Memory cards buying guide

The camera dictates what type of memory card you can use since all cameras come pre-configured with dedicated memory card slots. Higher-featured cameras may have two card slots; sometimes with both accepting the same type of card but at other times one slot is designed for a different type of card.

The top camera in this illustration, the Sony Alpha 1, has dual card slots that both accept CFexpress Type A cards but can also accept SD cards, while the lower camera, the Sony Alpha7 Mark IV card slots are configured so one slot only accepts SD cards, while the other accepts CFexpress Type A cards but can also handle SD cards. (Source: Sony.) 

The market has changed in the past 10-12 years. CompactFlash (CF) cards predominated in the late 1990s and early 2000s but, since the technical specifications were last updated in November 2010, they’ve fallen out of favour. Fortunately for owners of older cameras, they’re still being sold, although not at ‘bargain’ prices and available capacities are limited.

A variant of CompactFlash known as CFast was launched in late 2009, with the main usage being in video cameras. But since 2017, many cameras and video recorders have supported faster data rates than CFast media can offer so in September 2016, the CompactFlash Association introduced the much faster CFexpress standard.

Factors affecting memory card prices

Our market survey showed huge differences in prices for cards, including cards with identical formats and capacities, while also revealing the main factor influencing card prices: data transfer speeds. As with the SSDs we examined in the previous issue of Photo Review, you’ll pay premium prices for high speeds as well as high capacities.

The bus speed (UHS-I or UHS-II) dictates the maximum data transfer speed of both cards and cameras. If your camera is rated for UHS-I, a UHS-II card is limited to the maximum UHS-I speed.

If you only shoot stills and your camera has a modest resolution of 20 to 30 megapixels, there’s no sense in paying top dollars for the fastest cards – although it may be worth paying more for greater storage capacity (although that can be debatable). Fast cards are only worth investing in if you’re shooting a lot of 4K 50p video or if you’re a sports or wildlife (including birds) photographer who records long bursts of high-resolution JPEG or raw files.

The table below provides a guide to the approximate capacities you need for a day’s shooting and the speeds required if more than 50% of your files are 4K 50p video.

How Much Data Can You Store on Your Memory Card?
Capacity	JPEGs at 20 megapixels	Uncompressed RAW files from 20 MP camera	Minutes of 4K 25p video	Minutes of 4K 50p video
16GB	6,500	200	32	16
32GB	13,000	400	65	32
64GB	26,000	800	130	65
128GB	53,000	1,600	260	130
256GB	107,000	3,200	520	260
512GB	214,000	6,400	1,000	520
1TB	420,000	12,800	2,000	1,200

Around the same time, the SD (Secure Digital) card format began dominating the market for in-camera storage media, particularly in smaller cameras. Being based on solid state memory they have no moving parts, so they are highly reliable and also relatively cheap to manufacture. Until recently, they have offered an adequate range of speeds for most cameras.

Shows the different speed class ratings in use. (Images sourced from Lexar.)

SD cards

Most cameras today are designed to use SD cards, which come in several ‘flavours’ and sizes and should be fine for recording the image and video files produced by today’s consumer-level and enthusiast-level cameras. The ‘standard’ SD card measures 32 x 24 x 2.1 mm in size and weighs around two grams. There are also smaller miniSD and microSD form factors measuring 21.5 x 20 x 2.1 mm and 15 x 11 x 1 mm, respectively, which have been developed mainly for use in mobile phones and are usually sold with an adapter housing that fits into a standard SD memory card slot.

This illustration provides a comparison of SD cards by showing a microSD-to-SD adapter (left), microSD-to-miniSD adapter (middle) and microSD card (right). (Source: Wikipedia.)

While a few cameras have subsidiary slots for the smaller cards, most use the standard form factor. The original SD specification had a read speed of 12.5 MB/s but a later version (V. 2.0) introduced a High-speed bus mode that doubled the speed to 25 MB/s. Even so, standard SD cards have virtually disappeared from the market as their data transfer speeds are too slow. All SD cards are backwards compatible.

SDHC cards, which were launched in January 2006, are physically and electrically identical to standard-capacity SD cards and come pre-formatted with the FAT32 file system. They are sold with capacities from 2GB to 32GB.

Launched three years later, the SDXC format, which covers capacities from 32 GB to 2 TB. Some SDXC cards have an additional row of contacts and adopted Microsoft’s exFAT file system so they’re not fully compatible with the original standard. They also introduced the Ultra High Speed (UHS-I) bus with interface speeds from 50 MB/s to 104 MB/s for both SDHC and SDXC cards.

Three SDXC cards with identical capacities but different form factors and speed ratings. (Images sourced from SanDisk.)

Version 4.0 of this standard, introduced in June 2011, brought in the four-lane UHS-II bus, which allows speeds of 156 MB/s to 312 MB/s. Version 5.0, announced in February 2016 at CP+ 2016, added Video Speed Class (V) ratings for UHS cards with a minimum write speed of 90 MB/s to cater for higher resolution video formats.

The SD Express standard was announced in June 2019 to meet the demands of 8K and 3D video by offering speeds up to 4GB/s for SD Express and almost 2 GB/s for microSD Express cards. A subsequent SD 9.1 specification, released in October 2023, added multi-stream access and improved power and thermal management.

