ISO

ISO is similar to film sensitivity back in the days before digital. The higher the ISO number the more sensitive the sensor becomes. Usually the ISO starts at 100 and can go up to 64,000. As the ISO number becomes higher it becomes more sensitive to light. Increasing the ISO is great when you are in low light. However increasing the sensitivity comes at a cost to image quality.

Think of ISO settings as the volume knob on a cheap stereo system. The more the stereo volume gets cranked the more crackly and distorted the sound gets. That is basically what is happening to the sensor in the dSLR camera. As the the ISOs are cranked up the image becomes more noisy and grainy.

The picture was taken at ISO 1600. Look at the zoomed area. What should be a normal solid colour background is full of noise and colour speckles.

However that above shot is an extreme example. Most photographers will keep the ISO between 100-800. 100 is for sunny days and 800 for low light.

Mainly, adjusting ISO is when you have a desired aperture or shutter speed but you need to either increase or decrease the exposure of the shot.

Look at these 3 examples. Each shot had identical aperture and shutter speed values in the same room with the same lighting conditions. The only thing that was changed was the ISO settings.

Do you notice how the picture seems to get brighter as your ISOs are increased. Again, this is because the sensitivity of the sensor is being increased making it more sensitive to the same lighting situation.