Written by 11:48 pm Canon, Photography • 60 Comments

Quick Tip: Optimize Canon 5D Mark III Write Speeds – Avoid Using SD Cards

Photographer Jeff Cable discovered something interesting about the 5D Mark III when he was doing some high-speed shooting – it’s slow, but only under certain circumstances. Read more about when you should not use the SD slot in the 5D Mark III.

Buffer Sluggish on 5D Mark III? Ditch the SD Card!

Why is this happening? The 5D Mark III defaults to the slowest card that is in the camera at the time. If you want to take full advantage of your professional CF card, leave the SD slot alone – save it for times when speed isn’t important but having backups or more storage is.

On any camera with dual card slots (this includes pro bodies, such as the Canon 1D x and Nikon D4, as well as the Nikon D800 and Sony a99), it will default to the lowest card speed that is currently in the camera. What is different about the 5D Mark III, as observed by me, is that the SD card slot won’t write any faster than 133x no matter what kind of SD card is inserted.

When You Should Not Use an SD Card in Your Canon 5D Mark III

I am going to start this by saying that I really like my Canon 5D Mark III cameras and use them for shooting everything including Bar and Bat Mitzvahs, landscapes, portraits, sports and more. But just after receiving my two cameras I started to play with the memory slots and experimented with the best way to use both memory card slots. My first thought was…it would be great to shoot RAW images to both cards as a backup measure, and my second thought was…I could shoot RAW to the CompactFlash card and JPG to the SD card.

Here is what I found after some testing. Pass on using the SD slot if you happen to be writing the same image to both cards and care about clearing your buffer quickly. In certain circumstances, you do not want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the UDMA7 protocol and a standard SD card slot which does not support the high speed standard (called UHS for Ultra High Speed).

This is really strange because many other cameras have come out with UHS-I compatible slots over the last year. Without UHS support, the top speed that can be achieved by the SD card is 133x. This is true even if you purchase a 600x SD card and insert it in the camera. The best you will get is 133x.

So, the only reason to use a really fast SD card is for faster downloading after the shoot.

You might be thinking, “Why would the SD card slow down all of the data transfer of the camera including the CF card?” It turns out that the camera will default to the slowest card inserted. If you have a 1000x CF card in slot one and any SD card in the second slot, the very best buffer clear that will achieve is 133x. When shooting sports or any type of images with burst mode (6 frames per second), this is crippling.

I want to shoot a bunch of images, have the camera clear the buffer as quickly as possible, and keeping shooting. Why would I want to clear data at 20Mbps when I could be transferring at 90Mbps or better? For this reason, I almost never use the SD slot in the camera. I want to take full advantage of my Lexar 1000x Professional CompactFlash card.

This is not a problem if you are a photographer who shoots in a studio. It’s also not much of a problem for any photography who does not shoot in burst mode. But if you care about clearing your buffer, you need to be aware of this.

Tags: , Last modified: July 7, 2021