The Different Methods Of Cleaning Memory Cards
Copyright Robert S. Donovan
"Just delete the photos you don't want," one friend suggests when you ask what to do when you've finished downloading a memory card to your computer.
"No, no. You want to format it in the camera to be safe," chimes in another
And still a third friend offers, "What's best is low level formatting, if you camera offers it."
For a lot of people starting out in digital photography, all these bits of advice can seem both conflicting and confusing. What is low level and why is it better? What happens if I just delete? And will anyone make fun of me if I do the `wrong' thing?
First, let me belay the last question. We are all here to learn and at some point or another everyone faces this question. So don't sweat not knowing because this post will help set the record straight on how each method works. It's my hope you will then be able to speak intelligently about the different methods and use that knowledge for the greater good of digital photographers everywhere.
For the techies out there, I suggest you look away from this next part. I'm going to over simplify things a bit in order to make sure the basics of data storage are understood. I'm not going to get into bits and bytes and instead try to make this post accessible for all. And for the sake of argument, we'll assume all digital cameras function essentially the same when it comes to card formatting, etc… while admitting different models and brands do things ever so slightly differently. That's not really in the scope of this post either.
Square One
Let's start with some basics and use an analogy. Imagine your memory card is a book. A real simple book with only a table of contents and the pages. Maybe it's 50 pages of Table Of Contents (TOC) and another 450 of actual story, so 500 pages in all. Now then, when you take a picture, essentially what the camera does is write all the information of the picture (all the information from the camera's sensor) onto a page in the book and then notes that page in the TOC. Pretty simple. The camera writes out a page that describes a flower in massive detail and then writes in the TOC that page 342 contains "flower, shot on March 12th, etc…"
You finish shooting for the day. You've downloaded all the information from the card to your computer and are prepared to take more pictures. Here's where the different methods come into play.
Deleting Photos
When you delete photos you are, more or less, going into the TOC and erasing the entry for "flower, shot on March 12th, etc…". This then lets the camera know that page 342 is available to be written over. Either one at a time, or all at once, you're only affecting the TOC, not the actual pages when you delete. On page 342 there still exists a massively detailed description of a flower, but your camera only knows what's on each page by looking at the TOC, NOT the individual pages. As far as it knows, there is nothing on page 342 and it can reuse that page
Basic Or High Level Format
Until the last couple of years this was the only type of formatting available on cameras. And you might not have even know it because it was only called "Format". This was the quick way to delete a whole card and start fresh. Just like the Deleting option above, it wipes clean the TOC only but does it all at once. Nice and efficient and according to a camera looking ONLY at the TOC, you have a completely clean book on which to write. (NOTE: in these examples the camera will actually take a page from the book and, when it's ready to write a new picture, erase each letter, one at a time while it writes in the new information. We'll get to why this is important in a bit.) Again, it's only a TOC function, not a book function when formatting a card at high level.
Low Level Format
Now we have the real destructor. A Low Level Format not only wipes out the TOC but it will go through and erases each and every letter on each and every page and types in little zeros in all those spots. What you end up in the end is a book with no TOC and zeros everywhere. There's no way to find out what was there beforehand.
What Does This All Mean?
What this all means is if you want to reclaim the maximum amount of space and have the cleanest card, use a Low Level format if your camera can do it.
If not, be aware that your camera will be constantly writing to pages with text already on them, sometimes taking more than a page to write a single picture. And if you only delete some pictures and then start shooting again, you may be telling the camera it's ok to use pages 342, 355 and 398. The camera might then use all of page 342 and then some of 355 for the next shot. This is the classic definition of fragmentation and it can slow performance over time (although less so with solid state media like memory cards than it can with moving hard drives). The next picture you shoot then straddles page 355 where the last image left off and part of 398. And on and on and on.
Go back and delete that first picture and the camera now thinks all of 342 and part of 355 is available, while the next picture taken may only use part of that. Confused? This is why a fragmented hard drive on your main computer runs slower because of having to `think' and hunt for all those little bits and pieces that aren't all written on consecutive pages, but instead scattered depending on where available pages popped up when it was time to write information.
The method of deleting photos or running a High Level Format does have one advantage though: it's (somewhat) undoable. Meaning, if you accidentally ran a High Level Format on your card and, GASP!, realized you didn't want to, chances are all those pictures are still there on all those page. If you followed along above, all those descriptions of what the camera saw with each picture are, in fact, still in the book, there's just no Table Of Contents to help the camera (or home computer) find them. This is where photo recovery software comes in.
Photo recover software works on the principle that all the info is there, but you need to read the entire book in order to write out a new TOC. And that's what it does. While not perfect, photo recovery software will read that original page 342, understand where the description of the flower starts and stops and then present you with the image, letting you decide to keep it or recover it. Most programs do not rewrite the TOC for you and instead keep a virtual copy resident as long as the program runs.
And that concludes today's over simplified analogy lesson of how data is stored and destroyed on flash memory cards. I'm not here to tell you which is best (ok, I kind of did, just a little) but hope the info in this post will help you make an informed decision in the future! What works best for you is completely up to you.
