Reply To: Pixel Value and Exposure

[Mr Giovanni Di Giovanni]

Hi, Grant
It is actually not a good idea to do photometry with jpg formats. Unfortunately I have few resources and have to make do with those. However, with 256 levels you span a range of about 6 stellar magnitudes. And apart from the smallest variation still observable (sensitivity), 6 mag is not exactly small. For what you say, you are absolutely right, but I think that if, for example, I have to measure the height of a tree but don’t have a tape measure, what do I do? Do I leave it alone? No, I do it by eye: the tree is 8 m high + or – 2 m; I can tell at least that it is a cherry tree and not an olive tree. Also, PV-exposure linearity would be necessary to reduce PV measurements all at the same exposure time.
In my case, everything is more complicated. The attached graph shows PV-time for two iso values 100 and 800. I think I can solve the problem by observing that the two curves are similar to the charge function in the RC circuit. Of this it is not difficult to calculate the time constant, which is done for these two graphs. The awkward problem is having to apply the same mathematical algorithm to each photograph in order to use it. Robin’s graphs would be ideal for me, but only because the equation of a line goes in place of the charge law of a capacitor.
Of course, I will have to get the soft stuff you suggest.
Thank you.
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