How to choose the right memory card

The three main SD card ‘families’ used in digital cameras, from left: the ‘plain vanilla’ SD card with relatively slow transfer speeds and low storage capacities; the High-Capacity (SDHC) card, which has higher capacity but not necessarily significantly faster transfer speeds and the eXtended-Capacity (SDXC)card, which is significantly faster and can hold more data. (Source: SanDisk.)

Solid state flash memory data storage devices have been with us since the 1990s, but only gained wide acceptance in the current century. During their evolution we have seen shifts in card formats, capacities and read/write speeds, along with an extension of their use into mobile phones and portable smart devices.

Today their usage spreads across digital cameras, mobile phones, laptop computers, MP3 players and video game consoles, largely because they are small, re-recordable, and able to retain data without power. In this feature we’ll look at the main changes in the market in the past few years and explain how photographers are benefiting from them. We’ll also provide a guide to choosing and using memory cards.

Market Position

Currently, two card formats dominate the camera market: Secure Digital (SD) and CompactFlash (CF). For most cameras, the SD format is dominant and it looks set to remain so for the foreseeable future. Only the highest-specified camera models in the dominant manufacturers’ ranges include CF slots, and there is often an SD card slot as well.

The illustration above shows the main differences between CompactFlash and Secure Digital. Apart from size, the data transfer speeds for the CF card are more than double the speed of the SD card, despite their identical capacities and the fact that both have speed ratings that make them suitable for video recordings. (Source: SanDisk.)

Despite its many advantages, CF seems likely to fade away as a result of its limitations, which include:

1. The relatively large size of the cards limits their use in slim devices. However, it also makes CF cards easier to handle and less likely to be mislaid.

2. CF cards have many more interface points than SD cards (50 vs 9) and they use a pin-and-socket contact system, rather than exposed connections. Both are supposed to be ‘self-wiping’ but if a CF card is exposed to dust or other contaminants, its sockets can become blocked. Careless insertion of a CF card into a camera can cause the pins in the camera to become bent. Both will prevent data transfer and both problems can be costly to fix.

The latest UHS Speed Class 3 (U3) SDXC cards, which are designed for recording 4K video, can match the data transfer speeds of CF cards. The maximum capacity of SDXC cards is currently 256GB, while CF cards extending to 512GB will be released this year.

SD Card Options

The SD format includes four card families, which are available in three different form factors. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage and is rarely used in today’s devices, being replaced by USB interfaces.

The three form factors are the original size, the ‘mini’ size, and the ‘micro’ size. Mini cards are relatively uncommon, having been largely replaced by the smaller Micro SD cards, which are a better match for modern smart devices. Electrically passive adapters (normally sold with the cards) allow the use of smaller cards in a host device designed for larger cards.

Adaptors allow smaller Micro SD cards to be used in devices designed for larger cards. This is handy when you want to transfer images or movie clips to a computer via a card reader.

Some of the more popular brands of Wi-Fi SD cards, with a reverse view included to show compatibility with standard SD card slots.

Several companies produce SD cards with built-in Wi-Fi transceivers that support static security (WEP 40; 104; and 128, WPA-PSK, and WPA2-PSK). The card lets any digital camera with an SD slot transmit captured images over a wireless network and will store the images until it is in range of a wireless network. Examples include Eye-Fi, FlashAir, EZ Share, Transcend and FluCard.

All SD cards let the host device determine how much information the card can hold within the manufacturer’s specifications. Regular SD cards are specified for up to 2GB using the FAT 12 and 16 file systems; SDHC cards can hold between 2GB and 32GB using the FAT32 file system; SHXC cards are specified for 32GB to 2TB using the exFAT file system.

Speed

Card manufacturers often focus upon speed when pricing and marketing their products and, for some photographers, the data read and write speeds are important performance characteristics. Cards with fast write speeds work best when shooting rapid bursts (10 fps or more) and recording video. They also provide time savings when transferring files to computers for storage and/or post-production editing.

Unfortunately, regardless of how fast a memory card is, the camera’s write speed will limit the maximum speed at which it can shoot while the memory buffer is filling. If the camera can write out data to the card fast enough, the available buffer capacity can be fully utilised.

