Restricted amount of light
_DSC4860, originally uploaded by shutterhack.
Taken with a Nikon D50 and AF Zoom-Nikkor 70-300mm f/4-5.6G lens
ISO stands for 'International Organization for Standardization' and their film speed ratings are used to indicate the relative amount of light necessary to give a proper exposure. A normal film will be rated at ISO 100. A film rated at ISO 200 will give a proper exposure with only half the amount of light compared to the ISO 100 film, enabling you to shoot in lower light or with a smaller aperture or faster shutter speed. The ISO 200 film would be referred to as a 'faster' film. There are films available that range in speed from ISO 25 to ISO 1600.
So why not use the faster films all the time, what are the advantages of slower films?
The faster films have a more prominent grain structure the individual grains clump together to form spots that are visible to the naked eye, especially when you blow the photo up to A4 or larger from a 35mm negative. In certain circumstances this effect can be used creatively especially in black and white photography but mostly it is undesirable.
How does all this effect digital cameras?
In the digital photography world the phenomenon is called 'noise' not 'grain', the cause of the problem is slightly different. When light levels are low, the sensor has trouble reading the scene properly and pixels of random colour are thrown into the picture. However to us photographers the end result is the same or very similar.
The 'better' digital cameras have, usually hidden away among the manual settings, a sort of simulation of the film speed effect. My camera for instance, the Nikon D50, has an ISO range from 200 to 1600. This feature is not available when you are in fully auto mode but is available in all the other modes. I sometimes keep it set to ISO 400 as the grain structure is not significantly worse than ISO 200 and it gives me that extra f-stop to play with. When I am shooting fast action and I want to freeze the action (not always the case - see shutter speeds and apertures) then I'll select ISO 400 or 800. If the light is very bad ie night time or indoors then a shot at ISO 1600 is often better than a blurred shot caused by using too slow a shutter speed or no shot at all.
Labels: AF Zoom-Nikkor 70-300mm f/4-5.6G, Nikon D50, Photography
