Ever been shopping for a memory card for your camera or even looked at one you already own and wonder what the heck all the designations mean? Well there certainly are a ton of them so if your a bit confused sit back I’m going to help you sort it all out.

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Lets start with something called “speed class”. This is one of the categories that has morphed over the years but it’s very straightforward. First there was the basic Speed Class, signified by a number inside a circle. If you look closely you’ll notice that the right side of the circle is incomplete. It’s because it’s not a circle but a “C” for class. These classes ranged from 2 up to 10 and we’re a direct correlation to write speed with Class 2 meaning 2MB/s (Megabytes per second) max transfer rate and Class 10 meaning 10MB/s max.

Now lets back up for just a second to explain what MB stands for. Across the world of electronics and data transfer in general MB stands for Megabytes; not to be confused with Mb which stands for Megabits. Simply put a Megabyte is equivalent to 8 Megabits. So you can see if you get them confused you’ll grossly be over or underestimating your data transfer rates so keep track of that upper and lower case “B”.

Now, back to speed classes. The standard speed class is now pretty much obsolete because they stopped way back at class 10 and any card manufactured in the last several years will be well beyond 10MB/Sec. So yes, feel free to ignore this rating unless your looking at a very old card.

Likewise the “U” rating, short for Ultra High Speed, designated by a roman numeral 1 or 3 inside of a “U”, is also pretty much obsolete. The “U” rating topped out at U3 and like the standard speed class can easily be converted to a MB/Sec. rating. U3 corresponding to a transfer rate of 30MB/Sec. and UI to a transfer rate of 10MB/Sec. Again, any modern card is going to have the highest rating of U3 even if it happens to be much faster.

The final “Speed Class” is thankfully not yet obsolete and very much something you should be aware of. It’s the V-Rating short for “Video Speed Class”. This rating is designated by the letter “V” followed by a two digit number currently ranging from v30 to v90. Like the previous speed classes a rating correlates to a speed in MB/Sec. v90 corresponding to a transfer rate of 90/MB/Sec. The interesting about V-rating however is that unlike it’s predecessors it signifies a sustained transfer rate. This is important for video because in video if you dip below your necessary transfer rate you may experience loss of data whereas if your shooting stills your camera will most likely just bog down as it buffers. In essence if your shooting stills your camera will stop letting you take new images until the card catches up; something that it can’t do for video. Bottom line, if your a video shooter it’s good to know the transfer rates you need for your desired video mode and buy your card accordingly. Though the “V” rating is meant for video shooters it’s also a handy benchmark if your only shooting stills.

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Confused yet? Lets get a simple one out of the way; one you probably already knew. Every card will have a number on it followed by MB. MB stands for Megabytes or 1 million bytes. Simply put larger numbers mean higher capacity so again, know your camera and shooting needs and buy accordingly. As I am writing this 32MB to 64MB is usually a sweet spot both for price and capacity needed for todays cameras. For video, especially 4K video or ultra high megapixel cameras aim a little higher.

Although we’ve already used the term Ultra High Speed, we’re going to use it again in a different way. Pictured below are two SD card pin configurations.

If you flip over a modern SD card they will look like the image above and they are referred as Ultra High Speed I or II. Once again these correlate to a theoretical max transfer rate. UHS-I has a theoretical max of 104MB/s though you will most likely see that the fastest UHS-I cards will claim 95MB/s and will deliver a real world speed somewhere far below that. UHS-II card have a theoretical max write speed of of 312MB/s. The fastest of the UHS-II cards will advertise speeds around 299MB/s but will again deliver far less. Unlike the V-ratings we already discussed the UHS ratings are not sustained and will in fact often peak and dip within a given transfer.

The take away once again is to know your camera and your needs because although UHS-I and II are cross compatible the speeds are dependent on your equipment weather that is your camera or card reader. One last thing I will mention is that there are rumored to be a UHS-III cards under development but as of now none have seemed to hit the market.

One important thing to note when shopping for a memory card is “write” speed vs. “read” speed. Companies often will list a single speed (the faster one) hoping that you will not distinguish between the two. This is because the vast majority of cards and memory storage devices in general will “read” faster than they will “write”. Knowing only the “read” speed of a given card may give you a false impression on how fast a card actually is. In the higher end cards the discrepancy between read and write speeds will be diminished but will still be there. If a card is only showing you a single speed there is reason to be skeptical. Bottom line is do not buy a card unless you know how fast it writes.

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We’re in the home stretch now. As far as I know this designation will only be vital to you Sony video shooters. For some reason every Sony camera as far as I can remember will only shoot 4K to an XC or extended capacity cards. Cards above 64GB will be the ones designated as XC and will be the ones required for 4K shooting on a Sony camera. Those below 32GB will be designated HC for high capacity and will only work for 1080p and below. It could be that Sony simply wanted to force you to use a larger card because 4K takes up so much space but more like it’s something technical and complicated having something to do with max file sizes and file allocation tables etc. *shrugs*

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If you are familiar with Lexar cards you may have notice they have a designation which is basically a larger number, usually in the 100’s followed by an “X”. This may be the most archaic designation of them all and the main reason I saved it for the end. But if your still reading then basically you asked for it.

In the above case the 633x means that this card is six hounded and thirty three times faster than an original CD-ROM computer disk / drive. Yep…

That about summs it up.