The buffer is a storage where imaged are stored after (or before, depending on the system) they cross the image processor (e.g DigicII in my 20D) and before they are stored in a storage card. Comparable to a RAM-chip in the computer, it saves the picture before it will be written on the actual storage. This is useful for burst pictures, where many pictures need to be buffered, because they can’t be saved on the card immediately.

Depending on the size of the buffer and the processor, you can shoot more or less pictures in a burst (e.g: 5images/second on my 20D with a max. of 6 Raw / 28 JPG images)

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