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ASA

Summary

ASA

ASA

ASA is the rating of sensitivity to light  for chemical based, emulsion film stocks used in motion picture film cameras. 

Higher numbers represent film emulsion that is more sensitive to light. Lower numbers represent less sensitive emulsion. 

The most common average range of ASA sensitivity used are 100, 200, 400, 800. 

Description: 

ASA-ISO

In the ever increasingly rare situation of actual celluloid based film running through a film camera, each roll of film or Film Stock, has a specific chemical make-up. This chemical specification dictates the film roll's properties of sensitivity to light levels. In the days of film, the spec of ASA was used to specifically describe the light sensitivity range of the film stock. The ASA rating was also referred to as the "Film Speed". 

In the early days of film, there were very low ASA number ratings for film such as 25 or 50 ASA. This meant that a lot of light was needed to properly expose an image on these early film stocks. This was fine when shooting outside, but any interior shots would require an extremely large amount of lights for the camera to be be able to properly expose an image. As technologies improved and film stock began to gain higher and higher ASA ratings, many film stocks could be used with lower light levels because the ASA ratings were more in the ranges of 200, 400 and 800. The earlier ASA ratings of 50, 100 and 200 were then considered "Slow" film stocks, whereas the newer batches of 400, 500 and 800 ASA were considered "Fast" film stocks. The 'slow' and 'fast' deliniations of film stocks are a bit confusing when you refer to what the nickname means. Slow and Fast are better descriptors for the shutter angle of the camera as the shutter is what controls the amount of Motion Blur that the camera sees as it is running. The Slow and Fast description of film speed really only refers to the fact that a 'slow film stock' is less sensitive to light that a 'fast film stock'.

ASA ratings were and still are available at much higher numbers, but the general rule of thumb with film was that the higher you went in ASA rating, the more grain was introduced to the images that you would capture. This is a similar phenomenon when compared to the digital world of cameras with a video "Gain" setting. As the gain is increased, similar to higher ASA ratings, more noise (graininess) is introduced into the image.