Memory Card Basics: Choosing the Right Memory Card for Video and Time-Lapse
Choosing the right memory card for a video or time-lapse project is a critical decision. An under-powered, old, or even bad card can be the weakest link in your workflow chain and if it breaks your project will suffer. The wrong choice of card could even cause the unthinkable to happen – card failure at a critical moment during a wedding or sports shoot when the under-capable card can’t keep up with the flow of data from your camera.
So what is a UHS Class 1? Class 10? SDXC? Video Performance Guarantee? 1000x? 45 MB/s? Speed? Form factor? What are all of these things? I’ll demystify these terms in this post as well as illuminate basic media storage concepts such as SD Card Class and what the MB/s rating on SD cards really means for your video and time-lapse shoots.
Memory Card Basics: Form Factors, Capacities, and Speeds
Memory cards have three fundamental properties: form factor, card capacity, and card write/read speed. These are physical properties of specific cards themselves, as each can only perform within a particular class of data transmission and storage capacity.
Memory Card Form Factors and Compatibility
Camera memory cards come in a variety of sizes and shapes. Here are the most popular (listed in decreasing order of prevalence in the DSLR market):
● CompactFlash® (CF)
● Secure Digital (SD)
● XQD (next-generation CF card)
● Memory Stick Pro/Duo
● Multi-Media Card (MMC)
A few others worth mentioning: Ultra high-end professional video cameras, such as RED cameras, certain Sony EX cameras, and the Panasonic Varicam LT 4K S35, record onto manufacturer-dedicated solid state flash media. Cameras like the Blackmagic URSA 4K accept CFast cards, which are a variant of CompactFlash (CF) but are not physically interchangeable. Keep a lookout for this because CFast cards are very easy to confused with CF cards. Some cameras, like the Canon 1D X Mark II, accept one of each!
CF cards were the gold standard for DSLRs from the time of the Nikon D1 all the way to a few years ago, when the previously slower and smaller SD cards evolved and eventually became the standard for recording media. SD cards now offer the same or better data capacity and read/write speeds and are rapidly becoming the most common card type in today’s prosumer and consumer DSLR market. Many popular action cameras, such as the GoPro, make use of microSD cards because they have a much smaller form factor than their bigger cousins but share most (but not all) of their large capacities and speeds. These cards can be directly read by some card readers or they can be inserted into an SD card-sized adapter.
It’s also worth noting that some of the very newest professional DSLR cameras offer shooters a choice of media card form factor, such as the Nikon D5, which offers a body with dual CF slots and a body with dual XQD slots. One final thought about form factor: smaller things are easier to misplace and some shooters find the more robust, larger form factor cards (e.g. CF versus microSD) to be more physically resilient and robust in the face of extreme conditions.
Finding and Calculating Card Capacities for Video or Time-Lapse
Each memory card is capable of recording a certain amount of data. Data values are shown on the front of the card in gigabytes (GB) and, as one might expect, larger-capacity cards can hold more media but they cost more. In this amazing era of digital photography, huge-capacity SD cards seem to come down in price every day. 128 GB SD cards are now available for less than $50 or can be accessed as an affordable rental. But just how much video can I actually roll onto a card – especially 4K video?
The amount of video footage or still imagery (like for a time-lapse) that can be stored on each card really depends on the footage quality settings on the camera. Here is an example of how many still images an 8 GB card can hold at various settings on the Canon 5D Mark IV:
“Burst capacity”, or Maximum Burst Rate, has to do with how many shots in a row the camera can safely write to the card in high-speed shooting mode before it shuts down. Burst capacity in particular might only matter for still shooters but the read/write speed is a limiting factor that is applicable to both still and motion shooters. With respect to video files and card capacity, the same rules apply: different quality/frame rate settings affect how much media can be stored on a card.
With the example below again being an 8 GB card in the Canon 5D Mark IV, consider 4K video, where the bitrate is given as 3,587 megabytes a minute, or MB/min (this equates to 59.78 megabytes a second or 500 megabits a second). Two minutes of footage is not much:
Note that these capacities do not include recorded audio, which also takes up space on your card. In-camera video quality settings matter greatly here as well. Shooting 720p video generates only half as much data as shooting 1080p and 4K sends another ~2x as much information to the card! Your video capture frames per second (FPS) value also makes a difference. Higher frame rates send increasing amounts of data. Learn more about frame rates in Intro to Video Frame Rates and Frames Per Second Shooting Speeds.
