Instruction manual

The size (diameter) of a lens’s aperture at any given moment is called the f-

stop (also f/stop or f/number), which is expressed in several different ways: f8, f/8,

f-8, and 1:8. A lower f-stop number (f2.8) opens the aperture and admits more

light onto the camera sensor, while higher f-stop numbers (f11) make the

camera's aperture smaller so less light hits the sensor.

Just like shutter speeds, each whole f-stop represents a doubling or halving

of the amount of light hitting the sensor, depending on whether you widen or

narrow the aperture; for example, going from f8 to f11 will reduce the light hitting

the sensor by one-half.

The example below (not to scale) shows some sample f-stops:

The maximum aperture of a lens is also called its lens speed. A lens with a

large maximum aperture (f2.8) is called a "fast" lens because the large aperture

allows you to use high (fast) shutter speeds and still receive sufficient exposure.

The big advantage of these lenses fast lenses is that they are ideal for shooting

moving subjects in low light conditions. Zoom lenses specify the maximum

aperture at both the wide angle and telephoto ends (e.g. 28 - 100mm f3.5 - 5.6).

A specification like 28 - 100mm f2.8 implies that the maximum aperture is f2.8

throughout the zoom range.

Note: As you stop your lens down (i.e., set a higher f-stop number), diffraction

effects can occur that result in a loss of sharpness. Take a few test shots with

your camera to determine how much you can stop your lens down before you

notice an unacceptable loss of sharpness.

2.1.4 Relationships

Aperture and shutter speed are related, and effect the exposure value

(discussed below). As you reduce the shutter speed you need to increase the

diameter of the aperture in order to have enough light to take a properly exposed

picture. The wider you open the iris the shorter the shutter speed needed to get

the correct exposure, because the camera chooses shutter speed based on how

much light gets into the camera. For example, the following settings result in the

same amount of light reaching the sensor: f4 at 1/1000s, f8 at 1/250s, and f16 at

1/125s.

At a given ISO setting you can take a picture with a specific aperture and

shutter combination. If you double the ISO setting without changing the lighting

conditions, you have to adjust the aperture and/or shutter speed so you still get a

properly exposed picture. For example, at ISO 100 you may have the aperture

set at f4 and the shutter speed at 1/250s; if you double the ISO to 200 you will

need to increase the shutter speed to 1/500s to get the same exposure under the

same lighting conditions.

Large apertures like f4 will tend to have just one thing in focus, while a smaller

aperture like f11 will tend to have everything in focus. How much of the image

that is in focus is called “depth of field,” and is also referred to as the contrast

between the foreground and the background. You can adjust the depth of field,

for example, to make the background more or less sharp compared to the