Do you ever wonder why some SD cards perform better than others? Or why newer SD cards are slower than older ones? Well, you're at the right place :) Let's get started.
When SDHC was first introduced, they brought upon a new Standard of measuring SD Card speeds: the Class specification. There are (now) 4 Speeds available.
The Class specification only indicates a minimum sequential write speed for the card.
The Class speeds are as follows:
Class 2 = 2mb/s minimum sequential write
Class 4 = 4mb/s minimum sequential write
Class 6 = 6mb/s minimum sequential write
Class 10 = 10mb/s minimum sequential write
There is currently no Class speed rating to measure sequential reading of the card.
Class 10 came at a later time. However, there were plenty of Class 6 cards that performed well enough to exceed Class 10 no problem.
In the past, SD card manufacturers typically overrated their cards. It was common to see a Class 2 get 6mb/s, a Class 4 to get 10mb/s and a Class 6 to get 15mb/s.
However, it seems that manufacturers are only trying to achieve the minimum requirements to achieve their class rating.
Example:
I had a Kingston 8gb SDHC Class 4 that performed @ 15mb/s
Recently I purchased a Kingston 8gb SDHC Class 4. It's performance is 4mb/s.
Should I be angry? The rational side of me says no. However, the demanding side says yes!
But there is no such thing as "I have a Class 4 card, it should do at least 8mb/s". The Class ratings are there to show the absolute minimum, there is no guarantee of any extra performance.
So you should hope for the best, but expect the worse, when you purchase an SD card w/ that specific class rating ☺
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