A Photographer's Memory Card Guide: What the Speed Classes Mean

The memory card is the most boring-looking part of a photo kit — until the card throws an error during the first dance at a wedding, or the camera freezes with “buffer full” the moment the bird takes off. That’s when you realize the card is the most critical link in the chain: an entire thousand-frame day lives inside a small piece of plastic in your palm. This guide decodes the cryptic codes on the card label and separates what’s really needed for which type of shooting.

Translating the codes on the label

A typical SD card reads: 128GB, U3, V30, UHS-I, “170 MB/s”. Let’s unpack them one by one:

• Capacity (GB): how many photos will fit. A 24MP RAW file is roughly 25–35 MB; a 128 GB card holds about 3,000–4,000 RAW frames.
• V class (V30/V60/V90): the card’s guaranteed minimum sustained write speed — V30 is the promise “I’ll never drop below 30 MB/s.” This is the most important number on the card.
• UHS bus (I/II): the card’s connection standard. UHS-I carries a theoretical ~104 MB/s; UHS-II rises to ~312 MB/s with a second row of pins. UHS-II cards also work in a UHS-I slot (at UHS-I speed).
• The marketing speed (“170 MB/s”): the label’s largest-font, least meaningful number — usually the read speed. What stresses the card while shooting is the write speed, and that’s hidden in the small print on the back.
• Old markings (Class 10, U1/U3): the ancestors of the V classes; U3 ≈ V30. When buying a new card, look straight at the V value.

Where does speed actually matter?

If you shoot single frames, almost nowhere. Landscape, portrait, street — the camera takes a frame into its buffer instantly and writes to the card in the background. You won’t feel a difference even with a slow card.

Speed becomes vital in two scenarios:

1. Burst shooting: 10–20 RAW frames per second fill the buffer within seconds; the camera can then only shoot as fast as the card empties it. With a slow card the camera locks up and the moment you meant to freeze in mid-air is lost behind the buffer. If you shoot sports, birds or a moving child, a fast-writing card (preferably UHS-II) directly wins you frames.
2. High bit-rate video: 4K/6K video is like an uninterrupted stream of water; if the card falls behind, recording stops. Video classes exist exactly for this: V30 is the practical floor for 4K, V60 at high bit rates, V90 for 8K and ProRes-class work.

A practical table:

• Landscape, portrait, travel, single-frame work: V30 UHS-I — more just eats your money.
• Burst-heavy photography: V60 UHS-II and up (if your camera’s slot is UHS-II).
• Serious video: V60; V90 or the CFexpress your camera demands for professional codecs.

Beyond SD: CFexpress

Top-segment bodies moved to CFexpress cards that exceed SD’s physical limits (Type B and the more compact Type A). Writing 1000+ MB/s with NVMe SSD technology, these cards exist for extremes like 8K video and 30 RAW frames per second. If your body doesn’t want them, you don’t need to think about them; if it does, its manual already says which class is required.

Reliability: more important than speed

• Fakes are a real problem: a significant share of “512 GB super cheap” cards online are 32 GB fakes made to report a high capacity in software — they work until full, then silently swallow files. Buy the card only from an authorized seller; testing it with capacity-verification tools on first use is a good habit.
• Several medium cards instead of one giant: a single 512 GB card ties your whole archive to one point of failure. Two or three 128 GB cards split the risk; if one fails, the day is saved. Running the second slot in backup mode on a dual-slot body is insurance for irreplaceable work like a wedding.
• Format in the camera: format the card not on a computer but in the camera it will be used in — the file system is set up the way the camera expects and error risk drops. Formatting after each transfer instead of “delete” is also better for card health.
• Physical care: SD cards are durable, but the lock slider breaks and the pins get dirty. Cards should live in a card wallet, not the bottom of a bag.

How many GB should you buy?

The math is simple: your camera’s RAW file size × the number of frames you expect on a shoot day × 1.5 for safety margin. A weekend photographer wandering with a 24MP body is comfortable with 64–128 GB; someone mixing burst and video rises into the 128–256 GB band. If you’re just starting to shoot RAW (which you definitely should, for editing flexibility), account for files taking 3–4 times the space of JPEG.

One last reminder: the card is a transport tool, not an archive. The permanent home of photos is your computer + backup drive; the card is the courier that gets formatted after each transfer and returns to the field.

Common mistakes

• Paying for the big number (read speed) on the label: write speed determines shooting performance; read the small print on the back, look at the V class.
• Buying an expensive UHS-II card for a UHS-I-slot camera: it works but at UHS-I speed — the price difference goes to waste. Learn your camera’s slot first.
• Entrusting your whole archive to one card: the card is temporary storage until backed up. Don’t delay transfer day.
• Buying a “bargain” card from a suspicious seller: a fake card is the most expensive saving — you pay for it with your photos.
• Formatting the card on a computer and getting an error in the camera: always format from the menu of the camera it will be used in.