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I have found various online comparisons of write speed for memory cards.

How can I figure what limitations a given write speed will impose? For example, in the above linked comparison, the highest ranked card is listed as having "91.6 MB/s" with 64GB of storage. How can I translate this into photographic quality and length (video and burst). For example, if I am shooting 1080p HD, will this card be fast enough? What about 4K? How much time will I get?

I assume there are equations to figure this out, what are they?

  • With video it will be highly dependent upon the type of compression the camera uses as well as the contents of the scene (uniform colored walls take much less data per frame than highly detailed scenes). Not all HD video uses the same number of MB/S, even for the same scene. Ditto for 4K. It all depends on how the camera is compressing the data. – Michael C Aug 3 '17 at 7:12
  • Compression and scene contents are also applicable with still images, but with stills every frame has to include the values of the entire scene. With video typically only the parts that have changed from the previous frame need to be recorded. Of course stills are also usually much higher in resolution than video. – Michael C Aug 3 '17 at 7:14
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There are two things going on when you're shooting a video. Capturing/Processing and writing to the card.

Capturing/Processing -- You don't have to worry about this part. A manufacturer of any camera will guarantee that it will be sufficient. So it is a given.

Writing -- This is when the data is actually written to the card, and of course not cards are created equal. Will it be fast enough for your purpose? Whichever your purpose may be, capturing videos at various settings or capturing RAW images, the speed you need can easily be calculated from the image/video size.

There is no standard on file sizes of videos or images. So how much space 1 minute video will occupy or how big a RAW file will be, strictly depends on a particular camera in question. This answer, however, is generic and will work for every camera.

Videos -- Take any card, doesn't matter how slow it is. Put it in your camera shoot a video at max setting for a minute. Put the card into your computer and see the size of the video. For example, if the file size of 1 minute video is 60MB, the minimum writing speed you need is 1MB/Sec. Keep in mind this is a close estimate. And the speeds manufacturers of cards provide are also inexact. Because for some very slow cards frames can be skipped, once the video is finished recording briefly examine it. If there are no skipped frames or very few, this means that all or vast majority of frames are good. So in this example, getting a card of 1.5MB/sec write speed should be sufficient to compensate for possible skips and manufacturer margin of error. Getting a card with a speed of 3MB/sec would not improve video recording performance/quality, but is likely to cost extra.

Images -- RAW images need much more Write speed than videos if you shoot in burst mode. Your camera will have a specification for its buffer size, specifically how many JPEG and RAW it can store. Buffer is where photos are stored prior to writing to the card. It's very fast giving the camera ability to take images as fast as you can press or hold a shutter. But also this buffer is usually small. So lets say your buffer is 8 RAW images. If you press and hold shutter button you can max it out in couple of seconds. At which point camera will not take another shot until there's room (or on some older cameras - until the whole buffer is cleared).

So how do you measure the writing speed you need for images? -- Again put any card you have, and take a couple of RAW images of different things. A blank wall, and something with lots of detail. (This is because not all images are the same size, you want to know maximum size it can get). Put the card into the comp and look at the RAW file sizes. If your RAW file is 60MB, and your card's writing speed is 60MB per second, well you can take one photo per second. But keep in mind that having a buffer makes it seem faster because in many situations you need to take a burst, then pause and take some more. For me writing speed of 2 RAW images per second is fast enough. When I had less speed I often found myself in situation when I couldn't take another shot when I wanted to. How fast of a camera and card you need is subjective.

As for capacity, just look at file sizes of the samples you took in this example and apply your requirements. That is, how many 30MB RAW files can fit on a particular size card.

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The speed formula is simply bytes per second. But the maximum speed depends on both the card choice and the camera body. Write is typically slower then read. Camera processors are small and a bit slow, so the same card very likely measures faster to a PC instead of in a camera (and likely different in different cameras). The advertised card speed rating may never be seen in the camera.

A camera taking one JPG every second or two has no card speed concerns. Files are small and buffering more than handles it.

