What is ISO in Digital Photography ?

In traditional Photography, ISO was meant for the sensitivity of the photographic sensor.

In Digital photography ISO means the same however the photographic sensor is inbuilt and you need not change it anytime. If your sensor is more sensitive, it means you will have have more light imprints and hence an effect called Grains develop in the photograph.

To understand this assume a picture being made up of large number of pixels. Now suppose there be another picture with fewer number of pixels. This would make the other image looks grainy. Another simplified example of ISO is like assume a picture imprinted on a rubber balloon. Now if we fill in the balloon with a fluid, it inflates out. Now if you look at the imprint on the balloon, its a little bit fragmented into multiple pieces. This again is called grainy look. Similarly with an ISO. An ISO setting is a setting for the photographic sensor inside your camera measuring its sensitivity to light. A higher ISO means more grainy picture and a lower ISO means a less grainy and hence finer picture.

Image at 100-ISO and 3200-ISO (note: there are grains in the 3200-ISO image on the right)

Personally, while taking a print till ISO 400, it won't matter much (unless you want to take a really large print). Moreover by adjusting ISO you can have pictures even at low lights (may be a sunset scene or a person's image in dark). Another important thing about ISO is that it gives you an ability to increase your shutter speed without compromising much with the quality. With higher ISO, you can achieve higher shutter speeds. Do not worry if you are not understanding this concept right now, just move on to Shutter Speed section. We will now move to the Aperture in Digital Photography section.

A short, simple advised reference :

ISO 50-100. Suitable for bright light (like outdoors on a sunny day).

ISO 400 and 800. Use these values when the light is getting dim but it is not yet night.

ISO 1600 and above. Use for indoor or night shots. Also useful to freeze the action in sports shots. These values will produce the most noise.

For further explanation regarding anything related to ISO, its settings, types and any practical problems, just leave a comment below and i will try my best to elaborate it here itself.

Cheers,

Kumar