You just got a brand new camera and you’re ready to use everything it has to offer. You start by taking photos of everything and anything, testing out how good your pictures will look. Maybe you switch to burst mode and see how fast the camera can shoot - wow! It’s like a machine gun firing! Finally, you press record to capture some video. Then a few seconds later, it just stops recording. You think “huh, that’s weird” then try again. A few more seconds record, then it stops again. Maybe you exchange a few explicit words out of frustration and contemplate returning the camera - it must be broken, right? Or, if you’re here, you went online searching for answers instead. You want to know why it keeps stopping. Are you doing something wrong or is the camera actually broken?
Why is it stopping?
To put it simply, the camera stops recording because the buffer is too full. To take it a step further, the memory card cant write the data from what’s being recorded fast enough to empty the buffer. The short answer: the memory card is too slow.
How does it work?
When you capture anything with your camera, whether it’s a photo or video, that content takes up a certain amount of data to store. Once an image is captured, it’s processed and held in the camera’s buffer. The buffer acts like temporary storage, similar to RAM for a PC, but intended for images rather than applications. The buffer pushes the photo or video to the memory card and frees up space for another image or continued video to move in.
If your memory card can’t unload the buffer fast enough, things must come to a halt. For photos, it’s easy. The camera simply won’t take another photo until there’s room in the buffer for another photo. Since data is written instantly and photos are much smaller than video, the worst you’ll notice is a slower burst rate.
Things get a little more difficult with video if the buffer fills up. Video is a collection of still images captured over a period of time. This can be 24 frame per second (fps), 30 fps, or 60 fps (these would b e slightly different in other parts of the world). This means recording at 60 fps is equivalent to capturing 60 photos in one second, continuously until you stop recording. Two seconds of video is 120 photos,. Granted, video compression works a lot differently than how still images are stored, it still takes storage space. And once that buffer fills up, what can the camera do? You might suggest slowing down the image capture, but the number of images captured has less impact on this than you think. Storage size is what matters here.
How fast should my memory card be?
The answer to this isn’t simple. Since the issue is related to speed, you need to get some measurements. Put the ruler down, not those kind of measurements. You need to find a rate data is being recorded, this is referred to the bitrate. In video recording, both the audio and video pieces of the recording have their own bitrate. The bitrate is how much data is recorded or played back per second. The higher the bitrate, the more data is retained in every second of video, which translates to a higher quality image.
So how do you find the bitrate of your camera? That’s the complicated part. The bitrate can be different, depending on the video quality settings you use. Additionally, most manufacturers think the idea of a bitrate is too confusing for their customers and don’t usually specify the bitrate in-camera. Instead, they generically label the quality settings.
You can simply search online for the bitrate of your camera at specific settings, changes are someone already asked that question on a forum somewhere and got an answer. If you can’t find your answer online, it never hurts to check the manual or contact the manufacturer. They will be able to either tell you the speed of memory card to get or the bitrate the camera records at. Finally, you can record a sample video of any length and explore the metadata of the recording. Windows and Mac OS can retrieve this information in the file’s properties most of the time. If that doesn’t work, a free and open source tool I like to use is Media Info. When exploring the file’s metadata, add the video bitrate to the audio bitrate and that will give you the slowest write speed your memory card should be capable of.
Write speed vs Read speed
Finding a memory card for the speed you need isn’t as straight forward as it should be, however. One of the biggest problems is memory card manufacturers only advertise the read speed while hiding the write speed in fine print somewhere else on the package. That’s because read speed will always be faster than write speed. It’s easier to access the data than to create new data. Think about when you read and write. Do you read faster than you write? It’s a similar situation with storage. When you see a memory card advertise a speed of 95 Mbps, for example, stop and do a little research. Is that advertised speed for reading the data or writing the data?
What’s the difference? Read speed is how quickly data can be transferred from the memory card to the camera, computer, or whatever is accessing the data. For a 95 Mbps speed memory card, that means a file of 95 Mb will take 1 second to open or load under perfect conditions. This would include the speed of viewing or transferring the file.
Write speed is how quickly data can be stored on the memory card. Think of this as how quickly you can move the pen on paper when writing a letter. More literally, this means that a memory card that can write 95 Mbps will take 1 second to write a file that’s 95 Mb under perfect conditions. When buying a memory card, you should pay attention to both read and write speeds promised.
Mbps vs MB/s
It’s also important to compare values with the same units of measurement. When comparing the speed of two cars, we measure both in miles per hour, right? It would be ridiculous to compare one car in MPH and the other in kilometers per hour and expect the numbers to be comparable. With digital storage, there are also different units of measurement. Know the difference and compare the same units.
There are two units of measurement:
Mbps: This represents Mega Bits per Second. This is the smaller unit of measurement, meaning that this value should be significantly larger when compared to MB/s.
MBps (or MB/s): This represents Mega Bytes per Second. There are 8 bits in each byte of data. That means 1B (byte) = 8b (bits). Or 8MB (eight megabytes) = 1Mb (1 megabit). Dividing bytes by 8 will give you the bits.
This can be confusing in digital storage in general because one company may advertise a speed in one unit whereas another will use the other. Knowing there is a difference will help you choose the faster product.