Guide to camera memory card types: features and recommendations

Discover how to select the right memory card for photos and videos. Compare SD and CompactFlash cards and their speed classes.

Author: Marco Crupi

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

On the left, a CompactFlash card; on the right, an SD card.

Memory cards, due to their compact size and high storage capacity, are used across various domains and a wide range of electronic devices. For instance, apart from cameras, they are widely utilized in smartphones, with many models featuring a slot to expand storage using microSD.

They employ Flash-type memory, allowing data preservation even without power.

There’s a vast array of memory cards available, depending on their intended use. This guide will delve into all the types currently available in the market.

The aim of this article is to elucidate how memory cards operate and how to classify them, regardless of the technological advancements that will emerge in this arena.


Card Types:

  • SD – Now obsolete due to their very limited storage capacity, which maxes out at 2 GB.
  • SDHC – Secure Digital High Capacity. As the name suggests, they differ from standard SD cards in that their storage capacity is greater. They start at capacities just over 2 GB (4 GB) and can go up to a maximum of 32 GB (these limits will surely be surpassed as technology advances).
  • SDXC – Secure Digital eXtended Capacity. They start from a capacity of 32 GB and can theoretically reach up to 2048 GB, or 2 TB.

Card Formats:

  • Standard – 32x24x2.1 mm.
  • MiniSD – 21.5×20×1.4 mm.
  • MicroSD – 11×15×1 mm.

MicroSD card with SD adapter.


The data transfer speed of a card, both in reading and writing, is expressed in megabytes per second (MB/s). This metric is as crucial as its storage capacity (the more storage, the more photos and videos can be saved). This rate is vital for high-speed data storage. For instance, sports photographers greatly value this parameter. When using the camera’s burst mode, numerous photos must be saved onto the card within a short span. A card with slow write speeds, beyond a certain point, would prevent the photographer from taking further shots, only allowing them to resume once all images are saved properly.

Videographers also heavily consider card speed. The higher the video quality, the more essential it is to have a high-performing card. While I’m not an expert in the video domain, I recall that some cameras even prevent recording if they detect an unsuitable card.

SDXC Card Class 10, UHS-II U3.

The higher the class, the faster the write speed, making the card more efficient, and consequently, more expensive.

Beyond the class, there’s a speed rating, denoted by a number followed by a multiplication sign, like the card in the picture above labeled “1000x”. Sometimes, the speed is given in MB/s, referred to as the “Speed Rating“.

What do the notations on the card signify? If a card is labeled with “1000x,” it means the amount of data written in one second is 1000 times the reading speed used for audio CDs, which is 150KB/s (or 0.15 MB/s in megabytes). Specifically, we have: 1000 x 0.15 MB/s, which equals 150 MB/s.

For cards that directly display their speed in MB/s, no calculations are necessary. However, one must be cautious with these figures, as the speeds mentioned are peak rates, declared by the manufacturer and not verified impartially. Often, to avoid potential complaints, the manufacturer might include the term “up to” followed by the speed on the packaging or the card itself. Alternatively, you might even spot an asterisk next to the speed, indicating that the displayed speed is the maximum.

To classify their speeds, SD cards have been divided into classes. The speed class indicates the minimum continuous writing speed on an empty SDHC card, expressed in MB/s.

2 2 MB/s
4 4 MB/s
6 6 MB/s
8 8 MB/s
10 10 MB/s

In 2009, the UHS speed class was introduced by the SD Association, designed for SDHC and SDXC memory cards. To achieve higher data transfer speeds, UHS uses a new data bus that doesn’t operate on non-UHS devices. If a UHS memory card is used in a non-UHS slot, the standard data bus will be used by default, and the “Speed Class” rating will be applied instead of the “UHS Speed Class” rating.

UHS-I up to 104 MB/s
UHS-II up to 312 MB/s
UHS-III up to 624 MB/s

There are three UHS speed classes: UHS-I, UHS-II, and UHS-III.

