One question I’ve been getting a lot recently is: Which memory card is the best?

The answer, like every answer to a photography-related question, is: It’s complicated.

You don’t want to pay too much for an overpowered memory card you don’t need. But you also don’t want to be shooting and run into a problem of your card’s write-speed slowing down your shoot.

Part of the difficulties in selecting a memory card is that there are so many choices, and each card has its own positives and drawbacks.

The first place to start is distinguishing the two types of memory cards that your camera will use—CompactFlash (CF) and Secure Digital (SD).

CF cards are the faster of the two, with generally better build quality and a lower rate of failure. This is why pro DSLRs (e.g. Canon 5D Mark III or Nikon D800), will offer both CF and SD slots, while the more prosumer (e.g. Canon 70D and Nikon D7100) and consumer (e.g. Canon Rebel T5i and Nikon D3300) DSLRs will offer only the SD, since most pros need that faster write speed and are willing to pay more to make sure their photos are secure. That’s not to say that your SD card will fail tomorrow. It won’t (probably). It’s also why comparable CF cards are way more expensive than their SD counterparts, and generally more difficult to find.

That being said, pro shooters, don’t discount the power of those extra SD slots. If you’re getting paid for an assignment, it’s not a bad idea to have 4 to 6 memory cards on you, and having SD cards as backup will allow you to breathe easier and worry less when on a shoot. (You already have enough to worry about as it is.)

Even if the only card your camera takes is an SD card, you still have some decisions to make, and that’s mainly in the write speed and the card size.

Memory card manufacturers designate “speed” two different ways on the front of the box: MB/s and the “x” number.

You’ve seen both of these numbers on the box. A Lexar Professional 800x CF Card, for example, write at speeds up to 75 MB/s, but they also designate a “800x” number. Most photographers can understand the first number—if you have a camera that takes 5 MB JPEGs, the card will “write” that image in approximately one-fifteenth of a second. The “x” number is a little more technical. Generally, the “x” stands for 150 kb/s. So if you have a 800x card, it operates at a transfer speed of 800 * 150 kb/s. However, where this gets tricky is that the 800x could potentially mean that the card has a read speed of 120,000 kb/s, and read speed is much less important than write speed.

After all is said and done, however, the limiting factor for write speed generally depends more on your camera’s capabilities than your card’s, or in other words how fast your camera can process the image. You should be able to find your specific camera’s processor speed in your camera’s user manual. The main point to keep in mind here is that if your processor speed might be too slow for the faster cards anyway, so there’s no reason to spend that money.

Another factor that could affect your camera’s compatibility with SD cards is that, within the last seven years, the organization that maintains the standard for SD cards (the aptly named SD Card Association) have created new formats, SDHC and SDXC, the symbols of which you’ve probably seen before. Almost all cameras manufactured after 2006 can accept SDHC cards, if not also SDXC cards. But it’s best to consult the end of your user manual, where your camera manufacturer in all likelihood lists compatible memory card formats. Cameras that accept SDXC cards are backwards compatible with previous SD iterations (i.e. SD and SDHC cards). Ditto for SDHC cards.

When it comes to CF cards, there is another distinction to keep in mind, and that’s the UDMA, which stands for Ultra Direct Memory Access. This is basically another way to distinguish write speed, meaning that a UDMA 7 card will be faster than a UDMA 6 card. Again, it’s best to just use the actual write speed in MB/s to determine what card you should get.

So what card should you get? It really does depend on your specific shooting needs, but here are some decent rules of thumb:

Card speeds of 30 MB/s are good for your everyday shooter. They’ll be fine for short bursts of rapid-fire JPEGs (no guarantee on RAW) and can capture video at 720p or low-quality 1080p.

Card speeds of 50-60 MB/s (medium) are better for video and rapid-fire RAW stills.

Card speeds of 80-100 MB/s will be the best of high-speed, high-quality video recording, but they tend to cost hundreds of dollars more than medium cards of equivalent size.

In terms of memory capacity, if you’re shooting a lot of video, or taking a few hundred RAW shots, you’re going to want at least a 32GB card. If you shoot mostly in JPEG, 8 to 16GB would be just fine. For example, with my Nikon D7100, an 8GB card that costs $20 can hold 507 JPEG Fine images. That’s usually more than enough on any given shoot.

Finally, some advice: If you’re going on a trip, bring your portable hard drive, or at least download your pictures to your laptop every day, even if you know you won’t fill up your memory card.

And don’t select a memory card if the only reason you’re buying it is because it’s cheaper. You’re investing your time in grabbing that photo opportunity and your money in great gear, so why cheap out on the memory card, which is where your images will actually live?

Have more questions about memory cards? Let us know in the comments!