I am wondering how you compensate your exposure calculations in the early a.m. or late afternoon so as to not under expose or over expose.

For example, lets say you set up your shot and get a reading of F/16 - 2 second without the filter on. You calculate your expousure down to 1024 seconds for the 9 stop ND.
Now you put the filter on, switch to bulb mode grab your release and lock the shutter open.
The light is going to dim over the next 17 minutes, so how much longer do you expose past the 1024 seconds?
It must be a guessing game at that point. right?

It isn't like you can wait for the exposure to finish and try again. after 17 minutes if you under expose, you can't set up and try again because it will be too dark and the light and colors you want to include will be gone.

the same would obviously apply for an early a.m. shot, only the light obviously gets brighter over the long exposure.

Good Q.

Guess. Get a feel for it, then you can start making educated guesses. That's at least how I shoot long exposure pre-dawn and post-sunset. I really need to start bringing two tripods and two bodies with me. Shoot extreme ND on one and standard with the other.

Just build a multi head tripod, maybe a metal frame that will put one right on top of the other? That'd be badass.

Do you guys really calculate the exposure length like shawn mentions or even just guess that?

I guess.

I mostly guess. If light is really fading I'll increase exposure time. However I've rarely put an ND filter on when I'm already getting a 2 second exposure.

Why would you not calculate it down the 9 stops from your initial reading and guess from there? At least then, you have a good starting point.

I just start at 60 seconds, f/16 or so, then go from there.

I do the calculation, but then guess what would be a good adjustment.

Originally Posted by emorphien

I do the calculation, but then guess what would be a good adjustment.

+1

If you gave me a histogram at 60 seconds, f/16... I'm fairly confident I could tell you the exposure you need to get it right. So, pretty much, I'm doing the same thing you guys are, just without the calculation.

I think at those light levels, your going to be reaching into reciprocity failure too.. I know a lot of people think CCDs dont suffer from it, but they do, just at lot lower EV's than film used to..
as such, I would probably add 30-50% exposure, or just hope RAW will pull out any extra I need...

More info..
http://www.cinematography.net/ReciprocityFailure.htm

Scott, I tried your "expose for 60 seconds and guess from there" technique and man that works good.

For example


If your histogram looks like the one above 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 would be 240 seconds or 4 minutes, roughly.
Depending on how fast the light is fading, you may need to add a minute or more to that guess.

Originally Posted by thomps6s

Scott, I tried your "expose for 60 seconds and guess from there" technique and man that works good.

For example


If your histogram looks like the one above 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 would be 240 seconds or 4 minutes, roughly.
Depending on how fast the light is fading, you may need to add a minute or more to that guess.