Exercise 5: Sensor linear capture

The aim of this exercise is to take what we have learned about the sensor, linear capture and camera processing and simulate a linear image by applying the opposite kind of curve.

Any Tiff or Jpeg would suffice for this exercise.  I chose an image I had shot by the canal where I live that I had taken some months back.

When I opened the image in Photoshop I converted it to 16 bits per channel as instructed.  This is because I would have to make some strong adjustments which might create some banding in 8 bit.

I adjusted the curve to try and create what the image would have looked like when it was captured before the camera processor worked on it.

These are the images before and after the curve was adjusted. You can see that by adjusting the curve the image is a lot darker.

We can see from the histograms that the darker image has more tones to the left and the original image has more to the right and is more evenly balanced.  In the latter case, this is due to the fact that most of the levels available to represent tones are devoted to the brightest part of the image whilst the darkest parts - the shadows-  are represented by very few levels.  This has an implication for noise.  

The notes for this part of the exercise are incorrect because they tell you that the darker image should have the tones piled up in the right when it should be the left.  I have looked at the OCA website and this has been brought to their attention and will be re-written. 

Next I created a curve on the darker image in an attempt to get it looking as close as possible to the original image.  This is what the camera's processor does when we take an image.  

The most noticeable difference is to the darkest areas which have been lightened dramatically.

A note on Noise
If there is noise in an image it is concentrated in the shadows.  Before the camera processes the image it is buried from view within the dark areas of the image.  The strong curve that has to be applied to lighten the image to a normal appearance has the side-effect of exaggerating this noise as it lightens the shadow areas so strongly.

I examined a close up section of the images and noticed that there is some noise in the adjusted image.

Original

Adjusted image.  Noise is present.