Fast film is best for shooting in low-light situations or stopping motion in sports.
In photography, ASA and ISO are both measurements of film speed, or sensitivity to light. ASA is a scale created by the American Standards Association, but it is no longer widely used. Now, most film is labeled by ISO, which was created in 1987 by the International Organization for Standardization. There is no difference in the film itself.
Mathematical Basis
There have been many systems of measuring film speed, but two of the major ones before 1987 were ASA and DIN (created by the German Institute for Standardization). ASA was an arithmetic scale, meaning that doubling the sensitivity of the film also doubled its ASA number. DIN was a logarithmic scale, so doubling the sensitivity added 3 degrees to the DIN number. ISO basically combines these two methods into a single standard, so there is both an arithmetic and logarithmic scale within the ISO system. The ISO arithmetic scale, based on the ASA system, is more commonly used.
Choosing a Film Speed
Fast film, labeled with a high ISO or ASA number, like 1600, is very sensitive to light, so it allows you to shoot with faster shutter speeds or at higher f-stops. This is particularly useful in low-light situations. The drawback is that images shot with faster film often turn out grainy and with higher contrast. Slow film, labeled with a low ISO or ASA number, like 100, is less sensitive to light, so it requires you to shoot with slower shutter speeds or at lower f-stops. It also produces higher-quality photos. The same goes for higher ISO/ASA settings on digital cameras.
Conversion
There is no conversion needed between ASA and ISO numbers. ASA 100 is equal to ISO 100. If you need to convert between DIN and ISO/ASA, it is easiest to refer to a conversion chart.
Use In Film Photography
In film photography, ASA or ISO numbers help photographers select the appropriate type of film. The number appears on the box and on the film roll itself. On many cameras, there is a dial that should be changed to match the speed of the film you are using.
Use in Digital Photography
Digital cameras use an ISO setting too, but instead of indicating film speed, it controls the sensitivity of camera's image sensor to light. The same rules apply to digital ISO as film. It is rare for a digital camera to use ASA because the ISO standard replaced it before Kodak released the first consumer digital camera in 1991.
Tags: film labeled, film speed, arithmetic scale, digital camera, doubling sensitivity
