What's needed for a Long Exposure photo and How to Calculate.
My Setup for really Long Exposures (minutes).
Mirror Lockup or Live View to prevent Mirror slap which causes slight vibrations
Shoot in RAW (to correct any colour cast from filter in post + other corrections)
Camera with Bulb setting (Digital or Film)
With Film, watch out for Reciprocity failure Wiki
Setup camera to measure in 1/2 stop increments instead of 1/3 stop (easier to calculate)
10 stop ND filter. Plus others if needed.
Composition (can't stress this enough)
If using Auto Focus don't forget to turn it off otherwise the lens will seek if using a 10 stop ND filter and you'll have to take it off and focus again.
Measure Shutter Speed with Aperture Value set at f/16 first before ND is positioned.
What ever the shutter speed count back 10 stops of light hoping for 1/15 or lower but no longer than 1 second (too long for me)
eg 1/15 - 10 stops would be 1 minute
1/8 - 10 stops would be 2 minutes
1/4 - 10 stops would be 4 minutes
1/2 - 10 stops would be 8 minutes
1s - 10 stops would be 15 minutes
Then switch to Bulb mode on the Dial and set the Aperture to f/16 and ISO 100
Take a photo with cable release and lock for the time measured.
I learnt all this via http://www.bwvision.com/ by Joel Tjintjelaar and read the guides for Ultimate Guide For Long Exposures part 1 - 3
Another resource is http://www.bulbexposures.com/ by Marc Koegel
Any questions let me know.
*edit now that i have upgraded to a Canon EOS 6D i use ISO 50 so i get less Hot Pixels & Noise during the Long Exposure.