Since 2015, we’re in transition to faster, UHS-II SD memory cards from UHS-I cards. The new generation of cards has a second row of contacts enabling speeds that are twice as fast as UHS-I. But the camera and card reader must have the second row of contacts and associated components to deliver the faster speeds. UHS-II SD cards in a camera supporting UHS-II cards will have faster write speeds, but if the camera only supports UHS-I cards, it will use UHS-II cards in compatibility mode and have speeds at one half to one fourth the speed possible with UHS-II.
In general, a fast UHS-II card is going to be twice the speed of the fastest UHS-I SD card, but because SD UHS-I cards have been around a long time, there are a lot of configurations. Some of them are much slower depending on the components of the card. UHS-I cards are generally going to be cheaper than UHS-II cards.
Don’t confuse the published card speed with the actual speed in the camera. The actual write speed in the camera is usually 30-40% slower or more than the benchmark speed on the card. That means the fastest UHS-I card will probably have a write speed of around 70 MB/s while a fast UHS-II card has a speed of 140+ MB/s. These speeds have a big impact on the length of your maximum burst or series of bursts. This applies if you use the second slot regularly such as shooting RAW + JPEG or shooting video.
On the other hand, you can save money with a less expensive card in the second slot for overflow. So you might get a smaller UHS-II, or even a fast UHS-I card for the second slot. My approach is to buy a fast new card with any new camera, and then use my fastest older card as backup, but it depends on how current your older cards are. They do become more likely to fail over time, so after 5-6 years I just put the old cards in each camera bag for emergency use.
Right now I tend to stick with Sandisk, Sony, ProGrade Digital, or Delkin UHS-II SD cards. Lexar cards are okay but they are now owned by Longsys in China and service can be a challenge. ProGrade is the former Lexar management team.
When looking at a new card, I generally want a card size that handles a normal, high volume day of shooting. For me, that is 64 GB with a 24 megapixel camera if I shoot wildlife, and 32 GB if wildlife is occasional or uncommon. If you shoot a lot of video, or still photography with bracketing or a focus shift series of images, you might need larger cards. With a high resolution camera, I use a 128 GB card as my primary card.
Be careful of buying on Amazon or eBay. You may see counterfeits or older cards and it’s hard to tell the difference. Amazon cards that are sold by Amazon rather than third parties are normally fine.
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