Today’s Question: You mentioned capturing video with your digital SLR at 30 frames per second with a 1/30th of a second shutter speed. But that means the camera is constantly capturing frames. How is it that the buffer never gets full the way it does with still captures?
Tim’s Quick Answer: The short answer is that video is captured at a much lower resolution than the camera is capable of for still images, and so it is possible to capture video at a high frame rate for an extended period of time without filling the memory buffer in the camera.
More Detail: Let’s assume a somewhat typical (or at least popular) camera that is capturing images with a 22-megapixel sensor. That would involve maximum pixel dimensions for a still image of (for example) 5,760 pixels by 3,840 pixels. At a frame rate of six frames per second, the camera is capable of capturing 17 images before the buffer is full and capture speed will slow.
However, that full buffer represents 17 RAW captures of 22 megapixels. By contrast, full 1080p high definition (HD) video is “only” 1,920 pixels by 1,080 pixels, which is the equivalent of about 2 megapixels. In other words, there is a lot less information being captured when recording video rather than full-resolution still captures, and so you can capture many more frames without filling the buffer.
In general you will find that a digital camera that offers full HD video recording will be able to capture video continuously without filling the buffer. Of course, there are other factors to consider when it comes to recording video for and extended period of time, including overall file size, heat buildup, and other considerations. But you generally won’t need to worry at all about the buffer limitation of your digital SLR when you’re capturing video rather than still captures.