Aperture

What is aperture?

Aperture is the width of the opening in the camera lens.

The camera lens contains an iris diaphragm constructed of metal leaves that open and close to give a smaller or larger opening through which light enters the camera to reach the sensor.

It is can be compared to the iris of the human eye: when it is very sunny the pupils (lens) constrict to limit the amount of light reaching the retina (sensor); in the dark of night the pupil will dilate to increase the amount of light reaching the retina.

Aperture size is measured in f-stops, with the smallest f stop of the lens having the largest aperture (or opening) and the highest number being the smallest aperture. The reason for this is that f stops are measured in fractions:

 f-stop             =                                  focal length of the lens                  

                                                diameter measurement of the aperture

The common scale for apertures is:

1.4       2.0       2.8       4.0       5.6       8.0       11        16        22        32

wide open                                                                              very narrow

            f/2.8                                        f/8.0                            f/22

With each value quoted above the aperture size either halves or doubles. A change in f-stop from 4.0 to 5.6 will halve the aperture size and hence the amount of light reaching the sensor, whereas changing from 4.0 to 2.8 will double the aperture size.

In addition the Olympus E520 uses third-stop increments;

4.0       4.5       5.0       5.6       6.3       7.1       8.0       9.0       10        11        13        14        16

This allows more flexibility in aperture size, but the relationship between the f-stop numbers remains constant.

The 14-42mm Zuiko lens for the Olympus E520 camera has a maximum aperture of f/3.5 down to a minimum aperture of f/22. However since this lens has a variable focal length (14 up to 42 mm) the lowest f-stop value increases as the focal length increases. (f/ 3.5 at 14mm; f/5.6 at 42mm). Thus when the lens is fully extended the maximum aperture decreases (f-stop increases).

f-stop values are specific for individual lenses. The low values of 1.4 or 2.0 are generally only obtained with the most expensive lenses.

References:
Collins Complete Photography Course. John Garrett and Graeme Harrris. 2008. Harper Collins Publishers. ISBN: 978-0-00-727992-0
Get the most from your digital SLR. The Digital SLR Guide. Chris Roberts. 2008. Downloaded from the Internet March 2009.