Timelapse enthusiasts jokingly call the flawless transition from day to night 'The Holy Grail' due to it's demanding difficulty. In practice this is particularly sought after for sunrise and sunset sequences. There are a variety of methods out there, each with it’s pros and it’s cons. Some are basic and essentially free whereas some are more advanced and come with a price tag, let's break down the methods we know of from worse to ultimate:
Auto Settings (worse)
The most basic, or perhaps “newbie” approach. Simply set your camera in some combination of auto settings (be it full auto, aperture priority or auto-ISO) and let it go. One very important thing to do if you use this method with a DSLR is to make sure to cover the eye piece (gaffer’s tape will do the trick) to prevent light from spilling through the opening as it will affect the light meter.. DLSR camera's assume that your face will be there to block light from entering the eye piece.
So what’s the problem with this method? Flicker is the problem! There’s a good chance that the result will have ugly and distracting changes of exposure between frames. Why does this happen? Two reasons; first DSLR cameras are only capable of ⅛ - ⅓ eV granularity between shots and second the light meter is making all the decisions. With the first issue these gaps between exposure cause ‘exposure steps’ between shots rather than a gradual change in exposure to compensate for changing light conditions. With the second issue one could say the light meter is ‘wishy-washy’, zone metering is a wonderful thing in still photography but with timelapse one moment the meter may lean toward the over-exposed part of the frame and the next moment decide to lean toward the under-exposed side.
There are some software solutions that will reduce or fix flicker, including GBDeFlicker, LRTimelapse, GenArts RemoveFlicker or adjusting the 'CC TimeBlend' filter in After Effects. Unfortunately you may find that these post solutions may not work in every situation.
Stop and Re-adjust (ok)
This is a old school method that had its origins in film production. You start either over or under exposed (depending on whether it’s a sunset or sunrise) in full manual mode. Then as time goes by the shots will eventually travel through an exposure range. When the exposures start moving beyond a certain range say +/- 2EV, you stop the timelapse, manually readjust your exposure and re-start timelapse repeating this process as many times as needed. Then in post-production you take your multiple videos, split them up, carefully overlap each segment and cross fade between.
This can work in some situations, but the transition may not seem natural when certain variables are in place (such as clouds). It's also tricky to get right and time consuming in post production..
Stacking HDR Approach (ok)
With this method, you shoot manual using bracketing, as you would for an HDR Timelapse shot. Place each of the brackets into a folder, and from each folder create a video. Take each of those videos and stack them in different tracks in After Effects (or similar composting/editing program). Then fade the opacity across the tracks to keep the footage within an appropriate exposure range.
You can get decent results with this method, though it can be a time and data consuming process (as well as limiting interval speed to accommodate shooting all the brackets). Moreover, you'll be limited to the overall bracketing EV spread, so in other words if you can only bracket +/- 3EV then you only have 6EV of latitude which is not much in the real world.
Babysit Approach with Post Smoothing (better)
LR Timelapse has made this approach popular, it involves shooting RAW and watching your camera closely for changes in exposure and manually adjusting exposure and ISO between shots. Since you are in the driver seat there won't be any 'wishy-washy' effects introduced by the light meter, but there will be 'steps' due to the changes your making in exposure between shots. This is where post production comes to the rescue, again we can use 'de-flicker' software to help defeat these jumps in exposure (GBDeFlicker, GenArts RemoveFlicker) but LR Timelapse has a particularly interesting way of smoothing out the exposure changes by applying the smoothing to the XMP files prior to render time. When rendering the 'exposure steps' are smoothed out by creating exposure compensation in the XMP files. This technique works well and can accommodate a very wide latitude as long as you have the dedication to stick with the camera and carefully personally guide the exposure changes.
Bulb Ramping (better)
This technique uses the bulb settings on the camera to gradually change the exposure. This was pioneered back in 2008 by Thomas Bethel and his Little Bramper (now discontinued) which gradually changes exposure over time by sending slightly longer (or shorter) trigger signals. This was a revolutionary approach that inspired many modern solutions. Bulb Ramping does require also connecting to the camera's PC port so that the timing of the trigger can be accurately compensated for camera reaction time. Camera reaction time can vary from body to body and surprisingly also frame to frame.
One drawback is that to date Bulb Ramping only works with Canon DSLR bodies and the other drawback using bulb mode is the fastest shutter time achievable is 1/30” making very bright to dark situations difficult to achieve without shifting ISO. To defeat this many modern bulb ramping controllers can also connect to the camera via USB and shift the ISO and compensate the exposure accordingly. Some solutions even have incorporated their own light sensors to automate the process.
Modern & Available Bulb Ramping Controllers:
Ramper Pro (with USB camera control and external light sensor, also see Software Ramping below)
Timelapse+ (with USB camera control and external light sensor)
Promote Control (with USB camera control)
Software Controlled Ramping (ultimate)
The ultimate solution in a fully automated form comes in products that utilize a combination of many of the solutions above. At this time there are two highly sophisticated software solutions that automate the exposure process and effectively create perfect exposure transition with the most latitude:
GBTimelapse has done an amazing job of controlling all aspects of Canon cameras to achieve gradual adjustments of exposure over time (check out the 'Auto Ramping' feature). The drawback is the GBTimelapse software only works with Canons but the control level with this long trusted software is unmatched. It also requires taking a laptop or netbook into the field to control the camera.
Software/Hardware Controlled Ramping (ultimate)
RamperPro is a newer solution available in the market which is developing quickly. We're calling it a 'software/hardware solution' because although it's a dedicated compact hardware device it's also running a custom distribution of Linux with the ramper pro software installed. It's also listed in the 'Bulb Ramping' section because it's capable of bulb ramping but this device uses bulb ramping as an optional part of it's overall tool kit. In practice the RamperPro can create excellent results without activating bulb ramping, but Elysia Visuals advises activating the bulb-ramping feature if your planning on covering a very wide dynamic range. The unique feature of the RamperPro is it pre-calculates XMP files with smoothing applied and stores them on the device, these XMPs can simply be joined with the original RAW frames smoothing out the steps at render time. The other very unique feature of the Ramper Pro is it's fully compatible with most Canon, Nikon, Panasonic and Sony camera bodies.