Showing results for 
Show  only  | Search instead for 
Did you mean: 

Tip: Understanding data transfer speeds


Wanted to put together hopefully some helpful tips regarding the usage of media and copying data to computers. As there are numerous speeds of media, protocols, drives, etc., it's best to understand where your bottlenecks are and when it's wise to invest in faster media vs. saving your money.

IMPORTANT: All values below will be in bytes and not bits (e.g. MBps and not Mbps)


The following examples are theoretical maximums with this setup:

  • iMac Pro with internal SSD giving 3,000 MBps writes.
  • External Thunderbolt 3 RAID 10 giving 600 MBps writes.
  • External Thunderbolt 3 Compact Flash and SD card reader.  Thunderbolt 3 has a maximum transfer speed of 40,000 MBps.
  • Canon EOS 5D IV with 128 GB Compact Flash and 128 GB SDXC cards.
  • Canon Vixia HF G50 with dual 256 GB SDXC cards.

Note: I use SanDisk for all media.  All specific cards I own:


  • 256 GB SDXC: UHS I, U3, Class 10, 170 MBbps (burst read), 90 MBps (burst write), 30 MBps (sustained write)
  • 256 GB SDXC: UHS I, U3, Class 10, 95 MBbps (burst read), 90 MBps (burst write), 30 MBps (sustained write)
  • 128 GB SDXC: UHS I, U3, Class 10, 95 MBbps (burst read), 90 MBps (burst write), 30 MBps (sustained write)
  • 128 GB Compact Flash: UDMA 7, 160 MBps (burst read), 150 MBps (burst write), 65 MBps (sustained write)

Scenario #1: Capturing photos and DCI 4K on the DSLR.


I've seen some misleading information out there regarding the use of using SDXC cards to capture DCI 4K. The premise was to save cash over the more expensive compact flash cards. The codec uses 62.5 MBps. That far exceeds the sustained write value of 30 MBPs in V30-rated SD cards. The only faster SD cards are UHS-II cards which cannot be used in the 5D IV. Also note that the newer 170 MBps cards don't buy you anything since that's just a higher read speed. It's the same write speed as in the 95 MBps cards.


The compact flash card that I use can sustain 65 MBps writes, so that is now ample.


I've also found that if rapidly capturing images to both RAW + JPEG to both the compact flash and SD cards, that you can end waiting for the cards to write data. This is due to the lower sustained writes by the SD cards. If you want maximum performance, set up the camera to write RAW to the compact flash card and JPEG to the SD card.


Scenario #2: Capturing 4K video on the camcorder.


The camcorder records 4K at 18.75 MBps. Thus, any V30-rated SD card (sustained 30 MBps) is more than ample. This means you can use the older 95Mps-labeled SD cards. However, some more thoughts about this in Scenario 5 below.


Scenario #3: Transferring photos with camera connected via its USB 3.0 cable directly to the computer.


USB 3.0 offers around 625 MBps transfer speed. This far exceeds the Compact Flash's max read value of 160 MBps. It thus wouldn't matter if saving to the internal SSD or external RAID. Here, the bottleneck is the Compact Flash card itself.


To transfer 128 GB (around 2,200 RAW images), it would take roughly 13 to 14 minutes.


And, if using my compact flash card reader, the result would be the same since its the media that's the bottleneck.



Scenario #4: Transferring video with camcorder connected via its USB cable.


Note that the manual just states "High Speed" which is commonly used to describe USB 2.0.


USB 2.0 offers 60 MBps which is now less than the burst read of either the 95 or 170 MBps SD cards. To be fair, I have no info on sustained read performance of either card. But, let's just assume the bottleneck is now the cable.


And, it would not matter to which drive on the computer the data was written to.


Transferring 256 GB (around 3.5 hours of 4K footage) would take around 70 minutes.


Scenario #5: Transferring video from the camcorder by removing its SD card and using the computer's SD card reader.


Note: Either the reader directly built into the iMac Pro, or with the reader in the attached Thunderbolt 3 RAID.


Here, the USB cable is no longer the bottleneck. Instead, the bottleneck is once again the media.

Transferring 256 GB (around 3.5 hours of 4K footage) would take:

* 45 minutes with a 95 MBps-labeled card.
* 25 minutes with a 170 MBps-labeled card.

So in this case, investing in the higher read-speed rated SD cards will pay off.


Other notes:


I don't own any equipment that uses CFast or CFExpress media. Or, any equipment that would connect via Gigabit Ethernet. However, you can apply similar research on where your bottlenecks may be and figure out the best way to transfer data.  For example, Gigabit Ethernet is only 125 MBps. Compare that to USB-3 of 625 MBps. Newer USB-C is even faster.


Older computers (especially laptops with hard drives) could be the bottleneck.  As such drives probably only give around 100 MBps writes.



In summary, bottlenecks could be the media itself, how you are connecting equipment to your computer, and possibly even the drives on your computer.


Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers