Why does my Digital camera have different sensitivity settings, and how can I use them effectively?

ISO sensitivity expresses the speed of photographic film materials formerly ASA (American Standards Association) or commonly known as the film or digital sensor speeds. These range from 25, 64, 100, 200, 400, 800 - 3200 upwards.
The film or digital sensor speed affects the shutter speed/aperture combinations that you use to obtain correct exposure.

Film or digital sensor speed is a number that represents it's sensitively to light. The higher the number the more sensitive to light, in that the less light is needed to take a well exposed photo. The number is also an indicator as to the detail you will see in the final image. However, all this increase in sensitivity does not come free. There is a price to pay with your image appearing more graininess/noisy.

In a digital camera the size of the image sensor determines the ISO speed range that a digital camera can use without suffering from undue noise. A smaller sensor will exhibit more noise at higher ISO's than a large one. The bigger the sensor the better the end result.

Suggested ISO settings.

AUTO ISO – digital camera automatically sets the ISO speed according the the brightness of the scene, increasing or decreasing the sensitivity. User has no control over which ISO number is used.

ISO 80 – for taking photos in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality.

ISO 100 – for extra sensitivity with little, if any, reduced image quality.

ISO 200 – cloudy and overcast days. Acceptable image quality, with some visible noise.

ISO 400 – suitable for indoor photography whether or not a flash is used. Useful for “stop-action” and sports photographs. Most compact digital cameras produce high to very high image noise.

ISO 800, 1600 and above – useful for taking photos in very low light, or outside in good light when increased shutter speeds are required. Results can be disappointing when shooting at these high numbers with compact digital cameras, so take test photos before photographing an important event.

Most of the time you'll only need a 200 or 400 ASA speed film for basic snapshots. But it doesn't hurt to use the other speeds for special occasions, you'll notice a difference.

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