There is quite a lot of hoohah about crop factor but I’m not going to get into all that at the moment – just a brief description of what it is!
Simply, crop factor is the ratio of sensor size to a reference 35mm (or full frame) sensor which measures 36mm x 24mm.
|By MarcusGR [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons|
So the graphic above shows that the reference full frame sensor is considered to be a crop factor of 1. Any sensor bigger than this will have a crop factor less than one, and any sensor smaller than the full frame reference will have a crop factor greater than one.