Understand: sensitivity

Sensitivity is the last of three parameters that influence exposure. The first one is aperture that defines the amount of light that enters the lens. Then comes exposure time, which is the time during which the shutter remains open when taking a picture. These two parameters influence the amount of light that enters the camera to reach the photosensitive element (photographic film in analog cameras, or a digital sensor in digital cameras).

For given aperture and exposure time settings, the photosensitive element will detect more or less light depending on how sensitive it is. In other words, sensitivity is the measure of how sensitive the photosensitive element is to available light.

How to quantify sensitivity?

A photosensitive element's sensitivity to light is defined by sensitometry and measured on various numerical scales, the current international standard scale to measure light being the ISO system. Long story short, the higher ISO, the higher sensitivity. On most photographic films and digital cameras, common ISO stops are 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, etc. From each stop to the next or previous one, sensitivity is multiplied or divided by two. It means that if sensibility in increased one stop, the photosensitive element will detect twice as much available light. If sensibility is decreased one stop, the photosensitive element will detect twice as less available light. The lowest ISO sensitivity stop available on your digital camera (usually around 100 or 200) is called base ISO.

In a nutshell:

• Sensitivity is one of three parameters that you can use to change exposure.

• It corresponds to the sensitivity of the photosensitive element to available light.

• For a given amount of available light, changing sensitivity is changing the amount of light detected by the photosensitive element.

• Common ISO sensitivity stops are 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600 and so on.

• From each sensitivity stop to the previous or next one, the amount of light detected by the photosensitive element is divided or multiplied by two.

• The lowest ISO sensitivity available on your camera is called base ISO and is usually around 100 or 200 ISO.

How to change sensitivity?

In analog photography, the photosensitive support is photographic film, so if you want to change current sensitivity to another, you have to change the roll of film you are currently using. Some films are more sensitive than others.

In digital photography, push the ISO button to change sensitivity. You can also set ISO sensitivity in auto mode. In this mode, your camera will automatically calculate the proper ISO sensitivity depending on available light (i.e. depending on aperture and exposure time values).

What it does

Increasing sensitivity will increase the amount of available light detected by the photosensitive element inside the camera. As a result, the picture will appear brighter. Decreasing sensitivity will decrease the amount of available light detected by the photosensitive element inside the camera. As a result, the picture will appear darker. If you increase sensitivity one stop but want to keep the same exposure, you need to either decrease aperture one stop or to decrease exposure time one stop. Aperture and exposure time are calibrated as stops that match ISO sensitivity stops so if you increase one of them by a given amount of stops, you have to decrease the other two by the same total amount of stops to keep the same exposure and vice-versa. For instance, if you increase sensitivity four stops but you want to keep the same exposure, you can either :

1. Decrease aperture four stops.

2. Decrease exposure time four stops.

3. Decrease aperture two stops and exposure time two stops.

4. Decrease aperture one stop and exposure time three stops.

5. Decrease aperture three stops and exposure time one stop.

If you decrease sensitivity four stops but you want to keep the same exposure, you can either:

1. Increase aperture four stops.

2. Increase exposure time four stops.

3. Increase aperture two stops and exposure time two stops.

4. Increase aperture one stop and exposure time three stops.

5. Increase aperture three stops and exposure time one stop.

Noise

Unfortunately, increasing sensitivity brings noise (grain) into your picture. There are two different kinds of noise in digital photography: chroma noise and luminance noise. Chroma noise shows up as tiny blue, red and green spots particularly visible in dark areas. Luminance noise shows up as tiny luminous or dark spots particularly visible around edges. Note that noise is more and more present as you increase sensitivity.