There is one confusing technical challenge that some people experience when shooting at 4K. Ultra High Definition 4K video requires memory cards with faster write speeds than when shooting normal 1080p HD video. I read a lot from people in online camera forums talking about their DSLR and mirrorless cameras that stop recording while shooting 4K video and not knowing why.
Some of the issues are related to the temperature of the camera getting too hot and overheating. This is particularly a problem with some of the smaller Sony 4K mirrorless cameras, which often overheat and with the camera shutting off after about 20 minutes of 4K video recording.
But much of the time the issue isn’t about camera temperature. It is that the memory cards can’t write video data fast enough, which causes the camera to just stop recording. It can also be a combination of the camera’s buffer being too small and the memory card not being able to record the data it receives from the camera buffer fast enough to keep the camera processing the captured data smoothly.
Most people are using standard CF and SD memory cards that are rated at an average of 60MB to 95MB per second data speeds like these SanDisk 64GB SDXC UHS-1 cards. Cards like these are fast enough for most 1080p video shooting, bursts of still photos shot in RAW up to around 36MP, and for some 4K video recording at 24 frames per second. But often they aren’t fast enough if you want to shoot 4K video at 30-60 frames per second. With Sony mirrorless cameras it is sometimes less of a problem since Sony uses a different codec than some Canon DSLR cameras that create much larger uncompressed video files than Sony mirrorless cameras.
4K video shot with the newest Canon 5D Mark IV DSLR camera is a good example of where you can easily run into buffer and memory card problems. The 5D Mark IV shooting 4K video at 30 frames per second captures around 60 megabytes per second of recorded video data. Although it sounds like SD cards that are rated at 90MB per second should be enough for a camera shooting 60MB of video per second, they aren’t.
The problem is that the actual write speeds of most memory cards are usually only half, or at most, two thirds of the rated speeds that are printed on the face of the memory cards. This is because the stated card speeds are normally read speeds and not write speeds. So card manufacturers can be tricky when disclosing their card speeds. I recently experienced camera shut downs after just 5 seconds when using these SanDisk Extreme 64GB SDXC UHS-I cards rated at 90MB per second to record 4K video with my Canon 5D Mark IV. This can be disruptive on a shoot to say the least.
The simple solution to avoid any card speed write problems is to get yourself a memory card that states the speeds are up to 300 megabytes per second. Even if they only write at half that, data writing issues while shooting 4K video will be fully resolved. My personal choice are the SanDisk 64GB SDXC UHS-2 300MB/s Extreme Pro cards here because of both their high level of reliability and their fantastic warranty coverage if there is ever a problem. Before buying one of these memory cards though just make sure your camera model is compatible with the newer UHS-2 SD card format. If your camera doesn’t support UHS-2 cards then the next best thing are still the SanDisk Extreme Pro 64GB SDXC UHS-I memory cards here, which are rated at 95MB/s.
There are also other brands like Delkin, Sony, Lexar, and others which make memory cards that can also write at high speed. But warranty coverage with SanDisk is worldwide and they will send replacement cards to you no matter what country you are located in. I once had an experience with Lexar and a CF card that failed. Lexar told me they would only send a replacement card to me at an address in the USA, which is a problem since I reside outside the USA.
So now I only buy SanDisk. Plus I often travel to shoot video in other countries, and we all know that card failures can happen anywhere. If you are like me then you may also be best to stick with SanDisk cards. SanDisk is now a subsidiary of Western Digital (as of May 2016), which is the same company that makes a lot of the internal and external hard drives used with many of today’s computers.
If you have any questions, comments, or experiences of your own that you would like to share about memory cards and write speeds when shooting 4K video then please feel free to post them below.