These cards may physically look the same, but they'e all very different.
At first, choosing an SD card to go in your camera may seem like an easy task. After all, they all look more or less the same, so picking one should be as simple as going to the store, finding the highest-capacity one that fits in your budget, and putting it in your camera, right?
Unfortunately, that's not quite the case. While that strategy will probably work for most modern cameras, there are some considerations you'll want to take into account if you're using older cameras or if you're shooting video or bursts of photos.
The need for speed
SD card labels have a lot of information, but not all of it is relevant.
Looking at the label of a typical SD card can be an assault on the senses. They're often covered with numbers and letters, and while the meaning of some (such as the size) may be obvious, others are less so.
Many cards advertise a write and/or read speed, usually denoted in MBps. You can pretty much ignore this; often the card only shows the read speed, which isn't particularly important for cameras, as you'll mainly be writing to it, and even when it does include a write speed, it's almost always showing the best-case scenario rating, which won't be particularly applicable to real-world shooting.
You can ignore the listed read/write speeds
If that number isn't useful, what should you look at instead? It depends on your use case. If you just plan on taking single photos and maybe a few short video clips, most modern SD cards will work just fine; you don't need to pay extra for a super-fast model, which will often come with labels like V60 or V90.
If you have a high-resolution camera, plan on doing a lot of burst shooting with both Raws and JPEGs, or shooting long 4K60p clips or even 8K video, you'll want to spend the extra money to get one of the high-end cards. If you get a card with a low speed rating like V30 or even no V-rating at all, it could hinder your camera's performance.
What about the other ratings?
UHS-I cards (top) have fewer pins than UHS-II ones (bottom).
The SD standard has had several speed ratings over the years, enough that it could be its own separate article. If you're using an older camera, it may be worth checking if it recommends a certian C or U-rating, but nowadays the most important spec will be the card's UHS rating. Most cards you're likely to run into are UHS-I or UHS-II. It's easy to tell the difference between the two, as UHS-II cards have a second row of pins on the back.
If you have an older or lower-end camera, it'll likely have a UHS-I slot, and you won't get any benefit from a UHS-II card. It'll likely still work with your camera, but it'll only be able to run at the slower UHS-I speeds. Likewise, UHS-I cards will work in cameras with UHS-II slots, but as we noted in the speed section, they may bottleneck your camera's performance if your usage is particularly demanding.
Can an SD card have too much storage?
Some cameras won't be able to accept larger cards.
While high-capacity SD cards are always getting less expensive, there are situations where you may need to avoid getting the one with the most storage. For example, if you're using an older camera, it may not work with larger cards.
If you look closely at your card, you may notice that it has a few letters after "SD," such as "SDHC" or "SDXC." These actually denote different generations of cards, and while most newer cameras will be able to use any of them, older cameras may not be compatible with newer generations of cards.
Some cameras have a printed or engraved indicator that shows which version of SD they support; if not, that information should be available in the manual, or online. If your camera says "SDHC," for example, you'll probably only be able to use it with cards that are 32GB or smaller. Cameras that just support "SD," meanwhile, are limited to cards that are just 2GB or less, which can be difficult to find new.
SD eXtended Capacity, or SDXC, was introduced in 2009 and supports cards up to 2TB, so this will mainly be a concern for cameras released around that year or before, though some lower-end cameras from the 2010s may also lack SDXC support.
microSD vs SD
While it's not particularly common, a few cameras use microSD cards instead of full-size ones. The name makes the difference obvious: microSD cards are SD cards, but much smaller. The speed ratings and classes will all be the same, so the advice for buying a microSD card is much the same as it is for full-size ones; figure out how big and fast a card you need, and go from there.
If your computer only has a full-size SD card reader or you want to be able to use your card in a variety of cameras, fear not: microSD to SD adapters are widely available, and may even be included when you buy a microSD card. This only goes one way, though; you can't gracefully fit a full-size SD card into a microSD slot.
What is SD Express?
It's not particularly likely, but you may come across cards labeled as "SD Express." If you do, we don't recommend buying it, as there are currently no cameras on the market that support the standard. While SD Express cards are backwards compatible with standard SD slots, they'll only run at UHS-I speeds, so you'll be paying for a high-speed card without any benefit at all.
Wait, this isn't SD at all!
Higher-end cameras may have a CFexpress slot alongside an SD one... or even two CFexpress slots!
While SD cards have been the industry standard for years, if you buy a vintage point-and-shoot, you may wind up with something that doesn't take them. Unfortunately, there may not be much you can do about that, as finding obsolete cards such as xD or the original Memory Stick is increasingly difficult. In some cases, you may be able to find an adapter that will let you use SD or microSD cards in other slots, and some older formats like Compact Flash are still around, but generally it's best to stick to cameras with SD card slots.
On the opposite end of the spectrum are high-end modern cameras, which may use standards like CFexpress Type B. While most people won't have ended up with one of these cameras by accident, if you see a port that's far too big for an standard SD card, that may be what it's meant for.
When in doubt...
If you bought a new camera and are unsure what card to use with it, check the manual. Most come with recommendations for specific SD cards that the manufacturer knows will let you get the most out of your camera. Cards not on the list will almost certainly work with it, but if you want to be sure you're not getting something that will slow your camera down, it's a good place to start.