Card speed also affects the time taken to review images on the monitor, which influences how fast you can shoot. Typically, read speeds are a little faster than write speeds.

In the past, CF card manufacturers defined the speed of their products as a multiplier, such as 133x, 300x or 1000x. In contrast, SD cards were listed by ‘speed class’ ratings, based upon the minimum sustained speed required for recording an even rate of video onto the card.

Both have been replaced by a more precise speed rating in megabytes per second (MB/s), which is based upon the maximum transfer speed for reading and writing images to and from a memory card.

These are the fastest speeds at which the card can operate under ideal conditions with the fastest possible interface. If a camera can only output data at 10MB/s, having a card that could accept data at 150 MB/s won’t provide faster data transfer than a card that accepts data at 20 MB/s. To complicate matters, the efficiency of the interaction between the camera’s software and the card’s onboard controller must also be factored in.

If we examine the typical performance of some DSLR cameras with fast and slow CF cards when writing RAW files, the influence of the camera becomes clear. The figures in the table below come from Rob Galbraith’s CF/SD/XQD Performance Database (http://www.robgalbraith.com). As you can see, the fastest camera (Canon EOS 5D MkIII) gains more from a 1000x card than the others, though the Nikon D800 pair are fast enough to still gain some advantage from the fastest card. But with slower cards the difference between the cameras is reduced significantly, although the fastest camera retains a slight advantage.

	1000x	600x	300x	133x
Canon EOS 5D MkIII	80 MB/s	57 MB/s	39 MB/s	18 MB/s
Nikon D800/D800E	68 MB/s	55 MB/s	35 MB/s	19 MB/s
Canon EOS 7D	57 MB/s	53 MB/s	35 MB/s	17 MB/s
Nikon D700	41.9 MB/s	38 MB/s	26.5 MB/s	14.2 MB/s

 Now let’s look at some figures for SDXC cards in the two cameras that include SD card slots:

	Class 10 UHS-1 95 MB/s	Class 10 UHS-1  45 MB/s	Class 10  30 MB/s	Class 4
Canon EOS 5D MkIII	19.3 MB/s	17.1 MB/s	10.4 MB/s	4.1 MB/s
Nikon D800/D800E	42 MB/s	30 MB/s	25.6 MB/s	5.6 MB/s

 

There’s a big difference in performance between these two cameras that doesn’t reflect their performance with CF cards. It appears Canon decided to equip the EOS 5D MkIII with a CF slot that supports the newer UDMA7 protocol but a standard SD card slot which doesn’t support the UHS high speed standard. The Nikon camera, in contrast, has two UHS card slots.

UHS and UDMA

Several factors affect maximum read and write speeds of devices using flash memory cards, the most significant being the way the host device and memory card communicate. Both CF and SD cards include technologies that improve data transfer speeds.

The UHS speed class was introduced in 2009 by the SD Association for the new SDHC and SDXC memory cards. UHS utilises a new data bus that won’t work in non-UHS host devices. If you use a UHS card in a non-UHS host, it will default to the standard (slower) data bus.

CF cards originally supported a maximum transfer rate of 25MB/s but since 2003, a faster UDMA (Ultra Direct Memory Access) protocol has been implemented. This has been supported by most DSLRs and camcorders that use CF cards since 2006-2007. The maximum transfer rate for UDMA cards is 166MB/s. Unless both the camera and/or card reader support the same UDMA modes, the card will default to the lower speeds they sustain.

In 2008, a variant of CompactFlash known as CFast was announced but since it is based on the Serial ATA bus, rather than the Parallel ATA/IDE bus used for previous CF cards, it’s not physically or electrically compatible with CompactFlash cards. To date, it is only used in a few professional movie cameras.

Card Speed and Video

While card speed can be important for stills photographers who record long bursts of continuous shots, it is more important for photographers who shoot video. Compared to high-megapixel photography, video doesn’t need such a large data pipe because the video format is a smaller ‘fixed stream’ that uses only part of the data pipe available.

