What is A male 35 year old presents to the clinic with blackening and pain in upper anterior tooth. He also told that trauma to the tooth occurred 5 months ago. Intraoral findings reveal non-vital tooth #21. RCT was started and IOPA #21 taken for initial working length. After initial debridement of the canal was performed, another X-ray was captured with the initial root length measured and it appears that the ﬁle is approximately 4 mm from the apex. How could the doctor have obtained a more accurate initial length measurement? and explain in detail?
In this case image foreshortening is there.
Diagnostically unacceptable image distortion may require retaking the radiograph  with a change in vertical angulation. The pattern of distortion should guide the radiographer to select an appropriate angle. Image foreshortening indicates that the vertical angulation is too high. In contrast, image elongation indicates insufficient vertical angulation.
 
Changing the horizontal angulation of the cone in relation to the ﬁlm will not change the length of the image.
 
Reference: ORAL RADIOLOGY PRINCIPLES AND INTERPRETATION,8th EDITION, Stuart C. White,Michael J. Pharoah