Wednesday, July 9, 2014

How Fast Does A SD Card Need To Be For My Camera?

The memory card isle at your typical camera/computer shop can be often be a bewildering array of sizes and prices. This is a classic example of what is known in economics as price discrimination. Memory card product positioning does not have to be this complicated, but the wide array of price and selection is a mechanism designed to identify who is willing to pay how much for a memory card. The memory card manufacturers have to balance two sets of customers; those seeking value and those seeking performance. The key to product positioning and pricing is to maintain low prices for those that are the most cost-conscious while differentiating the high-end enough so that those who are willing to pay more will not be tempted to settle for the lower cost alternatives. This is also somewhat affectionately known as "Starbucks Pricing,"  as the strategy is to offer an overwhelming amount of choice without having the customer feel like he/she is being lead along a sliding price-scale experiment.Naturally, this makes finding the appropriate memory card for a given digital camera a little more than simple for the initiated. Fortunately, there's a very simple answer....

.... and that  is that for almost every camera aimed at non-working pros works just fine with normal speed Class 10 SD cards for casual JPEG shooting. This even covers 1080p HD video. In fact, stills shooting often places more demand on the memory card system than video shooting. Why? The reason is because 1080p video is actually not that demanding in modern terms: each frame is only the equivalent of a 2mp image and the data stream is compressed. If this seems like a low bar to cross, it is. Of course, that's for the large majority of users, but the full explanation is requires more explanation.

SD Card Types

For the sake of simplicity, we will cover only standard-sized SD cards, though there are some cameras that use the microUSB form factor (Nikon V3, Samsung Galaxy Camera).

  • SD: Memory card up to a maximum of 2GB. An obsolete standard, though there are many cameras in use that use this card type. Cameras that can only use standard SD are limited to 2GB of storage; there is no way to increase the memory capacity. These cards (like Fujifilm xD cards and Sony MemorySticks) are increasingly hard to to find for sale. If you need one, online is the best place to look for one, as local retailers would have lone since run out of stock.
  • SDHC: Cards  from  4GB to 32GB. The most common type of memory card sold. Colloquially referred to as "SD" and not by its full name
  • SDXC: Capacities of 32GB to 2TB, though the usual size is 64GB.

As the the standards have progressed through the years, backwards compatibility has been maintained on the camera side. If you wanted to, you could use an old 256MB SD card in a SDXC compatible camera. The only case of backwards compatibility being maintained with memory cards is with the UHS standard (see below).

SD Speed Ratings

"X" as in "48x" or "100x" refers to how many times faster than a 1x CD-ROM drive the memory card can sustain. For reference, the very first CD-ROM drives could only sustain data transfer speeds of 150kb/second, making this method of rating flash memory cards hilariously antiquated. There's no reason for this method of rating a memory card to persist other than the fact that it allows for an impressively big number to be printed on the packaging.

Speed Classes

"Class" refers to the minimum sustained ability to write to the card while recording video, as expressed in megabytes per second. This is the more useful way of reading a card's speed rating. In other words, a Class 6 SD card can sustain 6 MB/second, whereas a Class 10 card can sustain a continuous average write speed of 10 MB/second. The key word is "average" as the effective write speed can peak and dip.
However, the more useful terminology is the sustained write speed as expressed in megabytes per second. Anything in excess of 20MB/s is in excess of the Class 10 specification... sometimes by a fair margin in the case of the highest-end cards. Note that the class of the card is different from the speed rating of the card.  The speed rating is an additional figure that denotes the highest burst transfer rate. E.g, you can have a Class 10 card with a burst speed rating of 30MB/s. Think of it these terms: the class rating is like a guarantee in that it specifies the bare minimum that you should be getting, but the speed rating is like a promise in that it may not always be delivered as such.

UHS Speed Classes

The UHS standard was introduced in 2009 by the SD Association  for SDHC and SDXC cards. UHS uses a different data bus that does not work in non-UHS host devices. If a UHS memory card is used in a non-UHS host, it will default to the standard data bus and use the "Speed Class" rating instead of the "UHS Speed Class" rating.

  • UHS Speed Class 1 (UHS-I): 10MB/s minimum write speed
  • UHS Speed Class 3 (UHS-3): 30MB/s minimum write speed

Perhaps the most well-known of these types of cards is the Sandisk Extreme Pro line, which starts at 45MB/s. UHS-3 is the recommended rating for 4K video; for reference, the Panasonic GH4 produces a bitrate of 25MB/s. 