Verbatim provides an estimate of the potential recording time on its high-capacity SDXC memory card, which is Class 10 rated and UHS-1 compatible, making it ideal for video recordings. (Source: Verbatim.)

But it does require a minimum guaranteed speed to keep recordings smooth and uninterrupted. This is listed in the camera’s specifications. Without a fast enough memory card, HD video cannot be recorded continuously and most cameras and camcorders will display an error message and default to a lower definition setting.

Card Readers

With an increasing number of cameras using SD cards and laptop computers equipped with SD card slots, the need for dedicated card readers is reduced. However, if your camera uses CF cards and/or you edit on a desktop computer, having a card reader plugged into it can be handy.

While all card readers show variable performance, laptop card slots tend to be the most problematic. If your laptop doesn’t have a high-speed built-in card reader, any SDHC card you use will run slower than its speed rating and downloads will take longer than you would otherwise expect.

Using a really fast SD card with a slow host controller is a bit like plugging a USB 3.0 flash drive into a USB 1.0 port. A USB 3.0 card reader plugged into a USB 3.0 interface in the computer, should support close to the maximum speed for a high-end UHS-I SD or UDMA7 CF card.

Card Buying Guide

How much memory do you need? A lot depends on your camera’s resolution, the type of camera you use and whether you shoot JPEG or raw image files (or both together). Also factor in value for money because it’s often possible to double card capacity for only a few dollars more.

The table below provides a rough guide to the number of pictures you can store on three popular memory card sizes (8GB, 16GB and 32GB). Note the differences between capacities for JPEG and raw files.

Image format	Camera resolution	8GB	16GB	32GB
JPEG	12 megapixels	2000	4300	8600
	16 megapixels	1600	3200	6400
	21 megapixels	1100	2300	4600
Raw	12 megapixels	630	1200	2500
	16 megapixels	550	1100	2200
	21 megapixels	300	640	1300

 

A typical multi-card reader. (Source: Lexar.)

If you’re shooting video, a rough guide is to allow approximately 14 minutes for each 4GB of card storage when shooting Full HD movies at around 25Mbps and double that for HD movies. Most cameras place an upper limit of just under 30 minutes on clip lengths, although some restrict Full HD movie clips to less than 15 minutes.

Don’t feel you must buy only one card to accommodate your requirements. The highest capacity cards may not be supported in entry-level cameras and if the card fails or is mislaid, you lose everything you’ve recorded on it.

It’s safer to use several cards. You could use separate cards for different tasks, such as reserving one card for video recordings or allocating different cards to different shooting days. Otherwise, as one card fills it’s usually easy to swap it for a fresh one.

When shopping for memory cards, your first decision should be whether you really need to pay a premium for speed. If you shoot a lot of video and the camera can support fast cards, the answer will probably be ‘yes’; if not, you can shop on the basis of price.

Card prices are variable and can be dictated by the manufacturer’s brand name, storage capacity and speed class. Always check a range of capacities and speed classes for the card format you’re buying. Expect to pay slightly more for well-known brands with a good reputation for reliability and after-sale support. Generic brands can be cheap but may not match the reliability and support of known brands.

Card Failure

While memory cards are generally reliable, sometimes they can fail in much the same way as a computer drive crashes. To minimise this risk, back up files regularly to a computer or hard drive. (Set up a routine to back up every evening when you’re travelling.)

If a memory card fails before you have backed up its data, there are programs to help you recover lost files. SanDisk’s RescuePRO and Lexar’s Image Rescue 4 are included with some cards in each company’s range.

These programs can scan cards and may be able to recover all types of data, including image files as well as other document types. They work best when used with an external card reader and an external hard drive, which offer optimal security when transferring recovered data.

SanDisk’s RescuePRO is good insurance to have in case one of your memory cards fail.

Once the data has been recovered, re-format the card in the camera to reduce the likelihood of future crashes or failures due to lingering peculiar data structures. Cards should be re-formatted in the camera on a regular basis to remove ‘digital debris’ that may corrupt the card on subsequent use.

FEATURED LINKS:  

Lexar
SanDisk
Trek
Verbatim  

This is an excerpt from Photo Review Issue 60.

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