ISO

                                       

                                                                                                                                                                                                                                                                                                                                                                                                                                   

ISO is how sensitive a film is to the light.ISO is measured in numbers like on films that have 100,400,800, all of this specifically define ISO.The lower the number the sensitivity of the film and the finer the thing in the shot you're  taking.In digital photography ISO measures the sensitivity of the cameras sensor. The higher the ISO settings are generally used in the darker situations to get faster shutter speed.100 ISO is the regular shot because it will give you less l noise/grain.