What is ISO in photography?

ISO stands for International Standards Organization. How does that relate to cameras? ISO standardizes a lot of different industries, and photography as well.

It is used to standardize film sensitivity to light, in digital cameras a ISO equivalent is used. The lower the ISO value the less sensitive the film or image sensor is to light. The higher the value the more sensitive the film or image sensor is to light.

In cameras it affects the saperture and shutter speed setting combination to achieve a correct exposure.When light levels are low, the shutter speed will be slow, and that affects the images dramatically. So in order to increase the shutter speed you need to increase the ISO. With film you would need to change the roll to one with a higher ISO sensitivity. With digital cameras you just need to change the setting.

How does it affect the image quality? Well, with film it adds more grain to images, with higher sensitivities, it can add a mood to an image. With digital sensors it adds more noise, with higher sensitivities.

The bottom line is to use the lowest ISO setting possible in order to increase the image quality. Larger sensors produce less noise at higher sensitivities then smaller ones. So, a DSLR with a full frame sensor, such as Canon EOS 5D will produce a considerably higher quality image then a camera with a smaller image sensor.

Noise can be battled effectively with a help of software, such as Neat Image, so if you need to use high sensitivity, use it, its better to have the image to work on, then have no image.
One more way to avoid using high ISO sensitivities is to use a fast (wide aperture) lens, with a maximum aperture at least of f2,8 or faster on your DSLR. This will allow more light to reach the sensor, thus increase the shutter speed and you will be able to use a lower ISO setting.

In general, all new DSLR cameras have a very good noise performance up to ISO 400, above that results differ, with full frame models doing better. Point and shoot cameras perform poorly because of their tiny sensors and to much pixels crammed on them.
So, always try to keep the ISO setting as low as conditions allow and you will increase the quality of your images.