Your camera will determine the type of memory card you need. The camera’s resolution, video capabilities and the extent to which you use continuous shooting will dictate the card speed required.
This article will help ensure you choose a memory card that perfectly suits your photography.

Memory card speeds

Most modern cameras use Secure Digital (SD) cards, which come in three types: standard-capacity SD cards, high-capacity SDHC cards and high-speed SDXC cards. Standard-capacity SD cards have been largely phased out since their maximum capacity is only 2GB.

For all cards, data transfer speeds are currently rated in two ways: by a speed class defined by the SD Association or according to the UHS (Ultra High Speed) Speed Class rating. UHS ratings are only found on SDHC and SDXC memory cards and devices, with UHS 3 being more than twice as fast as UHS 1. Recently-released cameras should be able to use UHS cards.


Two UHS-rated SDXC cards from the same manufacturer show the speed difference between UHS 3 (left) and UHS 1 (right) cards. Lexar Card Selector  

The ‘x’ speed rating formerly used on memory cards has been made obsolete with the introduction of speed classes. However, some card manufacturers continue to use it for read speeds. (The ‘x’ equates to approximately 0.15 Megabytes/second.)

The table below compares the different speed class ratings for SDHC and SDXC cards.


Because card prices increase with higher speeds and capacities, photographers should ask themselves whether they really need the fastest cards. The answers are simple:

1. If you seldom (or never) record movies, you can happily settle on slower (or medium-speed) cards.

2. Large image files from high-resolution cameras (including raw files and RAW+JPEG pairs) will take less time to store on faster cards. This can be important for photographers who take action shots, particularly in rapid sequences.

3. If you regularly shoot continuous bursts of still pictures or record the occasional HD or FHD (1080p/1080i) movie, you should be able to use medium-speed cards; Class 10 is recommended as the minimum speed. UHS 1 cards will deliver smoother video footage.

4. UHS 3 SDXC cards are required for smooth consumer-standard (Ultra HD) 4K movie clips, which have a frame resolution of 3840 x 2160 pixels.

CompactFlash (CF) cards are mainly used in higher-specified DSLR cameras, some of which will have parallel SD card slots. Most cameras have Type I slots (the older Type II/Microdrive products have been largely superseded). The maximum read/write speed of current CF cards is 167 MB/second and you’ll need around 160 MB/second to record professional quality 4K movie clips with a frame resolution of 4096 x 2160 pixels.


This CompactFlash card carries both the ‘x’ speed rating and the UDMA 7 designation. The 1066x rating equates to almost 164 MB/second, which is close to the UDMA 7 rating.

Some manufacturers print UDMA (Ultra Direct Memory Access) ratings on their CF cards. The 0 to 7 range covers maximum transfer rates between 16.7 MB/second (UDMA 0) and 167 MB/second (UDMA 7).

Memory card capacities

While SD cards are made in capacities up to 512GB and CompactFlash cards are available with 256GB capacity, such high capacities are very expensive and only sold by specialist shops. The original SD (Secure Digital) memory cards, which were launched in 1999, had a maximum capacity of 2MB. This was expanded to 32GB using the FAT32 file system with the introduction of SDHC (High Capacity) cards in 2006.

Both capacity and speed increased again in 2009 when SDXC (eXtended Capacity) cards were introduced. They have a theoretical maximum capacity of 2TB, achieved by using the exFAT file system. Modern UDMA-7 CompactFlash cards support data transfer rates of up to 145 Mbytes/second.

The table below shows typical file sizes for the largest images at four popular sensor resolutions. The range of file sizes reflects different compression levels and whether raw files are compressed or uncompressed.


Use these figures to estimate the shooting capacity you will need. When shooting RAW+JPEG pairs, add the raw and JPEG file sizes, using data from your camera’s manual if you shoot reduced-size JPEGs or raw files.

It’s risky using a single card to store all your shots because it could be lost or mislaid and, although unlikely, any card can fail without notice through circumstances beyond a photographer’s control (inadvertent exposure to strong magnetic fields, for example). Consequently, many photographers prefer using several cards of lower capacity because they are more affordably priced. While you have to change cards more frequently and you can run the risk of losing a card, if that occurs ““ or the card ‘crashes’ ““ at least you won’t lose all your image files. Reputable manufacturers like Lexar, Kingston, SanDisk and Verbatim usually bundle their high-capacity cards with file recovery software that can retrieve images from a card if it fails.


Excerpt from Travel Photography pocket guide.