The sensitivity of a digital sensor works in a similar way to film ISO. With a film camera when it became necessary to shoot in low light without flash, the photographer purchased a film with a high ISO factor - usually 400 or 800. The problem was that everything regardless of light intensity had to be taken with the high speed film.

With a digital camera the ISO sensitivity can be adjusted just for the shots that require a higher value. Usually a sensor is working at its optimum best at 100 or 200 ISO, but a low light shot such as a church interior, would require a higher value of at least 400 for hand holding without camera shake.

The ISO values usually commence at 50 or 100 with each increment doubling in value - 200, 400, 800, 1600 and so on. At each stage the sensitivity of the CCD is doubled, so if at 100 (and with a constant aperture value) the shutter-speed is 1/8 second, at 800 it will be 1/60 second. In many cameras, particularly compacts, the ISO can be set to Auto, which should adjust automatically for differences in light intensity.

There is a possibility of a visual imperfection to the image called Noise when using higher ISO values. Recent improvements in digital technology has lessened this problem, but increasing the ISO value of the sensor should only be implemented when necessary.

Note: ISO Sensitivity should not be confused with EV Compensation.