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What is The single most effective method of reducing patient
somatic exposure when taking radiograph is to use: and explain in detail?
Intraoral dental X-ray film is available in two speed groups: D and E/F.
Clinically, film of speed group E/F is approximately twice as fast (sensitive) as film of group D and thus requires only half the exposure. Fast films are desirable from the standpoint of exposure reduction. Multiple studies have found that E/F-speed film is preferred, because it has the same useful density range, latitude, contrast, and image quality as D-speed films and can be used in routine intraoral radiographic examinations without sacrifice of diagnostic information. 
Current digital sensors offer equal or greater dose savings than E/F-speed film and comparable diagnostic utility.
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 33