One of the most common and annoying problems with time lapse videos is flicker: that strobe-like effect that makes your video look like a motion picture from the early 1900's. It's caused by sudden shifts in brightness between frames and solving the problem can be complicated. Since there are several factors that can affect exposure, no single solution works for every situation. Let's look at a few possible approaches to avoiding flicker and a few things to know about each one:
Fixed Manual Exposure
The most obvious approach to maintaining constant exposure settings is to lock them in manually. You simply meter the exposure, then set your ISO, shutter speed and aperture size manually and leave them there. Every frame is taken at the same settings.
This method is theoretically fine in situations where the lighting in the scene remains constant. Unfortunately, that isn't going to be the case in most actual shooting situations, especially outdoors. For studio sequences, such as decaying produce, etc. this approach may be the most effective way to reduce flicker.
Aperture Priority Auto-exposure
This is usually the preferred method for adjusting the exposure settings to match the lighting for each frame. You simply set the ISO and aperture settings for the results you want and let the camera meter the exposure and adjust the shutter speed accordingly for each frame.
Because the fixed aperture keeps the depth of field constant, there will be no distracting shifts in the focus depth of the finished video. On the other hand, it's possible that the metering may differ, especially if the camera is set to an averaging metering mode. Spot metering centered on a non-reflective object in the frame can help alleviate the problem. Of course, if a shadow falls on that object or the camera is moving, the metering will be affected.
This approach can yield good results and may minimize flicker in many situations. Its limitations become obvious in sequences where the lighting changes dramatically, such as night to day or vice-versa. In these sequences, the extreme change in lighting will probably exceed the camera's ability to adjust the shutter speed at some point. In addition, extreme differences in shutter speeds may create differences in motion blurring that make portions of the video appear to “jump”.
Undoubtedly the best way to ensure that the exposure for each frame is correct is to manually set the exposure for each frame. The most obvious problem with this approach is the amount of effort needed, especially when you're dealing with a great number of exposures. You also run the risk of disturbing the centering of the frame or the focus setting.
This method gives you the most control, but can be impractical. It also tends to create somewhat “jerky” lighting transitions that need to be smoothed in post processing.
An alternative to all of the above approaches is known as ramping, and can be done manually or with the aid of software and/or hardware. The method involves incrementally adjusting one setting, usually shutter speed, over the course of the sequence. A typical manual method involves using the camera's “bulb” shutter setting and a remote release while timing each shot manually.
Auto-ramping is much easier and can yield excellent results for day to night and night to day sequences, however the wide range of software, hard ware and methods used, as well as camera compatibility make covering the topic in depth in this article impractical. We'll cover it thoroughly in an upcoming article.
Cover Your Viewfinder
A common cause of flicker in time lapse videos is errors in metering caused by light entering the camera through the viewfinder. When you're using any auto-exposure method, be sure to cover your viewfinder with the factory cover or a piece of black tape.
No matter what method you use when shooting your sequences, flicker is bound to creep into your videos. There are a number of electronic and/or mechanical causes, such as the fact that a modern DSLR camera lens diaphragm opens and closes with each exposure and there may be minor differences in the size of the opening each time.
Regardless of the cause, chances are good that you'll need to use software to reduce or eliminate flicker in your videos. The number of applications for deflickering time-lapse sequences is surprisingly high and there are free applications as well as those you'll need to pay for. Additionally, some applications can help you balance exposures in your stills before producing the finished video.
As you can see, the task of avoiding flicker in your videos isn't an easy one and each shooting situation presents its own set of challenges. I hope that this article will help you determine which approaches are best suited for your sequences. Have fun shooting!