Which combination will result in the lowest absorbed dose from a 20-film series of dental radiographs?

The choice of rectangular collimation combined with F-speed film is optimal for achieving the lowest absorbed dose in a series of dental radiographs. Rectangular collimation produces a more focused beam of radiation that limits exposure to the area being imaged, thereby reducing the amount of scatter radiation that could contribute to the overall dose. This targeted approach minimizes unnecessary radiation to surrounding tissues.

F-speed film is designed to be more sensitive to radiation compared to other film types, such as D-speed film. This higher sensitivity means that F-speed film requires less exposure to radiation to produce a diagnostic-quality image. By using this film in conjunction with rectangular collimation, the total radiation exposure is significantly lessened when compared to other combinations that might either not optimize beam width or require a longer exposure time.

In summary, the combination of rectangular collimation and F-speed film effectively lowers the absorbed dose by focusing the beam more precisely on the target area and necessitating less radiation exposure to achieve diagnostic images.