Have you ever got to a location, started shooting and realised after a few shots that you hadn't formatted the memory card and it still has images you've already downloaded? If you're short on memory cards (and if your using an EOS-1Ds Mark III or EOS 5D Mark II then you're always short of memory cards!) it can be a real pain as you realise you need to go through and delete each image one by one. However, on the EOS-1Series cameras, there is a solution: File protection.

Below the LCD screen is a button you've almost certainly never pressed - the protect/sound button. This button allows you to stop certain images on the card being erased, or record a short (up to 30secs) clip of sound with an image. 

So how do you use it? Simply scroll through the images in playback mode and press the protect button at each of the images you've just taken. Now go to erase settings and choose "Erase all". This will delete any image that is not marked for protection leaving you with a memory card that  contains the images you've just taken. When an image is protected, you will see a small key on the display for each protected image.

If you're on a camera other than the EOS-1Series models, then you don't have the protect function on a button. However, you do have a protect function in the menu system. Simply find protect in the menu (it is in one of the blue tabs) and then scroll through each image pressing the SET button when you get to an image you want to protect. 

There is one thing you need to remember about this though: it will not stop images being deleted if you FORMAT the memory card. Formatting will clear everything off the card including the protected images, so if you only want to get rid of those images you've not protected, make sure you use "Delete".