Video has very low speed concerns. Nikon says their DLSR cameras are fine with Class 6 cards for any HD 1080 video. Class 6 is a pretty slow card today, 6 MB/second.

However, for video or otherwise, one concern is later transferring a few GB of files to a PC, when then a faster card and a USB 3.0 reader will seem important at times.

Burst mode of JPG is not likely a huge issue, for a decent card, within reason. Again, bytes per second measured is the criteria.

But burst mode of large raw files is where card speed will become a factor. You can measure your actual rate then as bytes per second. If you take 6 frames per second, and if your file size is (7, 16, 40 MB each), then long burst card speed needs to be 6 x (7, 15, 40) = (42, 90, 240) MB/second. Buffering handles it for a few shots.

  • Where I have noticed the difference in card speeds is in the cycle time between bursts. Slower memory cards take more time to flush the buffer and introduce a delay between the last shot of a burst and the next shot. – user50888 Aug 2 '17 at 20:31
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For video, check your manual :). The manufacturer's documentation should tell you how fast a card you will need; since the compressed size typically varies with the subject, and the manufacturer implements the compression, they know what the maximum data rate should be.

For stills, generally the faster the card, the faster the data gets written to it (assuming the camera is capable of feeding the card at its maximum rate), and the more pictures you can take in a long burst. Typically, the camera has enough internal memory to buffer several shots - when that fills up, it can't take another picture until it's written a picture out to the card. For short bursts, that fit in the camera's internal buffer, you're fine. But for long sustained burst, or for multiple short bursts before the last one is written out, the card will be important.

If you know your typical image file size, and the card's write speed, you can figure out what the maximum sustained shooting rate is (rate <= file size / write speed, in pics/sec).

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SD cards are classified according to sequential write speed. Class 4 means minimum 4MB/s sequential write speed. Class 10 means minimum of 10.

Most high end cards support far in excess of Class 10 now.

Then there is the UHS number, which is a more comprehensive speed requirement. UHS-1 (written as U1 on the card has a minimum 10MB/s sequential write speed, and UHS-3 has a minimum 30MB/s, however the testing is more rigorous so something that is class 10 is not necessarily UHS-1. Under the hood, the controller in your device will have a maxmimum rate it can communicate with a card, so an old camera may not necessarily get a speed boost from a faster card beyond its own capability.

Sequential write speed is not the whole story with cards, as it can conceal the fact that random write speed can be orders of magnitude slower than this. If you have a fragmented card (one that is near full and might have had photos and clips randomly deleted from it then the card may struggle to make writes very sequential. In the absense of knowing the random write speed of a card, it makes sense to overcater on the sequential write speed based on the logic that as sequential write speed scales up, so will random write speed, all else being equal.

Note that all flash drives have a fast read speed, so you usually don't need to worry about this. For marketing purposes the often put read speed prominently on the packaging, which is misleading. It may be quoted as, say, 45MB/s, 90MB/s or 140MB/s (which can be hard to reach practically due to the host controller, but nonetheless the card could support it). Read speed isn't very important for photographing/filming, so let's not get confused by talking about it here.

For write speed, aim for about triple the bitrate of the video you're filming. Remember than a 24Mbit/s video requires 3MB/s transfer, as a Byte is 8 bits. So for filming 24Mbit/s video choose a class 10 card at minimum, as that's approximately triple the video rate. If you're doing higher bitrate video go for a UHS-3 card. Your camera's manual will tell you what bitrate it uses for each video mode.

For photos, it only becomes relevant if you do burst photography of more then around 7 shots in a burst, because before that the camera is able to buffer the images and write them to the card later. If you going to do longer bursts, class 10 cards or better will perform decently. Remember that even with an infinitely fast card, the camera is still likely to take them at a lower rate once the buffer is exhausted, so if it can do, say, 7 shots per second for the first 7 shots, subsequent shots in the same burst may still be slower ie 4-5 shots per second no matter how fast the card is. So if you get a class 10 card and find this, don't assume that a faster card can better it. For any decent camera it should state in the manual what its maximum burst rate is both for the first (buffered) shots and subsequent shots.

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