Final answer:
Negative angulation results in elongation of the X-ray image by directing the beam downward in relation to the object being imaged, which is a critical consideration in fields like dental radiography.
Step-by-step explanation:
The vertical angulation that results in elongation is negative angulation. When the X-ray beam is angled negatively (downward) relative to the object being imaged, the resulting image appears to be lengthened or elongated, which is not an accurate representation of the object's actual dimensions. It's critical to adjust the vertical angulation correctly to prevent such distortion in the resulting image. For instance, in dental radiography, precise angulation is key to obtaining diagnostic images. If the angle of elevation (A) is set negatively above the horizontal line, an elongated image is produced. Conversely, for shortening the image, positive angulation is used where the X-ray beam is directed upward.