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[QUOTE=Shiraz;1035056]Hi Raymo. The meaning of the ISO speed rating of a camera is largely up to it's maker and it can be defined in a number of ways, so there is no reliable way to directly relate ISO to CCD capability.
If you want to get some idea of how to relate your DSLR system exposure times to the ones used in the book, maybe choose a bright astro object from the book and take some images of it yourself. Use ISO400 and try about half an hour of 4 minute subs to start with. You are going to have to do stacking of the multiple subs and a histogram stretch to get anything useful. When you have an image, you will be able to see how much different it is from the published one of the same object and be able to increase the total imaging time to get a closer representation. Then you will have at least a crude idea of how an exposure time given in the book relates to your requirements.
Whilst it is true ISO (International Standards Organisation) is able to be measured several ways by the camera makers ISO images are heavily compared at sites like DP review. If they are too different the camera maker gets a lot of criticism. So I think they tend to be closer than that in practice. Certainly Nikon and Canon ISO's would be pretty reliable. Certainly Fuji fudges it a bit where ISO6400 is really only about ISO4800 or less. Sony's seems accurate.
Greg.
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