EOS 250D What is the actual memory card writing speed?

hidet — Tue, 27 Feb 2024 14:00:53 GMT

In the manual for my Canon EOS 250D it says: "MP4/H.264 UHD 4K (3840 x 2160) at 23.976p/25p [120 Mb/s]"

But a reasonable SD card has a writing speed of half that value (60-70Mb/s).

My questions are:

What do those [120 Mb/s] mean?

And what is the minimum writing speed to the SD card to be able to film in UHD 4K (3840 x 2160) at 23.976p/25p without any problems?

Re: What is the actual writing speed?

AtticusLake — Mon, 26 Feb 2024 11:17:27 GMT

Unfortunately you have -- very understandably -- been tripped up by the terminology, which is confusing.

The difference is between the letter "b" -- that's lower-case -- which stands for "bit"; and "B" -- upper-case -- which stands for "Byte", where a byte is 8 bits. Camera record speeds are generally in bits, and memory card speeds are generally given in bytes. You will see a lot of people online get this wrong.

So your camera does actually record at 120 Mb/s -- that's mega bits per second. Divide by 8, and we can see that that is 15 MB/s -- mega bytes per second. If you have an SD card with a v30 speed rating, that is 30 MB/s -- mega-bytes per second, which is easily enough.

BTW when you say "a reasonable SD card has a writing speed of half that value (60-70Mb/s)" -- I think you mean MB/s, but in any case you need to know that the speed numbers printed on the card are useless, misleading garbage. That number is the FASTEST possible speed that the card can ever go at, which is completely irrelevant for shooting video, where we care about the WORST-CASE speed. That's why you should always look at the "v" speed rating, which is actually designed for video.

I made a series of videos which try to explain all this, which may help: https://moonblink.info/MudLake/tech

The videos "Bit Rates for Video" and "Memory Cards for Video" might be relevant here.

topic EOS 250D What is the actual memory card writing speed? in EOS DSLR & Mirrorless Cameras

EOS 250D What is the actual memory card writing speed?

Re: What is the actual writing speed?