How would i go about calculating how much ND i need and what aperture to select when i want to make an exposure past 30 seconds, all the way to 30 minutes or more? is it up to mathematics at that point or is there a formula? which i guess goes back to mathematics but anyhoo..
Anyone know?
Thanks,
Mike
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This is how I do it...
Set my camera to its highest ISO and biggest apeture, then the cameras light meter show how long shutter time I should use.
Lets take an exemple :
1Ds Mark II on ISO 3200 f/2.8 with 5second shutter time.
f/2.8 ISO 1600 = 10sec
f/2.8 ISO 800 = 20sec
f/2.8 ISO 400 = 40sec
f/2.8 ISO 200 = 80sec
f/2.8 ISO 100 = 160sec
f/4 ISO 100 = 320sec
f/5.6 ISO 100 = 640sec
f/8 ISO 100 = 1280sec
f/11 ISO 100 = 2560sec
f/16 ISO 100 = 5120sec
f/22 ISO 100 = 10240sec
f/32 ISO 100 = 20480sec
Then I just devide resault into 60 to get minuts.
Might be many other better ways to do this, but this works well for me...
Here is a shot I made here in Denmark on a Phase One P30 digital back, 45sec exposure.