Shutter speeds and ASA/DIN ratings
Much like f-stops, the shutter speeds available on your camera will vary. The numbers listed are fractions of one second. Therefore 2 is 1/2 and 4 is 1/4, etc. You will notice that the shutter speeds increase or decrease by a factor of 2. When you increase your shutter speed by one increment you are decreasing the time the shutter is open by half. This means half of much light will reach the film.
When choosing your shutter speed for a particular shot you will want to consider how the speed will affect the appearance of the image. If the object is in motion you will obviously need a faster shutter speed so that is won't appear blurred but there is a tradeoff. When you open the shutter for less time not as much light will reach the film. To compensate for this you have to open your aperature to allow more light in during that time. This means your depth of field will become shallower.
Depending on the light available, you might have to find a happy medium between having lots of depth of field and preventing motion blur. If you have a low light situation combined with a subject in motion you might find this close to impossible. The only way around this is to either change one of these coniditions or change the speed of the film you are using.
ASA and DIN ratings
ASA and DIN are ratings given to film to show their speed. The higher the number, the faster the film and the less light it needs to get a correct exposure. These two systems are the most common. ASA speeds give an idea of the amount of light needed to expose properly. An ASA 200 film will only need half as much light as one rated ASA 100. A DIN rating (most common in Europe) adds 3 to the film rating to show a doubling of speed. These two rating are often combined into a speed indicator called and ISO. An ISO shows both of these ratings together seperated by a backslash (eg. ISO 100/20). To make things even more unnecessarily confusing sometimes the ISO just lists the ASA speed (eg. ISO 100). You might think that the best thing to do is just buy all high speed film. As always there is a tradeoff. Faster films have a much more visible grain at larger magnifications. They don't produce uniform greys as nicely and will appear to be made up of little specks in enlargements.
When loading a film be sure to set the camera and your light meter (if seperate from the camera) for the speed of film you are using.
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