Memory Cards
Written by Abin Alex and Anu Joseph.
Abstract
A memory card is a removable flash storage device that holds the files a camera writes. Two families dominate: SD cards and CFexpress cards. This guide describes SD, CompactFlash, XQD and CFexpress cards, the units of storage and speed, and the meaning of the speed labels. It advises choosing a card by its capacity and its sustained write speed, not by its read speed alone, and it explains how to match a card to a camera and look after it. It is written for photographers and cinematographers.
Keywords: memory card; SD card; CFexpress; write speed; speed class
What is a memory card?
Short answer. A memory card, also called a flash card, is a removable storage device that holds photo, video and data files.
- Digital data is stored as bits, each 0 or 1. A byte is 8 bits and is the common unit of storage.
- Each step up the scale is 1024 times larger: kilobyte, megabyte, gigabyte, terabyte.
- Read speed is how fast a card gives data to a computer. Write speed is how fast the camera can save data on the card.
- Cards are portable and light, but they are for working storage, not long-term archive. See the Storing and Backing Up guide.
| Unit | Size |
|---|---|
| 1 byte | 8 bits |
| 1 KB | 1024 bytes |
| 1 MB | 1024 KB |
| 1 GB | 1024 MB |
| 1 TB | 1024 GB |
Table 1. Storage units.
Cham tipKnow the difference between reading and writing: your camera needs good write speed.
Cham taskLook at the capacity and the logos printed on one of your own cards. Write down what each mark means.
Check yourself: Which speed matters for a camera saving a photo burst?
The sustained write speed.
What are SD cards?
Short answer. SD (Secure Digital) cards are the most common memory cards. SD, SDHC, SDXC and SDUC differ by capacity.
- SD was developed by SanDisk, Panasonic and Toshiba and introduced in 1999. The three formed the SD Card Association to maintain the standard.
- A full-size SD card measures 24.0 × 32.0 × 2.1 mm.
- SD holds up to 2 GB. SDHC holds more than 2 GB and up to 32 GB. SDXC holds more than 32 GB and up to 2 TB. SDUC holds more than 2 TB and up to 128 TB.
- Check that your camera supports the card family. An older camera may not read SDXC or SDUC cards.
| Family | Capacity | Note |
|---|---|---|
| SD | Up to 2 GB | Oldest type |
| SDHC | Above 2 GB to 32 GB | Introduced 2006 |
| SDXC | Above 32 GB to 2 TB | Most common today |
| SDUC | Above 2 TB to 128 TB | Newest |
Table 2. SD card families.
Cham tipRead the camera manual for the supported card families before you buy.
Cham taskFind out which SD family your camera accepts and its maximum capacity.
Check yourself: Which SD family holds up to 2 TB?
SDXC.
What are reading speed, writing speed and the units KB, MB, GB and TB?
Short answer. Storage size is counted in bytes and speed in bytes per second. Write speed is how fast the camera saves data to the card. Read speed is how fast the card gives data to a computer (Kingston, n.d.).
- A bit is a single 0 or 1. A byte is 8 bits. Sizes climb in steps: KB (kilobyte), MB (megabyte), GB (gigabyte), TB (terabyte) and PB (petabyte).
- Each step is 1024 times the one before. Card makers usually count 1 GB as 1000 MB, so a card shows slightly less space on a computer than its label says.
- Capital B means bytes and small b means bits. 1 MB/s (megabytes per second) equals 8 Mbps (megabits per second). Camera video bit rates are given in Mbps, and card speeds in MB/s.
- Write speed matters while shooting. A slow write speed fills the camera buffer and the burst slows down or stops. Sustained write speed is the number printed as U3 or V30 (Kingston, n.d.).
- Read speed matters when you copy files to the computer. A fast card reader can make offloading much shorter, but only if the card, reader and cable all support the speed.
- Time to copy = total size ÷ speed. For example, 64 GB at 100 MB/s takes about 640 seconds, or about 11 minutes. This uses 1 GB = 1000 MB, so it is approximate.
- Number of photos = card size ÷ file size. For example, a 64 GB card holds about 2000 RAW photos of 30 MB each (64 000 ÷ 30), and fewer if the files are larger.
| Unit | Short form | Size | Example |
|---|---|---|---|
| Bit | b | 0 or 1 | One on/off signal |
| Byte | B | 8 bits | One character of text |
| Kilobyte | KB | 1024 bytes | A small text file |
| Megabyte | MB | 1024 KB | One JPEG photo, a few MB |
| Gigabyte | GB | 1024 MB | A card or an hour of video |
| Terabyte | TB | 1024 GB | A hard drive |
| Petabyte | PB | 1024 TB | A data centre |
Table 3. Storage units.
| Measure | What it tells you | Typical unit |
|---|---|---|
| Capacity | How much the card holds | GB or TB |
| Write speed | How fast the camera saves | MB/s |
| Read speed | How fast files copy out | MB/s |
| Video bit rate | Data used per second of video | Mbps |
Table 4. Capacity and speed measures.
Cham tipCapital B is bytes. Small b is bits. Divide Mbps by 8 to get MB/s.
Cham taskFind your camera's highest video bit rate in Mbps. Divide by 8 and work out how many minutes a 64 GB card will hold.
Check yourself: A camera records at 200 Mbps. How many MB/s must the card write?
