I'm not jumping on the band wagon here. This link has appeared on IIS since 2005 - and here it is again. No doubt many have been there already. But I do hope it doesn't get lost in the mass of threads.
I found the article by Samir Kharusi on sub-exposure calculations extremely helpful. Working my way through it day-by-day.
http://www.ayton.id.au/gary/Science/...graphy_DSO.htm