German version of this article here
One method that is often named in the context of day-to-night time lapses is called 'bulb-ramping'. The idea behind this is using the camera in its so-called 'bulb' mode and an external device ('bramper') to change the exposure time automatically.
While using the 'bulb' mode the camera opens the shutter curtain as long as the shutter button is pressed and closes it as soon as the shutter button is released. The mode is the preferred one when using long exposure times in conjunction with cable releases, e.g. night photography.
'Bulb ramping', using an external device, is used to 'ramp' up or down exposure times during time lapses.
Ideally this would keep continuous exposure transitions throughout the whole sequence.
The idea is not a bad one but the problems that occur are by design.
To know how much the transitions between two shots has to change, the controller has to the exposure delta that it has to change during a certain time, for example: during the next 2 hours the exposure time has to change from 1/50 to 2 seconds.
But to get to these values you need to know in advance which exposure time you would need from now on in 2 hours!
This piece of information depends on a lot of different factors, like:
- weather (what, if the sky is cloudy in 2 hours?)
- geographical latitude (closer to the equator the sun sets a lot faster that in central europe for example)
- the season or time in the year
- time of day
just to name a few.
Even if you could repeat a capture on 2 consecutive days (measuring the scene on the first day and capturing it on the second) the problem of weather changes and many other factors would remain.
Another idea used by some of the 'bramper' devices is to use a light meter (photocell) to measure the exposure times needed - with similar disadvantages as using camera internal metering: it cannot distinguish between changes that are natural/wanted (like clouds moving across the sky) or unwanted ones (like a bird sitting on the photocell).
Even if such a 'bramper' could incorporate GPS for the location and pair with a Smatphone to get weather information and the path of the sun there would still be unforeseeable variables that would interfere with a precise control of exposure.
Leaving the technical points aside, a 'bramper' is another complex device which you have to carry with you and still have the (high) chance to mess up that one sequence (in contrary to digital photography with time lapses you generally don't have a second chance to redo it with adjusted settings)
All of these problems made me develop a more flexible and simpler solution for the so called 'holy grail of time lapse photography': seamless transitions between day and night which can be accomplished without external devices.
Using this method dealing with sequences containing extreme dynamic range is a breeze as it is not prone to any of the disadvantages named above: after years of being hard to realize, now you can get the so-called 'holy grail of time lapse photography': perfect transitions from day to night or vice-versa.
The basic idea of this method is to change the exposure time or the ISO setting during the sequence as soon as you realize that the picture taken is over or under exposed to a certain degree.
To determine if this is the case, you could use the histogram when playing back your images during shooting - another, even better method is using the build-in metering of the camera. Don't worry - we will use it manually!
Almost every camera will show a exposure meter (either in the viewfinder or on the top LCD) during operation in manual mode (M) to show if what the camera sees is over or under exposed. Often the meter is a bar, marked with '0' at the center to mark the 'correct' exposure.
After the initial setup the meter shows the desired value e.g. '0' - in this case '0' is an example because you might want to deliberately over or under expose the sequence to keep light areas from clipping, during a sunset for example.
Now you need to remember the initial value you just set. While the scene gets darker the indicator moves to the negative section. If this indicator is at '-1' (or 'initial value - 1') you know that the current image will be under exposed by one stop - this way you know that you can compensate for exactly one stop. Using this method has big advantages over the use of the histogram as you do not need to touch the camera more than necessary and even save on some battery.
The next step is essential - you need to know exactly how to change the exposure time or the ISO on your camera by exactly one stop. Of course every camera is different at this point. With Nikon you need to turn the rear command dial by 3 clicks to the left to elongate the exposure time by one stop. Increasing the ISO by one stop is accomplished by pressing the ISO-Button and turning the rear command dial 3 clicks to the left.
During post production with the 'holy grail' you will compensate for those jumps simply by using LRTimelapse. How this can be done exactly is explained in the tutorial video on the tutorial website
Using this procedure correctly you will get perfect results with relatively minimal effort.
A very warm 'thank you' goes to Markus (MarkusFelix) and Olaf (splitti) for helping me translating this article to English!