The target is inclined about 20 degrees to the central ray of the x-ray beam; this causes the effective focal spot to be approximately 1 mm × 1 mm, as opposed to the actual focal spot, which is about 1 mm × 3 mm.  This smaller effective focal spot results in a small apparent source of x rays and an increase in the sharpness of the image, with a larger actual focal spot size to improve heat dissipation. Ref: Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 6