An image processing apparatus comprises a registration unit configured to perform a first registration between a first set of image data and a second set of data, and a second registration between the first set of image data and the second set of data. The apparatus also comprises a registrability determination unit configured to determine a difference between the first registration and the second registration, and to determine a measure of registrability between the first set of image data and the second set of data based on the difference between the first registration and the second registration.