Today’s Question: Can you explain why a digital SLR that is limited to somewhere around ten frames per second for photos is able to shoot sixty frames per second for video?
Tim’s Quick Answer: There are two key factors here, which I’ll over-simplify in an effort to keep things as clear as possible. First, video is typically shot at a lower resolution than still photos, so there is less data to process per frame. Second, for still photos there is generally a lot more work being done by the camera, such as moving the mirror and potentially establishing autofocus for each frame.
More Detail: Let’s assume a “typical” digital SLR that has a 20-megapixel image sensor. That means that each photo you capture will contain 20 million pixels. If we assume a frame rate of ten frames per second, that translates into 200 million pixels recorded per second.
If we then assume full high definition (HD) video at 1080p, we would be recording frames that are 1920 by 1080 pixels in overall size. That translates into just over 2 million pixels per frame. Even if we assume a higher-than-typical frame rate of 60 frames per second, the total number of pixels being recorded per second with video is “only” about 124 million pixels. At 30 frames per second that value goes down to about 62 million pixels per second. So, fewer pixels being processed per second with video.
Of course, things do get a bit more complicated. For example, the new Canon EOS-1D X Mark II (http://timgrey.me/atg1dx2) supports 4K video up to 60 frames per second, while capturing still images with a 20.2 megapixel image sensor. The resolution of the 4K video in this case is 4096 by 2160 pixels. That is “only” about 8.8 megapixels. However, when shooting that video at 60 frames per second you are processing more pixels per second (about 530 million) that you would for still photos captured at the maximum frame rate of 16 frames per second (about 319,000 pixels per second).
However, there are additional complexities here that make this possible. As noted above, there is generally more work being done by the camera when capturing individual still images in sequence. In addition, the “more” data being recorded for video doesn’t always translate into more data being stored on a memory card.
In the case of the Canon EOS-1D X Mark II, 4K video is captured in the Motion JPEG format, which employs compression to reduce overall file size as well as the overall amount of data being handled. With HD (1080p) the result is an MPEG-4 video.
So again, in general the answer here is relatively simple, because in most cases video represents less information being processed compared to still photos with a typical digital SLR. But, of course, there are more complicated issues involved in some cases, especially when it relates to how the video frames are processed and what specific tasks the camera must perform for still photos compared to video.