Practical Considerations

Memory card speed rating only has a bearing on two aspects of digital camera usage:

  • Shot-to-shot times when the camera buffer is full
  • Download times when transferring files.

That's it. For still photography, memory card speed has absolutely no impact on image quality. However, an insufficiently fast memory card can cause the camera to drop frames during video recording, but as of the time of this writing (middle 2014), Class 10 cards are sufficient, cheap and plentiful. Retailers will still put out lower speed grade cards as a way of enticing customers with lower advertised prices. These are fine, especially if you accumulate a lot of cards and don't want to spend much money, but given the competitive nature of the market, forgoing the least expensive option will not hurt your bottom line in any significant way when it comes to memory cards.

In daily use, empty cards are faster than nearly-full ones, and for the same reason, larger capacity cards write faster than smaller capacity cards.  This is because the memory controller will try to write data to contiguous blocks of memory within the SD card. As the card fills up, the data tends to get fragmented (much like a hard drive) and the memory controller has to hunt for free blocks of memory.

When reading files to a computer, the limiting factor will be the speed of the USB system. USB version 1 is dreadfully slow at 12 Mbit/s (full-bandwidth mode). USB 2 has an effective transfer rate of 35MB/s, while USB 3 has a practical throughput of 4GB/s.

Typical Usage By

Compact Camera to Entry Level DSLR's:

Any Class 10 memory card will do for these cameras. These cameras are typically used in JPEG mode by their users... sometimes exclusively...  and though some cameras like the Canon G16 have fairly high shutter burst rates, the camera buffer is often enough for most people. Even in entry-level cameras that don't have exceedingly high burst rates, the ergonomics usually don't allow for work that requires a high amount of data throughput. Even for video work, a Class 10 card is enough to record full HD videe without dropping frames.

Enthusiast and Semi-pro DSLR's and Mirrorless Cameras. 

This group of users covers a large range of shooting styles. The majority of JPEG-only users in this category will once again do fine with a basic entry-level Class 10 card, but there are three situations where high speed cards will be of use:

  1. High volume of pictures to download. For the causal shooter, this will be a convenience more than anything else, as the time savings is a one-time event at the end of the day. If you are going on a long vacation and won't be encountering demanding shooting situations, for the same money a larger memory card will likely offer better utility than a faster one
  2. Heavy workload. This situation is like the first, but it the need to download pictures happens on a frequent basis. Think: wedding shooters. 
  3. Shooting past the buffer. Commonly encountered with bird photographers and sports shooters. Also a failing of spray-and-pray type shooters.

That last group pretty much typifies the Nikon D7100. As DPReview reported, that camera will fill the buffer within 6 shots when shooting in NEF mode. Even using a Sandisk Extreme Pro 8GB Class 10 SD card (95MB/s) the sustained shooting rate  of just under 3fps when the buffer is full. The situation becomes less dire with JPEG shooting, where the buffer capacity is 50 shots with a stained rate of 4fps thereafter.

4K Considerations

GoPro Hero3+ Black Edition:

GoPro recommends a Class 10 SD card that can sustain at least 45 MB/s. Mind you, the 3+ Black can only record 4K video at a maximum framerate of 15 fps. The list of officially recommended microSD cards as of July 2014:
  • SanDisk Extreme 32GB microSDHC (SDSDQXL-032G)
  • SanDisk Extreme 64GB microSDXC (SDSDQXL-064G)
  • Lexar 32GB SDHC 600x (LSDMI32GBSBNA600R)
  • Lexar 64GB SDXC 600x (LSDMI64GBSBNA600R)

GoPro notes that using slower memory cards will cause the device to downshift the memory stream from 45MB/s to 35MB/s.  As the Hero 3+ Silver Edition only records 1080p video, any Class 10 memory card will work .

Panasonic GH4 and FZ1000:

Panasonic recommends a UHS-3 card for it's highest bitrate quality of 200Mb/s ("mega-bits"). However, this is reserved for high bitrate 1080p output; 4K (3840x2160p, 29.97fps) is recorded at 100 Mb/s, which is roughly 20 minutes per 16GB.

Additional Resources

  • Though on longer activity (on the web anyway) as of 2012, Rob Galbraith has perhaps the most comprehensive set of test data on SD and compact card performance. Go here for the numbers.
  • Toms Hardware maintains an active database of current cards here.
  • Chuck Steenburgh has an in-depth look at the Nikon D7100 and memory cards here. This is extremely useful for RAW shooters frustrated by the small-ish buffer of that camera.

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