction curve brings the image back to a normal appearance.



Histogram after gamma correction

Although not exactly the same as the original, the tonal values are spread more evenly across the histogram. A strange exercise to do was my first impression, but it has shown me the extent of processing within camera that occurs. I knew it happened, or rather I didn't really think much about it but appreciated something technical went on inside ;o) Now I know what :o)