Exercise 5: SENSOR LINEAR CAPTURE

This exercise is aimed at explaining the simple but very important concept of the way light is captured in digital cameras. Although it is very basic, but understanding this concept is very important. Jeff Schewe and Bruce Fraser have very nicely explained this topic in their book “Camera RAW with Adobe CS5”. 

Prior to this exercise, I was aware of sensor’s linear capture. However, I had never paid so much attention to it (and definitely not done an exercise on it). I am kind of happy that the camera is taking care of the basic processing, but I also cannot deny that at times this can go against the photographer (especially while shooting in very contrast light conditions). That is why I only shoot in RAW format, which gives me more control over the captured images.

For this exercise, I have used an unprocessed and unused image (I had clicked this during the exercise 4). This image is straight from the camera; so only in-camera gamma correction has been done before viewing.

Ex5_Original

As per the exercise requirement, we need to reverse process the image and then try to restore it back. See the process, I followed in the exercise.

    • Opened the image in TIFF format in Photoshop CS4
    • Checked it for 16BITS
    • Opened the “Curves” and made the required adjustment. See the image below.

Ex5_Curves

    • Saved the dark image for the reference.
    • Opened the original image. Now I had 2 images on the screen tiled together (original and one with modified curves). Check the difference below, in the histogram for both the images.

Ex5_Histogram_Comparision

Kindly notice that in the both the images have very contrasting histogram levels. By now we have already learnt, that more levels on the left side of the histogram represents dark parts and right side represents bright parts.

    • Saved the darker image with another name (now we have 3 images, original, a dark version and a light version).
    • Now we have to apply another curves adjustment to this dark image. If we produce a curve that results in a similar image to the original then, we’ve duplicated the processing that goes on in camera.

Ex5_Reverted_Curves

Basically I have tried reverting back the adjustment, I had made to the curves. The curve adjustment shown here is somewhat similar to the camera’s processing

Conclusion:

This exercise reinforced the importance of exposing the image correctly. If the correct exposure compensation is used, the camera processor will not have to do extreme adjustments to compensate the image, that way we can make better quality images. While reverting the curve I noticed, that the most effect was on the darker parts. That means if we try to brighten the image, there will be a noticeable amount of noise.

Now onwards, I will make sure to choose the right amount of exposure compensation, when I am intentionally under-exposing an image.

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