Mark Metternich
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bobek13 wrote:
Another great one!
If I may ask, how do you calculate these extreme long exposure times? Since after sunset light conditions change quickly - a 15m exposure now can turn into 20m exposure 10m later :-)
Cheers, A.
Thank you very much! 
It's not a good way to teach others but I actually guess (and rarely miss)! But its much more than that though. The basic methodology is that I will first take a quick test shot of either 1 or 2 minutes. Then after that, I can guess it almost perfectly. Just remember that 1 more stop of light is double the exposure time.
So:
1min becomes 2.
2 min becomes 4.
4 to 8.
8 to 16...
Then at that point, you have pretty much reached your limit because 32 minutes would be too much for the sensor.
So, I take my first exposure with the settings I am going for (low ISO) and usually a sharp f/stop (like f/11 or f/8) then after exposing, I look at the histogram! Based on how far away the data is from the right wall of the histogram I guess how long I need to go next. So, as an example, let's say the data is somewhere up to the middle of the histogram. Well, if I just took a 2-minute shot, I am going to take the next one at, at least, 8 minutes (I want the data close to the right). BUT, as you rightly pointed out light is changing all the time (usually getting darker) as night comes. So actually I might go longer (10-12 minutes?) with the exposure time to make up for the loss of light. When I'm exposing the photo I watch to see what the light is doing. Sometimes it gets dark FAST. Sometimes a lot slower...
On workshops, I usually call out audibles, or several of us do. So I'll do a test and then call out what I think will be the best exposure length (and often settings) and also help others with their settings.
Once in awhile your test shot might be a bit dark (and I absolutely do not recommend basing your image off of your LCD - if you do rely on what the image looks like on your LCD it may look magnificent, but the file will be hideous - rely on the histogram only!). So if my image is quite dark, I usually take one QUICK last test shot (blue hour goes by pretty fast) and maybe open up the aperture by a stop (three clicks is a stop) so f/8 would go to f5.6 and double the 1 minute to 2...
If I did not figure it out on the first test, I always do it by the second try. Most times it takes me one.
Note: I have used plenty of apps over the years, but have found them less reliable.
CRITICAL SECOND NOTE: most people do not know that Adobe Raw Profiles (all of them) can shave off as much as 1-2 stops of highlights! The profiles have been through a pretty heavy S-Curve BEFORE they arrived in the Raw Converter (Lightroom / Camera Raw). This is data that a person can only get back if they create a LINEAR RAW PROFILE. But that is a more complicated subject matter more suited for Robert Park and my "The Ultimate Mastering Fine Art Printmaking Workshop...
Also, another idea is to choose the best f/stop for the land on the first shot (assuming a blend). Then take a variety of different lengths of exposure for the sky... Skies are soft, so any f/stop will work. Sometimes you might be doing it early and need to go to f/22 for the sky, then back to something like f/11 for the land...
Sometimes clouds are moving very fast, and sometimes slow...
That's about all I can tell you sitting at my desk and typing.
I also ENJOY NOT knowing everything beforehand. I even often count manually. But during workshops, this does NOT work. Thus the iPhone...
Have fun and please share your experiments with us all!
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