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How It Works - Memory Cards

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One of the most common questions I get asked, usually immediately after “what camera should I buy?” is “how many pictures can I take with it?” Assuming I don’t just punch the questioner straight in the throat, it’s remotely possible that I may actually attempt to explain about memory cards, file sizes and data compression. However to save myself the bother, here’s quick look at memory cards, where they came from, what they look like and what they do.

Over the past five years or so, the maximum resolution of digital cameras has grown from 3 or 4 megapixels to 10 or 12 megapixels, even more if digital SLRs are included. Canon’s EOS 1Ds Mark II weighs in at an amazing 16.7 megapixels, and there’s no reason to suppose that even larger sensors won’t appear in the near future. One side-effect of this ever-increasing power is the growing size of the image files that they produce and need to store. RAW files from the EOS 1Ds Mk II are over 11MB each (converting to 50MB TIFF files), and even JPEG files produced by normal compact cameras are frequently well over 3MB. Clearly storing more than a handful of pictures needs a substantial amount of storage capacity.



The solution used by virtually all digital cameras (and many other modern digital media devices) is of course the removable memory card. Although there are several different types of card in common use, they all work in much the same way. They contain chips of a type of computer memory called Flash memory, a technology first developed by Toshiba in the early 1980s. The main advantage with Flash memory is that it can store data for very long periods without being connected to a power supply (it is non-volatile). In fact modern memory cards can potentially store data for as long as 10 years without degradation (compare this with the durability of exposed film, which is often no more than a year). This means that a memory card can be removed from the camera when full and replaced with an empty one.



The other big advantage of Flash memory is that it is reusable. Once you’ve copied the image files from your memory card onto a computer or other permanent storage device, the contents of the card can be deleted and the card can be used again.

As if that weren’t enough, Flash memory cards are also incredibly durable. There are many stories of cameras and other devices being damaged or destroyed, but the pictures stored on the memory card surviving unharmed. I have personally accidentally run an SD card full of photographs through a full cycle in a washing machine (it was in a trouser pocket) with no ill effects.

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