As one might expect, in a camera shooting 4K video at 60 FPS, the amount (and rate) of data being written to the card can be exponentially greater than a camera set to 720p at 24 FPS. Figuring out how much video you can store on a card involves many factors. Different manufacturers use different compression codec and bitrates, resulting in differently-sized files. Go into your camera’s manual (or find specs online) to find out what bitrate your camera shoots at. Bitrates are defined by megabits per second (Mbps). In general, the higher the bitrate, the better the quality. The bitrate is the amount of data being written to the card each second when shooting video.
For example, the Rebel T5i is listed as shooting about 330 megabytes per minute for 1080p. I used Google to convert 330 megabytes per minute to megabits per second for me and the result is 44 Mbps.
For a very rough estimate of capacity, take a memory card’s capacity in gigabytes (almost all cards nowadays), divide it by the bitrate, and multiply the result by 2. For a 32 GB card recording at 44 Mbps, I can fit about an hour and half of footage. You can sanity check this with what your camera’s specs say. For the Rebel T5i, highest-quality 1080p footage is estimated to be about 44 minutes of footage per 16 GB. For a 32 GB card, this is 88 minutes, or nearly an hour and a half.
Going back to our chart and the 4K example above, an 8 GB card shooting at 500 Mbps:
8 / 500 = 0.016
0.016 x 2 = 0.032
0.032 hours is about 1.92 minutes. To fit more, you’ll want to either shoot at a lower bitrate or use a larger card.
For stills, a Nikon D810 produces 12 bit NEF raw files that are around 56 megabytes. How many stills can I fit on a 32 GB card when shooting a time-lapse, for example? First, let’s consider what 32 GB is in megabytes.
32 GB = 32768 MB (in binary – 32000 in decimal, which is close enough)
32000 MB / 56 MB = ~ 571 frames to fill my card.
To learn more about time-lapse intervals and how much space you’ll really need for a given length, see How to Shoot a Time-Lapse: Intro to Interval and Exposure Settings and Techniques.
These are the roughest of calculations that will change depending on your settings and the content you’re shooting. For stills, simpler scenes take up less data than complex ones, for example; it’s more of a range than an exact number. Each camera’s user manual will usually contain a chart similar to that shown above to help you learn what the available space will be with various cards.
Card Write/Read Speed and the Anatomy of a Memory Card
How good are enormous volumes of space if your card can’t handle the massive influx of data streaming to it from your camera’s sensor? The final fundamental property of media cards to consider is read/write speed. Things can get complicated at this point but fear not. Each individual media card has a physical limit to the amount of information it can reliably store under normal conditions. Higher end (read: more expensive) cards can handle larger streams of data. This is the critically important part of cards for video shooters, especially those who shoot 4K.
Right on the front of each card are many markings, some of which have to do with capacity and some of which provide information about card write speed. Let’s look at an SD card’s properties and what they mean for today’s video and time-lapse shooters.
A & G = UHS Speed Class Rating
Each card lies in a particular “class” of speed rating. There are six classes: Class 2, Class 4, Class 6, Class 10, U1 and U3.
Class 2 is the slowest, U3 is the fastest. If you are shooting 4K video, you will almost always need a class U3 card. 1080p shooters can get away with U1 class cards. As of last year, some newer SD cards use a “Video Class Rating” (often called, more simply, “VSC-rated cards”), such as V10, which means the card is built with ultra-fast modern NAND flash memory and supports simultaneous multi-file recording; these cards are ideal for video applications. The number after the V corresponds to the amount of data in MB/s that can be reliably written to the card at speeds up to 120 FPS. V90 is the highest and is suitable for 8K video recordings.
Cards labeled U1, Class 10 (like in the example card above) can record up to 10 MB/s and are suitable for HD video recording (1080p), or full-size raw recording for time-lapse in most applications (but not all); massive sensors, like the one in the Canon 5Ds, may overwhelm even these fast cards. The speed performance of these cards also varies based on the type of SD bus in the camera – whether it is high speed or ultra-high speed (UHS). Newer camera hardware will typically be UHS; the Canon 5D Mark IV SD card slot, for example, is UHS-I but not UHS-II.
Above is an example of the Nikon D810 listing its SD bus type. You can find this information in most camera manuals. If you are renting a camera, check BL’s “Manual” tab on the camera’s listing. They try to provide manuals for each camera (when available). Most can be found online somewhere.
Other than capacity, this is the most important property of SD cards – if you are shooting a 4K video project where you can’t afford to miss a shot, this is the class of SD card you need. The SD Card Association chart below provides a helpful graphic to visually demonstrate what speed-classes of SD cards are appropriate for different types of video projects:
Shooting tight intervals is a common occurrence in the time-lapse world. When I’m shooting 30 second long astrophotos, I often want to have only a one second interval between frames to minimize gaps between stars and to make the smoothest possible time-lapse footage. Remember how the raw file size of the Nikon D810 is ~56 MB? If I truly want the shortest interval between frames, a UDMA1 CF card or a Class 6, 4, or 2 or SD card will be insufficient, as the camera will be sending more data to the card in the 1 second between frames than the card can handle. I either need to choose a longer interval or use a faster card.
