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Old 19-10-2007, 07:40 PM
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bojan
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Location: Mt Waverley, VIC
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Quote:
Originally Posted by fringe_dweller View Post
umm ..ermmm .. i'm embarrassed to say it Phil , i didnt use it astronomically until the warranty was over .. i was still mainly into film for astro work, other than quick twilight snaps, so is my fault to a degree,
here's an example of it, funny but the longer the exposure, the less trouble it is!? in fact you dont even notice it really, but anything up to and around 30 secs is really bad!
here's an example, ive exaggerated it and converted to grayscale to show it clearly
Guys, I think you are simply over-processing your pictures, IMHO...
I have the same thing with my (unmodified) 400D, when the hystogram is stretched such that the readout noise becomes visible. This is quite normal, actually. There is no such thing as noiseless sensor, or noiseless amplifier.
Longer exposure cures that "problem" nicely.... Lowering sensor temperature (or the temperature of the whole camera) will also help a lot. Stacking (in DSS, for example) also dramatically reduces this effect because the noise is averaged, and the signal is left as is.
The best idea is to use the whole dynamic range of the sensor and amplifier, not just the range of signal levels close to the noise floor. That means, lower ISO settings (this lowers the noise) and exposure duration such that the only the brightest parts, which are not needed or wanted, are saturated. In astro-photography this is of course tricky because the dynamic range of the subject is huge.. but it all depends what we want to show on the final photo.
The analogous principle applies to film photography as well, the only difference is that the noise here is of different nature (it appears more random because there is no read-out noise which in electronic cameras is not random but rather in sync with the readout process, line by line and pixel by pixel. Also we have grains here which are analogous to pixelization in digital pictures). Also, in the chemical photographic process it is not as easy to stretch the histogram that much (it can be done by using high contrast paper).

Last edited by bojan; 19-10-2007 at 08:46 PM.
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