25 MB/s, because 200 ÷ 8 = 25.
What do the speed labels on a card mean?
Short answer. Speed labels state the minimum sustained write speed. Use them to match a card to your recording format (Kingston, n.d.).
- Speed Class: C2, C4, C6 and C10 mean minimum sustained write speeds of 2, 4, 6 and 10 MB/s (Kingston, n.d.).
- UHS Speed Class: U1 is 10 MB/s and U3 is 30 MB/s (Kingston, n.d.).
- Video Speed Class: V6, V10, V30, V60 and V90 mean 6, 10, 30, 60 and 90 MB/s. V60 and V90 need UHS-II (Kingston, n.d.).
- UHS bus speed (the roman numeral on the card): UHS-I has a theoretical maximum of 104 MB/s, UHS-II 312 MB/s and UHS-III 624 MB/s (Kingston, n.d.).
- Bus speed is a ceiling. Speed class is a floor for sustained writing. Both matter (Kingston, n.d.).
- To estimate the speed you need, multiply file size by frames per second. As an example, 30 MB RAW files at 10 frames per second need 300 MB/s. A video bit rate of 100 Mbps is 12.5 MB/s, because 8 bits make a byte.
| Label | Minimum sustained write |
|---|---|
| C10 | 10 MB/s |
| U1 | 10 MB/s |
| U3 | 30 MB/s |
| V30 | 30 MB/s |
| V60 | 60 MB/s (UHS-II) |
| V90 | 90 MB/s (UHS-II) |
Table 5. Common speed labels (Kingston, n.d.).
Cham tipBuy for sustained write speed and match it to the highest bit rate your camera records.
Cham taskFind the highest video bit rate your camera records. Convert it to MB/s and choose a card.
Check yourself: What does V30 mean?
A minimum sustained write speed of 30 MB/s.
What are CompactFlash, XQD and CFexpress cards?
Short answer. CompactFlash led for years. CFexpress, built on the XQD shape, is now the professional standard for fast bursts and high bit rate video (CompactFlash Association, n.d.).
- CompactFlash was introduced by SanDisk in 1994. Its large size and pins were prone to bending in readers.
- CFast was a faster CompactFlash version used in a few cameras.
- XQD was a smaller and sturdier card with up to 250 MB/s bus speed. CFexpress Type B has the same size and connector as XQD (CompactFlash Association, n.d.).
- CFexpress uses the PCIe and NVMe interfaces. Type A is the smallest with one PCIe lane. Type B has two lanes and about double the speed (CompactFlash Association, n.d.).
- Some XQD cameras can accept CFexpress Type B cards after a firmware update (CompactFlash Association, n.d.).
- The CFexpress standard has had versions 1.0 (2017), 2.0 (2019) and 4.0 (2023) (CompactFlash Association, n.d.).
| Card | Lanes | Note |
|---|---|---|
| XQD | PCIe Gen 1 | Up to 250 MB/s bus speed |
| CFexpress Type A | 1 PCIe lane | Smallest, in some Sony cameras |
| CFexpress Type B | 2 PCIe lanes | Same shape as XQD |
Table 6. CFexpress compared with XQD (CompactFlash Association, n.d.).
Cham tipRead what your camera slots accept. Do not buy a faster card than the slot supports.
Cham taskIdentify the card slots of your camera and the formats they take.
Check yourself: Which CFexpress type has two PCIe lanes?
Type B.
How do you choose and look after cards?
Short answer. Choose a card by camera support, capacity and sustained write speed. Care for it with a fixed routine.
- Match the family to the slot, then match the write speed to the highest bit rate you will record.
- Use several smaller cards rather than one huge card for important jobs, so one failure loses less.
- Never format a card before you have verified your backup.
- Format the card in the camera, not in the computer, when you are ready to reuse it.
- Keep cards in a case and away from heat, water and static.
Cham tipFollow the same routine every time: shoot, copy, verify, then format.
Cham taskWrite your own card routine on one page. Include who verifies the copy.
Check yourself: When should you format a card?
Only after the files are copied and the backup is verified.
Questions beginners ask
What is a memory card?
A memory card, or flash card, is a removable storage device that holds the files a camera records.
What is the difference between SD, SDHC and SDXC?
They differ by capacity. SD holds up to 2 GB, SDHC up to 32 GB, SDXC up to 2 TB and SDUC up to 128 TB.
What does U3 or V30 mean on a card?
Both mean a minimum sustained write speed of 30 MB/s.
Do I need CFexpress?
Only if your camera has a CFexpress slot, which is used in high-end cameras for fast bursts and high bit rate video.
Is a faster card always better?
A card only helps up to what the camera and slot can use. Match the card to the camera.
When should I format a memory card?
After you have copied the files and verified the backup, and format it in the camera.
References
- CompactFlash Association (n.d.) Card Types. compactflash.org/card-types
- Kingston (n.d.) Memory card speed classes. www.kingston.com/blog/personal-storage/memory-card-speed-classes
How to cite this guide. Alex, A., & Joseph, A. (2026). Memory cards: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/memory-cards/
Make this your craft at Creative Hut Academy
Creative Hut Academy in Mattakkara, Kottayam, Kerala teaches photography, cinematography, editing, sound and AI filmmaking in one-year residential diplomas. Students come from across India.
Want to talk? Message us on WhatsApp, or ask Cham, our AI mentor, a study doubt.