Alex Weiss
New member
Hi,
first of all, this is a great, great forum, and I've already learnt a lot from reading here - heck, Doug Kerr's posts alone led me to register.
I bought a Canon 550D a while ago and would now like to measure its various characteristics, such as read noise and dynamic range (total and above/below middle gray). I've already measured read noise at various ISOs, and the results are consistent enough for me to conclude that my measurements were correct.
To calculate a theoretical dynamic range (theoretical because it is the engineering definition of the term), I simply want to find out the ratio between the highest possible signal level (i.e. at saturation) in ADU and the noise level.
But here's the problem: I get two different sets of readings for the highest possible ADU; in pictures with "normal" overexpose where only parts are blown out I get a number in the 13000s (don't remember the exact number, but it is always the SAME number, +/- 1). However, in pictures with severe overexposure (read: a white picture with pretty much no details at all), I always get a number in the 15000 (again, always the same one). I cannot explain this discrepancy, and it does not make sense at all to me. It also seems that these numbers, especially the first, are rather low, as these are the raw numbers that still include the offset Canon applies to all RAW data (2048 in the case of the 550D).
Has something similar ever happened to anybody?
Thanks,
Alex
first of all, this is a great, great forum, and I've already learnt a lot from reading here - heck, Doug Kerr's posts alone led me to register.
I bought a Canon 550D a while ago and would now like to measure its various characteristics, such as read noise and dynamic range (total and above/below middle gray). I've already measured read noise at various ISOs, and the results are consistent enough for me to conclude that my measurements were correct.
To calculate a theoretical dynamic range (theoretical because it is the engineering definition of the term), I simply want to find out the ratio between the highest possible signal level (i.e. at saturation) in ADU and the noise level.
But here's the problem: I get two different sets of readings for the highest possible ADU; in pictures with "normal" overexpose where only parts are blown out I get a number in the 13000s (don't remember the exact number, but it is always the SAME number, +/- 1). However, in pictures with severe overexposure (read: a white picture with pretty much no details at all), I always get a number in the 15000 (again, always the same one). I cannot explain this discrepancy, and it does not make sense at all to me. It also seems that these numbers, especially the first, are rather low, as these are the raw numbers that still include the offset Canon applies to all RAW data (2048 in the case of the 550D).
Has something similar ever happened to anybody?
Thanks,
Alex