Exposure

SECTION III

Exposure

To take a picture you need to control exposure. Exposure refers to the amount of light going through the lens to the sensor to create the image.

too much light = overexposed photo

not enough light = underexposed photo

 Exposure depends on Available Light.

Available Light - AL

Three factors control the exposure of the sensor:

1.The ISO number

2.The shutter speed

3.The aperture of the lens

Got to set them correctly to take a properly exposed picture (not too dark, not too bright).

The light meter built into the camera reads the amount of AL and sets

both values (shutter speed and aperture) to expose correctly, according with the ISO.

1. The ISO number

It indicates how sensitive the sensor is. In the past ISO was named ASA or Film Speed.

THE LARGER THE NUMBER THE MORE SENITIVE THE SENSOR

ISO SCALE

100

200

400

800

1600

With a high ISO number you need less light to create your image. This is to be used in low lighting, but also when you need to stop action (sport photography).

Increase ISO:

- in low AL

- to take action photos

2. The shutter speed

THE SHUTTER SPEED CONTROLS THE LENGTH OF TIME OF THE EXPOSURE

THE LARGER THE NUMBER THE LESS LIGHT GOES TO THE SENSOR

3. The aperture scale

THE APERTURE CONTROLS THE INTENSITY OF THE LIGHT ONTO FILM

THE LARGER THE NUMBER THE LESS LIGHT GOES TO THE SENSOR

ALL THREE OF THESE EXPOSURE FACTORS ARE RECIPROCAL

The Law of Reciprocity

The law of reciprocity means that you can compensate a change of one exposure control with an opposite change in the other exposure control and still have the same total exposure onto the film.

Shutter Speed Aperture

30        32

60               22

125     16

250     11

500       8

1000 5.6

These couples are all equal exposures for the film – the same total quantity of light goes to the sensor. The difference created by setting the different combinations of aperture and shutter is aesthetic! It depends on how you want the picture to look. If you want lots of sharpness you must choose a closed down aperture and have a slower shutter speed; if you want “stopped action” you must choose a fast shutter speed and a more opened aperture.

NOTE: this is just an example; in different lighting conditions and with different ISO numbers the couples will be different too.

Controlling Both Motion and Depth of Field

Depth of field is an expression that means the zone of apparent image sharpness. While human eyes see everything in sharp focus, the camera sees in a selective way. Selective focusing means that only a portion of the entire picture will be sharp; the rest (in general both foreground and background) will be blurry. Depth of field is the distance including all sharp points. Depth of field can be increased rising the aperture number.

To take a picture of a landscape, as we want everything to be sharp, we will set a big aperture number: 16 or 22. Conversely to take a portrait we will focus on the person face and we will use a small aperture number to have a blurry background.

Setting both shutter and aperture you can control your image in terms of sharpness and motion.

A high shutter speed number gives stopped action

A low shutter speed number gives blurred motion

A high aperture number gives great depth of field (sharpness)

A small aperture number gives little depth of field (sharpness)

But it is the given light in which you are shooting that determines the limits in which you can work.