Although SD Express cards work in an existing camera with an SD slot, they’re not widely available and it’s not worth buying them as the camera’s slower transfer speeds determine the maximum speeds at which image data is recorded. Furthermore, no camera manufacturer has taken advantage of the high speeds of these cards, instead re-configuring one or more card slots for CFexpress cards.

All SD cards are classified according to capacity and Speed Class. SDHC cards cover capacities up to 32GB, with SDXC cards extending beyond that. For commonly-available SD cards, speed ratings are split into three areas: a speed class mark with a number between 2 and 10 inside a large ‘C’, a UHS Speed Class with a figure (1 or 3) enclosed in a capital ‘U’ and a Video Speed Class between V6 and V90.

The table below shows the speed class ratings for each type of SD card.

Interface     SD	Min. sequential write speed	Speed Class				Video Usage (Speeds vary with recording & playback device.)
		Speed Class	UHS Speed Class	Video Speed Class	Express Speed Class
	2MB/s	Class 2				VGA video
	4MB/s	Class 4
	6MB/s	Class 6
	10MB/s	Class 10	U1
	30MB/s		U3			HD/Full HD video
	60MB/s					4K video
	90MB/s					8K video
PCIe/NVMe	150MB/s					4K multi-streams, 4K Intra video
	300MB/s
	450MB/s					8K multi-streams, 8K Intra video
	600MB/s

Adapted from SD Association data.

CFexpress cards

The first cameras to use CFexpress cards were the Nikon Z6 and Z7 mirrorless cameras, which both supported XQD cards on their release in August 2018. A subsequent firmware update enabled support for CFexpress.

Faster data transfer speeds are the main reason camera manufacturers have adopted the CFexpress format, largely driven by demand for more cameras to be capable of recording high-resolution, high frame-rate movies. The table below shows the read/write capabilities of the currently-available CFexpress cards used in our cameras, along with how modifications to the standard have boosted transfer speeds.

Standard	Version	Launched	Bus	Speed (read/write)
CFexpress	1.0	Q2 2017	PCIe 3.0 x2	2.0 GB/s
	2.0	Q1 2019	PCIe 3.0 x1	1.0 GB/s (Type A)
			PCIe 3.0 x2	2.0 GB/s (Type B)
	4.0	Q3 2023	PCIe 4.0 x1	2.0 GB/s (Type A)
			PCIe 4.0 x2	4.0 GB/s (Type B)

As you can see from this table, Type B CFexpress cards are roughly twice the speed of Type A cards.

Interestingly, different camera manufacturers have adopted different versions of the CFexpress format – often for different reasons. Canon and Nikon cameras have so far chosen the Type B format, while Sony favours Type A, which has a similar form factor to SD cards. Using Type A has allowed Sony to make cameras with media slots that can accept both CFexpress and SD cards. This can be handy for some photographers as it increases the camera’s versatility.

The table below shows the cameras with one or more CFexpress slots as at the end of 2023, along with the type of card they use.

Brand	Models	Type A	Type B	SD card slot
Canon	EOS-1D X Mark III EOS R5EOS R3EOS R5 C		Dual slots 1x CFexpress 1x CFexpress	1x SD UHS-II 1x SD UHS-II 2x SD*
Nikon	D6 Z9 Z8		Dual slots Dual slots 1x CFexpress	1x SD UHS-II
Sony	α7S III Sony α1 FX3 α7 IV α9 III	1x CFexpress 1x CFexpress 1x CFexpress		1x SD UHS-II 1x SD UHS-II 1x SD UHS-II 2x SD UHS-II** 2x SD UHS-II**

*One slot supports CFexpress Type A cards

** Both slots support CFexpress Type A cards

Using dual card slots

If your camera comes with two card slots, there are several ways in which you can use them, regardless of whether both cards are the same type or if they’re different. Your camera’s menu should provide ways to access the options for directing files.

The menu page from a Canon camera showing the settings for dual slot recording. (Source: Canon.)

Because different camera manufacturers put these settings in different parts of the menu and use different terminology to label them, we’ve used generic terms that describe what actually happens with your image files for each of the settings.

1. Overflow (or Auto switch card), the normal default setting, directs the camera to save all files on Card 1 and then swap to Card 2 once the first card is full.

2. Backup (or Rec. to multiple) sets both cards to record the same files each time you shoot. This setting is popular with wedding and event photographers as it provides a safety net in case their primary memory card fails. Make sure it works in your camera for both photo and video files if you record both types on the same card because some cameras will only duplicate still image files.

3. RAW + JPG (or Rec. separately) sets the camera to record raw files to one card and JPEGs to the other. This is handy if you like to store the files separately in your image database. It also lets you change the size and quality of each type of file independently of the other, which can be handy when storage space is reduced.

4. Photos/Videos is similar to RAW + JPG but lets you save your stills and movies to different cards. As well as providing the same benefits as the RAW + JPG setting, this can be handy if your camera uses different media formats as you can set it to save the video clips to the faster memory card.

CFA Video Performance Guarantee

The CompactFlash Association has recently announced a Video Performance Guarantee (VPG) standard that ensures media cards can handle the high data rates required for video recording by guaranteeing a minimum sustained write speed necessary for recording high-quality video without dropping frames.

This is crucial for professionals and videographers who rely on uninterrupted, high-resolution video capture – but not necessarily relevant for everyday videographers. Media cards without VPG support from CFA Members will work perfectly well in such situations. Refer to the camera manual to see if a VPG-supported card is required for your situation. Click here for full details.

 

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 99

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