SD Cards, Explained (and what you need to shoot 4K)

One of the most important pieces of equipment in a digital photographer's kit is the memory device which the photos and video will be recorded to, and these days more often than not that’ll be an SD card. When shopping for an SD card, it’s important to know which cards will be conducive to the type of shooting you’ll be doing with your camera.

There are three main pieces of information printed on the face of an SD card: the capacity, the maximum read speed, and the minimum write speed. The minimum write speed is the most important number to pay attention to, because this will determine whether or not this particular card will be able to record the type of photos/video you’re trying to shoot. Importantly, to shoot 4K video you must have a minimum write speed of 30MB/s.

Minimum Write Speed

Minimum write speed is indicated by up to three symbols printed on the upper-right of the SD card: the speed class, UHS speed class, and video speed class. This may look complicated, but all three of these refer to the same thing and are in fact redundant. The reason all three are printed on newer cards is that the manufacturers were afraid people wouldn’t adapt to the new numbering systems as they came out, and therefore just kept listing all three on there.

Speed class ranges from 2 to 10, 2MB/s to 10MB/s respectively.
UHS speed class can be 1 or 3, with 1 writing at 10MB/s and 3 at 30MB/s (so you just multiply the number inside the “U” by 10)
Video speed class ranges from 6 to 90, 6MB/s to 90MB/s respectively

4K video requires a minimum write speed of 30MB/s. So, to shoot 4K, you need an SD card that says U3 and/or V30 or higher.

Read Speed

The number you see in the top-left refers to the maximum possible speed at which data on the card can be read by your device. Listing this speed is largely a marketing tactic, since it is the maximum possible speed and not a number you can rely on. However, the possibility of being read at 170MB/s vs 20MB/s does sound good to me, so get a higher one if you can. Bottom line: the higher the better, but don’t pay any extra for increased capacity in this particular area.

Capacity

This is the simplest piece of info to understand, it’s simply the amount of data that can be stored on this device. A general guideline is that a 64GB SD card can hold about 90mins of 4K footage. It’s hard to say a number of photos it can store, since there are so many factors that change the file size of a photo. I’d say for the most part a 32GB card is sufficient for me when I’m shooting a 2hr event on my Sony A7s II (about 1,000 RAW photos) but 64GB gives plenty of breathing room which I find more comfortable. Nowadays with storage being cheaper than ever I wouldn’t buy a card with less than 32GB capacity. On the flipside I think any more than 128GB on an SD card is overkill. With it being such a tiny, high-use piece of equipment, I simply don’t want to leave that much data on it in case of loss, malfunction, etc. I’d say 64-128GB is the sweet spot whether you're recording photos or video.

The SD card pictured above works perfectly with my Sony A7s II, and can be purchased for just $20 here. If you have a different camera and want advice on which SD card to buy, feel free to ask questions in the comments below!