Why slow film is used instead of Fast film in Radiography

[purode333]

Dear sirs,

Why slow film is used instead of Fast film in Radiography.

i.e. WE commonly used D4 instead of D7.

But fast film having course grain gives less exposure time, which is beneficial to us in project or duration, point of view.

Then, why we preferred slow film, which will take more exposure time.

Pls help/provide any technical or genuine or logical reason , behind to consider or finalized this.

Regards
Prasad

 

[weldstan]

Fine grain, slow film provides greater sensitivity and contrast to that will provide a greater ability to find pertinent discontinuities.

 

[purode333]

Is it , given in any code , i.e. film verses thickness chart.