If the central ray is perpendicular to the film but not to the object, then:
Image shape distortion is minimized when the long axes of the image receptor and tooth are parallel. 
Figure A shows that the central ray of the X-ray beam is perpendicular to the image receptor, but the object is not parallel to the image receptor. The resultant image is distorted because of the unequal distances of the various parts of the object from the image receptor. This type of shape distortion is called foreshortening, because it causes the radiographic image to be shorter than the object. 
Figure B shows the situation when the X-ray beam is oriented at right angles to the object, but not to the image receptor. This results in elongation, with the object appearing longer on the image receptor than its actual length.
Elongation of image occurs
Foreshortening of the image occurs
Blurring of the image occurs
None