On UHS cards, another classification, U1 and U3, can also be observed, which serves to identify the minimum write speed.

U1 between 10 MB/s and up to 30 MB/s
UHS-II between 30 MB/s and up to 90 MB/s

The Video Speed Class, also known as “Class V“, was created by the SD Association. It’s a certification that indicates the card’s guaranteed performance, essentially its ability to maintain a consistent write speed during video recording.

V6 6 MB/s
V10 10 MB/s
V30 30 MB/s
V60 60 MB/s
V90 90 MB/s


CompactFlash (CF) are solid-state memory cards that utilize flash memory. Like SD cards, CFs may display their maximum speed in MB/s on the device. The CompactFlash system isn’t under the control of the SD Association, hence it has a distinct speed level classification. There are two types of classifications, often both found on the same card.

  • UDMA Classification: Ultra Direct Memory Access allows for faster read and write speeds, enabling the card to support HD and 4K images. The cards are ranked from 0 to 7, with 7 offering peak performance.
  • VPG (Video Performance Guarantee) Classification: There are two classifications, VPG-20 and VPG-65. The former means the CF card is certified to record videos at a minimum guaranteed speed of 20 MB/s, while the latter assures recording at a minimum of 65 MB/s and guarantees no frame drops.

They measure 42.8 x 36.4 mm, with thickness varying based on the type; indeed, there are two versions of this model:

  • CompactFlash Type I with a thickness of 3.3 mm.
  • CompactFlash Type II, less common, with a thickness of 5 mm.

There are six main versions:

  • CF version 1.0 (1995) at 8.3 MB/s.
  • CF High Speed (also CF+ / CF2.0), (2003) at 16.6 MB/s.
  • CF+ 3.0 (2004) added UDMA 66 support with a theoretical peak speed of 66 MB/s.
  • CF+ 4.0 (2006) introduced UDMA 133 support with a theoretical peak speed of 133 MB/s.
  • CF 5.0 (2010) introduced 48-bit addressing support, allowing a theoretical capacity of 128 PetaBytes.
  • CF 6.0 (November 2010) added UDMA Mode 7 support, with a theoretical peak speed of 167 MB/s.


So, how does one navigate through this maze of acronyms and numbers? Certainly, the class designation is a pivotal reference, but one should also consider the brand. Some manufacturers are notably more reliable than others. Given the same brand, there are high-end products, often aimed at more demanding users labeled as “prosumer” (professional-consumer). Brand choice is subjective, but it’s undoubtedly preferable to opt for well-known brands with industry experience over unknown ones.

Don’t be swayed solely by price. Your card will store your photos, videos, work, and memories.

Fortunately, the internet can assist. A simple search like “SD card comparison” will yield hundreds of sites offering comparisons between various brands and different product tiers from the same manufacturer. Some provide comparisons using professional equipment, others with empirical methods, but one should always verify the credibility of the results presented.

Which card should you choose? Naturally, it depends on multiple factors. Firstly, consider the device with which the card will be used and your current needs. On a standard compact camera, a 128 GB class 10 SDHC card would likely be overkill. Conversely, for an entry-level DSLR or mirrorless camera, even if you’re a beginner, a 32 GB class 4 card might be limiting.

Class 10 card prices aren’t as steep as they once were. For advanced hobbyists, I’d recommend aiming for these models. Faster write speeds will benefit cameras during burst modes or sequences of shots. While cameras come equipped with sizeable buffers, to avoid filling them and slowing down write operations, it’s better to opt for higher-performing products. Moreover, if you’re into videography, a high-class card is a must.

Personally, I’d advise not going overboard with card capacity. The larger it is, the more files you risk losing if something goes wrong. Professionals often use multiple cards to ensure they don’t lose all their work. It’s also a good practice to download card data to a laptop whenever possible.

Data transfer from the card to a computer is another aspect to consider. Transferring 64 GB of data from a class 4 card is different from a class 10 card. With a lower-class card, it might take twice as long. Before purchasing a card, check your camera’s manual to ensure compatibility.

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