B = Max Read/Write Speed
When you see something like “45 MB/s” on a card, this is showing the maximum amount of data that can be reliably read from the card each second.
Some cards display this as a rate-of-data per second, some as a number followed by the letter “X”. They all refer to the same thing. But this can also be misleading, as it’s manufacturer-provided. Pay more attention to the independent SD Card Association-provided guaranteed write/read speed class ratings (see “A” above) than to the MB/s or x-speed class rating (i.e. 3500x). If you’re using a card that lists an x-speed rating rather than the clearer MB/s, know that each “x” represents .15 MB/s. Multiply, for example, a Lexar card with an x-rating of 2000x by .15 MB/s and you will get 300 MB/s.
C = Card Type
Like all storage media, such as hard drives, SD cards have a file format, which dictates the maximum amount of data that can be stored on the card. SDHC stands for SD High Capacity, which contains up to 32 GB in size and has a FAT32 file system. SDXC stands for SD eXtended Capacity, which holds up to 2TB and has an exFAT file system.
Note that FAT file systems won’t store single files (i.e. long movie recordings) larger than 4GB, so if you need to roll 4K video for long periods of time, make sure you have an SDXC type card.
D = UHS Rating
This refers to the type of UHS bus needed by the host hardware (i.e. your camera) to enable card reading at the highest speeds. There are three possible values here: Normal, High, and Ultra High I/II.
These are backwards compatible but you won’t get UHS-II speed out of a card unless the camera has a UHS-II bus.
E = Hours of HD Video
This is an outdated marking found on older cards that rather ambiguously gives an amount of “HD” video that can be stored on the card.
It’s ambiguous because HD video means 720p to some folks and 1080p to others, and other settings such as frame rate or the presence or absence of audio can be a factor in this actual value.
F = Capacity
This value gives the amount of data in total that the card can safely carry. Regular SD cards can only handle up to 2 GB (these are relative dinosaurs in today’s marketplace) and most cards that video or time-lapse shooters use today are SDHD, which range from 32 GB all the way up to a whopping 2 TB.
The markings on CF cards are more or less the same with the notable difference of speed class rating being given both as a value of MB/s and a UDMA rating. For those shooting 4K video, or shooting short-interval time-lapses on a modern full frame camera, look for UDMA 7 CF cards, which have a ceiling of ~150 MB/s.
Newer CF cards may even offer a “Video Performance Guarantee” rating of VPG-20 or VPG-65, which translates into continuous read/write data streams in that number of megabytes per second.
What Memory Card Should I Use?
Now that we have explored the various properties of the most common video memory cards, how do you chose which one to use for your next project? In a nutshell: if you are a 4K shooter and nailing the shot on the first take is make-or-break to the success of your application, seek out only the newest 64 GB U3 SDXC UHS-II card or a 128 GB 160 MB/s UDMA 7 CF card. For raw time-lapse shooters, or those shooting only 1080p HD video, slightly lower-performance cards should suffice, like this 64 GB U1 SDXC UHS-I SD card or this 64 GB UDMA 400x CF card.
To recap: go into your camera’s manual (or find specs online) to find out what bitrate your camera shoots at. Bitrates are defined by megabits per second (Mbps). In general, the higher the bitrate, the better the quality. The bitrate is the amount of data being written to the card each second when shooting video. Cross-check this value against the minimum speed rating of the card you are considering. For my Nikon D810 and 5D Mark IV examples, ~60 MB/s is the minimum requirement I need from an SD card or UDMA 7 CF card. If a card only lists speed by an x-rating instead of by MB/s, you can find the MB/s by multiplying an x-rating by .15 MB/s. For example, that 64 GB UDMA 400x CF card mentioned above has a speed of ~60 MB/s – the very minimum I need for a D810 or 5D Mark IV. Using something a little faster might be prudent.
There are a lot of cards out there. When it comes time to gear up for your next video or time-lapse project, choose wisely. Older, slower speed class cards can cause longer video shots to fail, hampering your creativity and interrupting workflow. There’s nothing worse than waking up after setting up an epic astro time-lapse on a mountaintop to find that it’s unusable because your intervals are irregular because your card couldn’t keep up. If smooth, uninterrupted recording is mission-critical to you, it is crucially important to test before your shoot; you do not want to have a card-write failure while you are in the middle of a once-only, live interview.