I have given-up with CHDK DNG and ImageMagick.I am using Reyalp's tool and looking at the data with ImageJ's histogram function.It indicates the median and other values and automatically only uses the area of the histogram that contains pixels to draw the screen image.If CS4 can do that I have not found how.
Now, I need to make the histogram as narrow as possible by using a steady, monochromatic, very diffuse light source.
Problem is, even if I find the true shutter values for my A620 the same factors may not apply to other cameras if it depends on how quickly the mechanical shutter can close.
But occassionally at faster shutter speeds, a single shot will show up that does have a lower exposure. I find this can be sometimes found if you set a static shutter speed and then take a series in burst mode. This may mean that occasionally, the camera will actually achieve a faster speed, but this is unreliable and can't be depended upon to be consistent.
Whoever first proposed this pixel value method doesn't know much about these cameras.
CCD cameras are routinely used to make extremely precise measurements of absolute luminosity. See the page I linked earlier.
Measuring by pixel values won't work. Due to the wide range of sensitivity due to ambient temperature of the sensor while it is being used from shot to shot. This is the same problem people ran into when trying to figure the top ISO range of their cameras. The first shot after powering-up the camera might look like ISO1000, but by the 20th shot it might look like it was shot at ISO1800, though the ISO override value was never changed. The only reliable method would be by photographing something at a known rate of motion and then figuring out how much angular distance it has moved during the exposure. Whoever first proposed this pixel value method doesn't know much about these cameras.
I would also prefer to use a more direct measurement of exposure time (like taking a frame of something with a fixed and known velocity).
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