Quote:
Originally Posted by sharptrack2
My 0-255 reference came from something I had just watched as I have been seeking more detail about what is a histogram and how to read it. They may have simply used the concept of a point and shoot camera that only puts out JPEGs. Interestingly, does that mean that the histogram on a DSLR camera is measuring the full resolution of a RAW file (is that range known?) Does it pick a format to display in, 12 or 16 bit?
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The analog signal is read off the sensor and converted to digital by the ADC, which is then stored in the RAW file. The resolution of the ADC in most Canons is 14-bit, but there are many cameras on the market that are 12-bit. Astro CCDs tend to be 16-bit. For 14-bit, that gives you values from 0-16383.
From what I understand, the raw image is converted to JPEG for the rear view screen and histogram...digital cameras are very efficient at doing that conversion, but that isn't to say that the JPEG is an accurate representation of the raw data.
Again for a Canon, you should expose to bring the main peak of the histogram off the left hand edge at least. The main peak of the histogram is the background sky (unless you're imaging something Eta Carinae and it fills the whole FOV
) and you want this to overwhelm the read noise of the camera as much as possible, as otherwise when you try to stretch your signal to bring out the fainter detail you will also be stretching the ugly noise too
Typically, the bias noise has values around 512 - 2048 (depending on the camera), so the range of values where you want to be recording signal is between that and the maximum. Of course, once you hit the maximum values the object (be it a star or otherwise) is over-exposed and there's no coming back from that! IMO find the sweet spot is where the stars in the frame are not over-exposed but where your signal is over the above noise. It's not easy to judge that from the back screen of the camera
