Guide to Camera Memory Cards: How to Choose the Right One for Your Photos and Videos

Learn how to select the best memory card for photography and videography. Compare SD and CompactFlash cards and understand speed classes.

Author: Marco Crupi

This article is a segment of the Digital Photography Course. Click here to return to the main course overview.

Memory cards, due to their compact size and high storage capacity, are widely used across various devices, including cameras and smartphones – many of which have slots for expanding storage with microSD cards. These cards use Flash memory, which allows data to be retained even without power.

There is a wide range of memory cards available depending on your needs. This guide will walk you through all the types currently on the market, explaining how they work and how to classify them, regardless of future technological advancements.

Card Types:

• SD – Now mostly obsolete due to their very low capacity, which maxes out at 2 GB.
• SDHC (Secure Digital High Capacity) – As the name suggests, these cards offer more storage than standard SD cards, ranging from 4 GB up to 32 GB.
• SDXC (Secure Digital eXtended Capacity) – These start at 32 GB and can go up to a theoretical maximum of 2 TB (2048 GB).

Card Formats:

• Standard SD – 32x24x2.1 mm
• MiniSD – 21.5x20x1.4 mm
• MicroSD – 11x15x1 mm

The speed of a memory card, measured in megabytes per second (MB/s), indicates how fast data can be read or written. This is just as important as the card’s capacity, especially for high-speed data storage. Sports photographers, for example, need fast write speeds because burst shooting can quickly fill the card’s buffer. A slower card will delay further shooting until the images are fully saved.

Videographers also pay close attention to card speed. Higher-quality videos require faster cards, and some cameras won’t even start recording if the card isn’t up to the task.

Higher classes indicate faster write speeds and better performance, though they also come at a higher cost. Besides class, you’ll often see a speed rating marked with a number followed by an “x” (e.g., 1000x). Sometimes, the speed is directly stated in MB/s, known as the “Speed Rating”.

What Do the Numbers on the Card Mean?
If a card is labeled 1000x, it means the data write speed is 1000 times the speed of reading audio CDs, which is 150 KB/s (or 0.15 MB/s). So, 1000 x 0.15 MB/s equals 150 MB/s. If the speed is directly stated in MB/s, no calculation is needed. However, keep in mind that these speeds are often the maximum rates claimed by manufacturers, not necessarily achieved in real-world use.

To classify speed, SD cards are divided into speed classes, which indicate the minimum continuous write speed on an empty SDHC card, expressed in MB/s.

In 2009, the SD Association introduced the UHS (Ultra High Speed) class for SDHC and SDXC cards, utilizing a new data bus that isn’t compatible with non-UHS devices. If a UHS card is used in a non-UHS slot, it defaults to the standard data bus and the regular speed class ratings apply instead.

In UHS cards, it is also possible to notice another type of classification, U1 and U3, which serve to identify the minimum writing speed.

There is also the Video Speed Class (V Class), designed by the SD Association, which guarantees that a card can maintain consistent write speeds during video recording.

CompactFlash (CF) cards are solid-state memory cards that use Flash memory. Like SD cards, CF cards also display their maximum speed in MB/s. However, CompactFlash cards are not regulated by the SD Association and have a different speed classification system. CF cards can be rated under two systems, often both present on the card.

Speed Classifications:

• UDMA Rating (Ultra Direct Memory Access) – Allows faster read/write speeds, supporting HD and 4K video. Ranges from 0 to 7, with 7 offering the best performance.
• VPG (Video Performance Guarantee) – Comes in two levels: VPG-20 (guarantees a minimum write speed of 20 MB/s) and VPG-65 (guarantees 65 MB/s), ensuring no frame loss during video recording.

CF cards are sized at 42.8 x 36.4 mm, with thickness varying by type:

• CompactFlash Type I – 3.3 mm thick.
• CompactFlash Type II – 5 mm thick, less common.

Main CF Card Versions:

• CF 1.0 (1995) – 8.3 MB/s.
• CF High Speed (2003) – 16.6 MB/s.
• CF+ 3.0 (2004) – Adds UDMA 66 support, 66 MB/s max speed.
• CF+ 4.0 (2006) – Adds UDMA 133 support, 133 MB/s max speed.
• CF 5.0 (2010) – Adds 48-bit addressing, up to 128 PB theoretical capacity.
• CF 6.0 (2010) – Adds UDMA Mode 7, 167 MB/s max speed.

How do you navigate the various specs and speed ratings? The speed class is a critical factor, but you should also consider brand reliability—some manufacturers are known for better quality and performance. Even within the same brand, high-end models labeled as “prosumer” (professional-consumer) offer better performance for demanding users.

Don’t let price alone guide your decision. Your memory card stores your photos, videos, work, and memories, so quality matters. Research online comparisons to see how different brands and models stack up. Some comparisons use professional testing, while others are more empirical, so verify the credibility of the results.

Which Card to Choose?
It depends on your device and needs. A high-capacity, Class 10 SDHC card would be overkill for a basic compact camera, while a low-capacity, Class 4 card might limit an entry-level DSLR or mirrorless camera. Class 10 cards have become more affordable, making them a good choice for advanced enthusiasts—especially for burst shooting or rapid sequences, where faster write speeds help prevent buffer slowdowns. If you’re shooting video, a high-speed video-rated card is essential.

It’s generally advisable not to use cards with excessive capacity, as losing large volumes of data in the event of a failure can be devastating. Many professionals prefer using multiple smaller cards to mitigate this risk, regularly backing up data to a laptop when possible.

Lastly, consider data transfer times from card to computer. Transferring 64 GB of data from a Class 4 card will take significantly longer than from a Class 10 card. Always check your camera’s manual for card compatibility before purchasing.