In endodontics, a long cone open-ended cylinder is used as an adjunct with paralleling technique in order to:
Image size distortion (magnification) is the increase in size of the image on the radiograph compared with the actual size of the object. The divergent paths of photons in an X-ray beam cause enlargement of the image on a radiograph. Image size distortion results from the relative distances of the focal spot-to-image receptor and object-to-image receptor. Increasing the focal spot-to-image receptor distance and decreasing the object-to-image receptor distance minimizes image magnification.  
Key Point:
The use of a long, open-ended cylinder as an aiming device on an X-ray machine thus reduces the magnification of image on a periapical view. As previously mentioned, this technique also improves image sharpness by increasing the distance between the focal spot and the object.
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 84
Increase the apparent size of the focal spot
To prevent distortion of image
To prevent elongation of the image
To prevent shortening of image