Lens – Aperture
Amount of Light
The diaphragm inside a lens regulates the aperture and thereby the amount of light entering the camera.
The light sensitive surface like an image sensor or film works optimal when it receives a measured amount of light. Light meters in cameras are usually calibrated to middle gray. The aperture, exposure time and sensitivity together determine the amount of light needed to achieve optimal exposure.
In the human eye the amount of light reaching the retnia is influenced by the iris and the pupil size.
Likewise, in photography, the amount of light reaching the light sensitive area is influenced by sliding diaphragm blades that regulate the size of the aperture.
Think of the iris as being the diaphragm and the pupil as being the aperture.
In older lenses the diaphragm blades are controlled manually by rotating the aperture ring.
In modern lenses the aperture is controlled manually by a command wheel (control dial) or chosen automatically by the camera.
The f-number ( N ) is determined from the focal length ( f ) and the aperture ( D ):
N = f D
| Gebruikelijke diafragmagetallen | ||||||
|---|---|---|---|---|---|---|
| Notation | f /1 | f /1,4 | f /2 | f /2,8 | f /4 | f /5,6 |
| f-number | 1 | 1,4 | 2 | 2,8 | 4 | 5,6 |
| Example |  |  |  |  | ||
| Notation | f /8 | f /11 | f /16 | f /22 | f /32 | f /64 |
| f-number | 8 | 11 | 16 | 22 | 32 | 64 |
| Example |  |  |  | |||
The f-number is found on the lens aperture ring or the LCD screen of your camera. The larger the number, the smaller the aperture.
The lowest available f-number for the largest aperture is called the lens speed. In the image above, the lens speed is 2.8.
As the f-number is related to the focal length you might find two lens speed notations on a zoom lens. E.g. on a 35-105 zoom lens it might be 1:3.5-4.5. This means that the lens speed is 3.5 at 35 mm zoom and increases to 4.5 at 105 mm zoom. The more you zoom in, the more light is needed to get the same exposure.
Zoom lenses in a higher price catergory might have a contstant lens speed of e.g. 2.8 over the entire zoom range.
