EXERCISE: Sensor Linear Capture

We can simulate a linear image quite simply by applying the opposite kind of curve.  This will not be exact, but close enough for our purposes.

Take any TIFF or JPEG image and open it in Photoshop Elements.  Convert it to 16 bits per channel (image >Mode >16 bits) because you will be making some strong adjustments that might create some banding in regular 8-bit.  Got to Image > Adjustments >Curves and make a curve that looks like the example.  You will need to create several points along the curve to keep it smooth.

Being very technically challenged when it comes to Photoshop, I searched my computer high and low to see if I had a version of Photoshop Elements, as the write up for this exercise (above), stated that this was the software programme to be used.  Unbeknown to me, I could in fact use Photoshop to conduct the same exercises.  After much research and deliberation I tried to simulate the requirements needed in both Adobe Bridge and Photoshop and found that I could complete the exercise as requested, so disaster averted and time to knuckle down to the task ahead.

I decided to use a photo I had taken whilst on a recent photography outing to London.  This image had received no post-processing manipulation and seemed to be fairly well exposed.  Due to the subject matter, and the shadows and highlights already present in the shot, I was interested what changes would occur once the Linear Simulation Curve had been implemented.

St Stephen Walbrook Church - Original

St Stephen Walbrook Church – Original

As I do not use (or yet understand) TIFF, I opened the JPEG image in Photoshop and changed the image mode to 16-bit, displayed both the Histogram and the Curve dialog boxes then set about working through the requirements of this exercise.

So that I could mimic the curve we received in our coursework, I changed the grid on the adjustment layer so that I had smaller squares to work with, and then set about making a four-point curve as required.

St Stephen Walbrook Church - Dark

St Stephen Walbrook Church – Dark

As expected, the dark areas of this image have now become very dark, with data lost completely in some areas, on the reverse side, the light areas are now very distinguishable and are a better representation of the scene when I took the original photograph.

There is also big difference in the histograms:

Original Photo Histogram

Original Photo Histogram

Dark Photo Histogram

Dark Photo Histogram

The original histogram dictates that the image was exposed relatively well with only a small amount of over-exposure, which can be seen in the sky of the image (and to the right of the histogram), but as the coursework advised, in the second image, the histogram data is squashed almost entirely to the left of the grid.  This tells us that most of the levels available to represent tone are contained within the brightest parts of the image, whilst the darkest part, or shadows, are represented by fewer levels.  This latter piece of information actually has implications towards noise, something that this module seems to have been building towards from the very beginning.

Before moving on from Linear capture, we have been requested to reverse this action and produce a curve that makes the image look as close as possible to the original.

When reversing the curve as shown in the coursework, I was not able to achieve the exact same exposure as I had seen in the original image, in fact the image is quiet a bit lighter than the original shot; although this has not produced such stark results as we saw in the initial test to produce what our camera saw before the image was processed.

Reverse Curve

Reverse Curve

I was not really happy with this result, as I knew it was not what I should have expected to see, and that is when I realized that I had used the wrong image file to produce the reverse curve, so to rectify this issue, I set about using the darkened copy of the image and run the test again.

Reverse Curve Pt.2

Reverse Curve Pt.2

This is a much better representation of the original image – although it is still not quiet right, and it is the closest I can get the result. They only thing that does not look right is the new histogram and although it has roughly the same shape, there are individual lines present and not a block of black as in previous representations.


I am really enjoying working though DPP as I feel that I am beginning to learn and understand how a camera works.

Doing the research, then conducting the exercises is very satisfying, especially when you can replicate the result AND understand what you have done in the process.  Reading up on the jargon is currently paying off; I just hope this module continues on the same vane.

I am also beginning to get to grips with Photoshop and find myself playing around with it more and more, so that is another plus side, although it is taking me quiet some time to complete the exercises as not only am I learning about photography, but I am on a crash course to learn and understand post-processing too.

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