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