Understanding ISO Sensitivity


Photography is built on the three pillars of exposure: shutter speed, aperture and sensitivity.

Shutter and aperture are controls for adjusting how much light comes into the camera.

How much light is needed is determined by the sensitivity of the medium used.

That was as true for glass plates as it is for film and now digital sensors.

Over the years that sensitivity has been expressed in various ways, most recently as ASA and now ISO.


The “normal” range of ISO is about 200 to 1600. With today’s digital cameras you can sometimes go as low as 50 or as high as 204,800.

The number chosen has two important qualities associated with it.

First, it sets the amount of light needed for a good exposure.

The lower the number, the more light required.

The more light that’s required, the more likely a slow shutter speed will have to be used.

That means low ISOs, like 100 or 200, are most often used in bright situations (like sunlight) or when the camera is mounted on a tripod.

If you don’t have a lot of light, or need a fast shutter speed, you would probably raise the ISO.


Understanding ISO Sensitivity

Understanding ISO Sensitivity


Each time you double the ISO (for example, from 200 to 400), the camera needs only half as much light for the same exposure.

So if you had a shutter speed of 1/250 at 200 ISO, going to 400 ISO would let you get the same exposure at 1/500 second (providing the aperture remains unchanged).

This is why high ISOs are so often used indoors, especially at sporting events.

Needing a fast shutter speed to stop action, photographers regularly choose ISO 1600 or above.

To read the rest of the artical please go here: Understanding ISO


Understanding ISO Sensitivity