Project: Dynamic Range, Your Camera’s Dynamic Range

This exercise was to measure the camera’s dynamic range by capturing a scene with a high enough dynamic range to exceed the camera’s limits. I waited for a sunny day then did as was suggested in the course notes. A large piece of what card was placed in direct sunlight by my front door, this provided the highlight and the low end would be provided by the dark door and/or the less well lit interior of the house. The upper limit of dynamic range would be indicated by a spot metered reading on the white card and I took a number of spot metered readings of the door and interior.

A number of exposures were taken, all at f6.7 and starting with 1/90 sec shutter speed, reducing until the card showed no clipping. This occurred at 1/350 sec. This was taken as the working shot and is shown here with the spotmeter readings:

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Photoshop pixel dropper was used on the card to show that the exposure was just below the clipping point.

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Values of R-253, G-252 and B-250 are satisfactory

The dark area was lightened and zoomed to pixel level to evaluate the noise:

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It is apparent that it is all fairly noisy, although it is difficult to determine what is the limiting noise as far as dynamic range is concerned; there is not a lot of detail to compare it with. Nevertheless, the bottom limit can be taken as the door.

Based on this, admittedly crude, experiment, the dynamic range of the camera is about 7.1 EV. This is a bit disappointing and does not compare particularly well with the result published by DxO for the same camera (http://www.dxomark.com/index.php/Cameras/Camera-Sensor-Database/Canon/EOS-40D) of 11.3 EV.

There are a number of things to be learnt from this:

1. This is not a very accurate way of measuring the dynamic range. The highlight is fairly accurate. Although the exposyres were half a stop apart, checking the levels of the three channels showed that it was not far from full white. The lower level was more subjective. Determining the noise/detail threshold is subjective anyway, but as it happened and due to the choice of scene, there was not a lot of detail to help make the choice.

2. Despite this, the exercise was another step towards understanding how a digital camera responds to light, especially at the extremes,

3. With this understanding, it should help to evaluate a scene with a view to how the camera will capture it. This is more the subject of the next exercise.

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