1. Developing a technique for the camera to measure flash light
A light meter can not measure the clipped flash output because it takes in all of the flash. In order to measure the clipped flash output, a technique was developed to use the camera to measure flash light. The Canon G9 compact digital camera and the Canon 430 EX II flash were tested in this project. Images of a Kodak grey card that reflects 18% of the incident light with 1% deviation were taken by the camera. Flash output level was varied from the maximum to minimum setting. Each flash output level was tested three times. A Gossen Variosix F2 light meter, with a repeatable precision of 0.1 stops, was placed beside the grey card to measure the flash output. The camera was set at an aperture of F5.6, ISO 100, a distance of 2 metres to the grey card, and a shutter speed of 1/125 second. The slow shutter speed was used to ensure that the flash output was completely captured by both the camera and the light meter. Figure 1 shows a typical shot of the setup.
The brightness (mean RGB value) of a centre square of the grey card was measured with Adobe Photoshop. Figure 2 shows the linear relationship section between the light meter reading and the brightness of the grey card. The linear relationship can be used to determine light output of high speed flash exposure.
2. Using the camera to measure clipped flash output
The Canon G9 camera was used to measure the clipped flash output of the Canon 430 EX II flash at various high shutter speeds. The same setup as before was used except that the shutter speed was varied and the flash was set at the full output level throughout the test at a distance of 3.5 metres for the Canon 430 EX II. Three trials were done at each shutter speed up to 1/2500 second. Images of the grey card were taken by the camera at each shutter speed. The brightness of a centre square of the grey card was measured with Adobe Photoshop. Using the equation determined previously for the linear response of camera image sensor, the clipped flash output was calculated. The data was fitted into Equation 7 developed in this project. Figure 3 shows the relationship between the measured clipped flash output and shutter speed, along with the best fit of Equation 7 with R2 = 0.9961 for the Canon 430EXII flash.
