Exercise: Sensor Linear Capture

“The Photoshop Curves tool is one of the most powerful commands you have in the digital darkroom. But it can be intimidating to some.” Digitalcameraworld.com

The Photoshop Curves tool, is a powerful tool which can be used to adjust tones in order to brighten, darken, add contrast and shift colours.

The curves control determines how the values of the pixels you start with (The input values) are to the changed or transferred to become the Output values. When you open up a curves control box, you will see a diagonal line running through the graph, from the bottom left corner, up to the top right hand corner.

Example taken from Google Images.


Dark pixels can be made lighter, and the lighter ones can be made darker, by simply dragging anchor points up or down on the Curve line to lighten or darken tones.

We can simulate a linear image quite simply, by applying the opposite kind of curve.

For this exercise, you were asked to take any of your photographs (JPEG, TIFF) and open it in Photoshop elements. Convert it to 16 bits per channel (Image>Mode>16bits) because you will be making some strong adjustments that might create some banding in regular 8-bit. Go to (Image>Adjustments>Curves) and make a curve.

Once you have added the curve, the image will look how it was captured before the camera’s processor got to work on it. Save the dark image that you have just made.

I will be using Photoshop Light room 4.4 for this exercise.

I began by choosing the image below because when I looked at the histogram, it was exposed well. Not too light, and not too dark.

Original Image:

Original Image histogram


I took the image and made a copy. I opened the copy in light room. The exercise at the beginning asks for us to convert the image from 8bits to 16bits, however, I was unable to convert my image as I had problems which I will explain below at the end. Therefore, my image remained at 8bits. I then opened the ‘develop’ area in light room and proceeded to use the ‘Tone Curve’ which allowed me to create the curve I needed in order to make a darker image.

Dark Image:


Dark curve graph

(Outcome of the curve)

Dark Curve

You were then asked to put the original image and the darker image side by side. With the darker image, create a curve that will adjust the image, making it look close enough to the original ‘Normal’ image. By doing so, you are effectively doing what the camera’s processor does each time you take a photograph. Notice that the biggest effect has been on the darkest parts, they would have been lightened by what looks like several stops.

I took the dark image and re opened the ‘Tone Curve’. I began moving the curve until I achieved what I believe to be a curve that produces a similar copy to the original image.

Light Curve Image:

Light Curve

Light Curve (2)


Original Image                                        Dark Curve Image

Original Image Histogram small   dark curve histogram small

IMG_0571  Dark Curve

Once the curve in the graph was applied, the histogram changed. Most of the tones have been squashed strongly towards the left hand side. This means that the levels available to represent tones are devoted to the brightest parts of the image, while the darkest parts, the shadows at the far left, are represented by very few levels.

I was also quite surprised to see just how dark the image had become. Given that the original image was averagely exposed, the linear image has become extremely dark, with some areas with complete data loss. There is also noise in the shadow areas.

Original Image                                           Light Curve Image

Original Image Histogram small  Light curve histogram small

IMG_0571  Light Curve (2)

When I took the dark image, and began moving the line on the graph, I had to move it in several directions before I reached what I thought was acceptable. However, looking at the histogram, I have managed to keep it similar to the original image, by having the tones remain ‘Averagely Exposed’, however, there is data loss in the light areas, which in my opinion has been caused by constant transformations, and moving the line on the graph. When I zoom into the image, I can see a lot of noise, and some data loss in the image itself.


Overall Opinion:

I must admit that I had a lot of problems with this exercise to begin with. I currently use Photoshop elements 9. When I attempted this exercise using Elements 9, I was unable to do it because I could not find the curve tool. I looked everywhere online to help me find it, but nothing helped. Thankfully I recently bought a new camera, which came with a Photoshop Light Room 4 disc. I uploaded this disc and realised that thankfully, I was able to finish this exercise, and the upcoming exercise’s, as elements 9 would have stopped me from continuing with a lot of the exercise’s.

What I have learned from this exercise is that you are able to completely change an image, then take it back to its original form. It has also made me play around with my Photoshop and now Light room, as I am not the best at using either of them, therefore by doing these exercise’s, I am learning how cameras work with processing, and I am also teaching myself how to use Photoshop.



Online Article, Photoshop Curves Tool: 6 techniques every photographer must know, By Jeff Meyer, March 18 2013.


Digital Photography Master Class, Tom Ang, Dorling Kindersley Limited, London 2008. Page 164

ISBN: 9781409333906

Online PDF, Raw Capture, Linear Gamma and Exposure, by Bruce Fraser. Adapted from his book Real World Camera Raw, published by Peachpit Press, in August, 2004.







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