Exposure Response and Noise in a Digital Imaging System

Pictures represent scenes by recording different values of brightness and color at different locations. A digital camera has a sensor with many pixels. Each pixel has stored electric charge that leaks out through a photodiode when light falls onto it. The amount of leaked charge indicates the exposure which corresponds to the amount of light energy that hit the pixel. Variations in exposure for the same light energy represent noise. Besides the scene\u27s luminance (brightness), exposure and noise depend on the camera settings of shutter speed, aperture, ISO, and resolution (megapixels). The first experiment’s goal was to see how accurate the proportionality between the exposure and light energy was. The second experiment’s goal was to test the principle of reciprocity which states that the same exposure can be achieved with different combinations of shutter speed and aperture as long as the same light energy is maintained. The third experiment’s goal was to see how noise is affected with an increase in ISO. The last experiment’s goal was to see how noise is affected with changes in resolution. The experiments used a Canon digital camera and a software called GetRGB which extracts the RGB values for every pixel in the picture. For each color value, the mean and standard deviation were calculated for all the pixels which reflected the exposure and the noise respectively. The following conclusions were found: 1) Even this basic digital camera proved to have a more linear exposure response and reciprocity than typical film. 2) As expected, the noise increased with ISO due to a higher amplification for a smaller amount of charge. 3) As expected, a decrease in the number of pixels resulted in less noise due to the merging of pixels and consequently more collected light energy and fractionally less variations. This contradicts the common misconception that more pixels are always better