Exercise: Sensor linear capture

One of the most appealing aspects of wet film is it’s natural response to light, in that it tends to mimic the capability of our eyes.  Not so the digital sensor.  Our eyes have the ability to compress the extremes of light and dark into a larger dynamic range than is available to the digital sensor.  This allows our eyes to cope with a wide range of light levels but to achieve the same with a digital camera we have to resort to electronic manipulation.

In the great majority of cases, our cameras process the raw digital interpretation of a scene before we are presented with it.  This ensures that the result is much more to our liking than the basic data would have appeared.  Of course it is possible to process this raw data ourselves using suitable software in such programmes such as Photoshop or Lightroom.

To examine how the raw data would be presented without processing this exercise asks us to open a picture and adjust it’s curves to simulate the raw data.

This is the original picture with it’s histogram attached.

With the curves of this picture adjusted like so…

the picture looks more like it would have done without any processing.

With the two images together I now return the dark one (left) back to resemble the normal one (right) by moving the curve up and left.

The result of this adjustment is to lift the dark areas of the image but the side effect is the noise that was previously hidden in the shadows had appeared.  Below is a comparison with the original image.

The noise is quite evident when viewed above with a section of the original image.

For all practical purposes, the processing done by the camera to create a good looking image is beyond the understanding of most photographers.  Even when converting a RAW image file to save as a TIF or JPG, the photographer is mainly tweaking a pre set series of values that have been chosen by the software designer.  However, having access to the RAW file allows us much greater artistic freedom.  Though we must endeavour to remember the limitations of the digital sensor, particularly when eyeing up a scene of high contrast and large dynamic range.  What our eye sees is not necessarily what the camera will be able to record!

Addendum:  The course notes relating to this exercise would seem to be in error.  They state that ‘for the linear image the histogram shows the tones to be squashed strongly to the right’, whereas in fact they are moved to the left as can be seen above.  In addition, the notes add that the linear image ‘has most of the levels available to represent the tones devoted to the brightest parts of the image’ whilst it can clearly be seen that the linear image has more dark tones.
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