I'm looking at upgrading my 95 MB/s SD card to a UDMA 7 CF on my 5D mk II; however, I know that eventually I'll hit an internal hardware bottle neck (after all the processor can't spit out images at an infinite rate).

Instead of asking specifically about my camera, how would I find that threshold where adding a faster card wouldn't make the camera any faster?

  • 2
    \$\begingroup\$ +1 for asking the general question rather than a very model specific one. I think, though, that this is already exactly covered at How can I know what speed card to get for my camera? \$\endgroup\$
    – mattdm
    Nov 20, 2015 at 17:21
  • \$\begingroup\$ @mattdm Thanks for think link. I want to close this question, but I'm not satisified with the answers. Michael Clark's sheads a lot of light, but I'm not happy with "...rarely publish the maximum speeds..." and che's formula doesn't seem too scientific. \$\endgroup\$
    – SailorCire
    Nov 20, 2015 at 20:19
  • \$\begingroup\$ Add a bounty to that question asking for more detailed answers? \$\endgroup\$
    – mattdm
    Nov 20, 2015 at 20:20
  • \$\begingroup\$ Sounds good..I can toss 50 or 100 up for it. \$\endgroup\$
    – SailorCire
    Nov 20, 2015 at 20:21
  • \$\begingroup\$ How are you using an SD card with a Canon 5D mark II? \$\endgroup\$
    – Michael C
    Nov 20, 2015 at 20:41

1 Answer 1


Just choose the largest raw file format that your camera will produce. Shoot off a burst of 5 or 10 of them, and time the duration that the card write LED is on. Download will show the size of the files, and N files x Size / seconds is the MBytes per second write speed. The cards 95 MB/second rating is probably read speed, write speed is often slower. And the camera might be slower than a fast card.

If just shooting a JPG every few seconds, then card speed is of very little concern.

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    \$\begingroup\$ The only part of this "answer" related to the question is "And the camera might be slower than a fast card." which is precisely what OP is asking - How can the maximum speed that a camera can write to a card (which is not yet owned) be determined so that a decision on whether to buy the card can be made? \$\endgroup\$
    – db9dreamer
    Nov 20, 2015 at 19:59
  • \$\begingroup\$ Sorry, the alternative for unanswerable things is to time different cards. Here is a good discontinued test: robgalbraith.com/camera_wb_multi_page9ec1.html?cid=6007-12451 --- You might ask the camera manufacturer for specs, but they won't tell you (it also depends on the card). The card has nebulous specs, generally read speed, but it is about the fastest USB 3.0 card reader on a computer, not about the camera - when speed can help us. --- You can estimate your actual needs, N files of S size in T seconds. Except for bursts of raw files, that won't be much. \$\endgroup\$
    – WayneF
    Nov 20, 2015 at 20:20
  • \$\begingroup\$ A fast card is helpful when reading a few hundred files into the computer with a fast reader. \$\endgroup\$
    – WayneF
    Nov 20, 2015 at 20:27

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