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If you have shot after sunset or before sunrise and see that your meter is blinking "30" that means it is too dark for the meter to get an accurate reading.
The following information will help you learn to expose manually to get the correct exposure.

Things needed:
a. Digital SLR
b. Tripod
c. Cable release or remote release

Assuming you are setup on your tripod, you have taken the time to line up your horizon making sure it is straight, locked in your focus point and are ready to click the shutter.
You see that at F/11 or higher the meter is showing a blinking 30 and not allowing you to take a picture.
Put your camera in bulb mode, lock the shutter and take a 60 second exposure.

For example, lets say your histogram looks like this after 60 seconds

farm3.static.flickr.com/2147/2069527216_444d348005.jpg

If your histogram looks like the one I linked to after a 60 second exposure, you can see it takes up approximately 1/4th of the histogram.
So in order to fill that histogram you would have to quadruple your exposure time.
Which in this case, you started with 60 seconds, now quadruple that time. You now have a 240 seconds or 4 minute exposure time, roughly.
I say roughly, because, depending on how fast the light is fading, you may need to add a minute or more to that guess.

Watch your histogram, you do not want the blob of data to be touching the very right end of the histogram. This allows you to not blow out your highlights. (this is very important)

If I have confused you or you have questions, please ask.
NorthernMinnesotaPhoto 10 years ago
what about reciprocity? With film and long exposures, say 3 minutes at f64 - f128, reciprocity was a process where you had to add time to the exposure because of the light saturation of the film.
It's another one of those calculations that old photographers had to do before every camera was a computer... kind of like bellows draw formula.
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Shawn Thompson - Lake Superior Photographer Posted 10 years ago. Edited by Shawn Thompson - Lake Superior Photographer (admin) 10 years ago
Please see above where it reads A. Digital SLR
Some argue that reciprocity is a bit of a factor in DSLR's most say it is not.

I don't want to make this into a Film question thread.
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JustinSinks 10 years ago
Very informative!
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K. Praslowicz 10 years ago
It's another one of those calculations that old photographers had to do before every camera was a computer... kind of like bellows draw formula.

Before every camera was a computer, and before Fuji Acros 100 existed. 1/2 stop extra for everything past 4 minutes is amazing.
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