Project: Linear Response

The idea behind this exercise was to simulate the response of the sensor to light. Unlike the human eye, the sensor’s response is linear, doubling the light falling on it will double its output. The eye on the other hand is more sensitive at low light intensities, with the response rolling off as the light level increases. The exercise involved working with a single jpeg image and reversing the processing the camera applies to attempt to show what comes off the sensor.

Here is my starting picture:

_MG_0310

I used Canon’s Digital Photo Professional to work on this image as it gave me more control over the curve function than my version of Photoshop Elements. Here is the reverse gamma curve I applied, shown with the resulting histogram:

clip_image002[4]

And this is the resulting picture:

_MG_0310 Linearised

This is overall much darker as the histogram predicted it would be.

This is the simulation of the picture as it came off the sensor and is the result of the sensor’s linear response. This is typified by the dark image.When the camera processes this data with the intent of matching the scene to how the eye would have perceived, it uses a gamma correction curve and the object of the next part of the exercise is to produce this.

Again using Digital Photo Professional, I used this curve:

image

And got this picture:

_MG_0310 Linearised recovered

The penalty is noise as shown in this magnification of the wall to the left, what was deep shadow in the linear response version of the picture. There is also some notable posterisation due to the intense processing of the 8 bit file.

image

This exercise was useful in developing understanding in the way camera sensors react to the light as its intensity varies. There is more that can be explored: the difference between CCD sensors and CMOS sensors, the development of CMOS sensors with programmable response and lin/log response but that is getting into a bit too much detail. I think for now, it is sufficient to understand that sensors responde to light in a different way to the eye and some processing is required to make the resulting image look “right”.

Another useful outcome was to expore the curves function of processing software. I particularly liked the Canon program I used. as well as allowing user selectable curve points, it displayed the RGB and master histograms with the curve and showed in real time the effect on the histogram of changing the curve. I found it very effective and gave considerable control over the process. And all this on a jpeg image!

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