Digital camera media, or any other computer storage, tries to store files contiguously, in one chunk.
However, it is not always possible.
As files are written and deleted from the media, free space gets fragmented,
and eventually there is no single free gap large enough to fit the entire file in.
Once this happens, subsequent files are stored in several non-adjacent chunks.
This is called fragmentation.
Since 2002 or so, ZAR uses the simple header-footer algorithm to do its digital image recovery.
This algorithm only recovers contigouos files; fragmented files are only recovered partially or come out distorted.
Fragmented image files come out distorted or missing their bottom parts.
Fragmented video files typically won't open at all.
How big is the problem? On a typical digital camera media, 10% to 20% of image files are fragmented,
and about half of videos are fragmented. Media after some heavy use, like in dash cam recorder, may well have all
video files fragmented.
To deal with that, I have developed a standalone software, which is by now more complex than ZAR.
ZAR is still free for image recovery, and will still recover 80% to 90% of the image files for you.
However, if this is not enough, or if you have a problem with fragmented videos, you can try my other
(paid, not free) product, Klennet Carver.