What is Film-Stock Speed?
Film speed represents the measurement of the film’s sensitivity to light. The system of measurement that is used to measure film stock speed was established by the American Standards Association (ASA). And represents the basis for the International Organization for Standardization (ISO). All film stock is assigned a value corresponding to its “speed”. Which is the basis of ISO? But exactly what is film-stock speed and how is it determined?
What is Film-Stock Speed?
Film stock speed represents the measurement of a film’s sensitivity to light. Measured in terms of ISO, all film-stock is represented by an ISO number. That indicates the sensitivity of that film to the light which enters the camera.
The concept of film speed may be challenging at first. But essentially film-stock speed represents the amount of light that the film can withstand without impacting quality and exposure.
Affects Quality
The “Speed” of the film stock directly affects the quality of the image that is captured. Film stock speeds will range from as low as 20 all the way up to 3200.
Film stock speed is part of a doubling system in which each of the film speed values is rated and doubled. Thus a a film stock speed of 400 is twice as fast as one that is 200.
Slow Film Stock vs Fast Film Stock
A lower ISO number, such as 20, is considered a slow film stock. Likewise, a higher ISO number, such as 3200, is considered a fast film stock. Slow film stocks are less sensitive to light whereas higher film stocks are more sensitive to light.
The ISO number or film-stock speed lets you, or the camera, know how to adjust aperture and shutter speed in order to maintain the best exposure.
Light Sensitivity
Fast film stock is incredibly sensitive to light, and produces distinct contrast between light and dark images within the frame.
Filmmakers that are shooting documentaries. Especially in situations where the addition of light is limited for the shoot. Often used fast film stock rather than slow stock which is less likely to result in the best images in lower light situations.
Film-Stock Speed & Exposure
The speed of the film indicates the amount of light that is required for the film to render the best exposure. Together, film-stock speed works with the aperture (controls light coming into the lens) and the shutter speed (controls how much light hits the film) to determine exposure.
Lower speed films also produce less grain than high speed films, especially when used on bright days. Likewise, high speed films or fast films will have noticeable fine grains, but can be used in very low light conditions.
Box Speed
When filmmakers choose to use the same ISO setting found on the film box, they are said to be at “box speed.”
When addressing film speed in this manner, it simply means that the filmmaker is using the recommended ISO settings for the film that they are using. Thus they’re shooting with the same ISO that is listed on the box.
So, what is film-stock speed? It’s the ISO numbering convention that dictates the film’s sensitivity to light in ISO terms representing the amount of light that the film can withstand while producing the most suitable exposure.