The Aperture, how Cameras Work the best?

When you open the back of a camera to change the film, look inside and then press the shutter release. As the shutter opens the aperture can be seen beyond it. This shows more clearly if you look through the front of the lens or, on a camera with interchangeable lenses, take the lens off and look through that. Turn the aperture control and the size of the aperture will alter within the lens as you change the settings.

As the size of the aperture changes, so does the brightness of the light allowed through the lens. Aperture size, and therefore brightness, is controlled by the iris diaphragm, made up of a number of thin, interleaving blades which rotate to make the aperture larger or smaller. Making it smaller (stopping down the lens) reduces the amount of light reaching the film; increasing the size allows more light through.

The f number system

The brightness of the image on the film depends on the combination of the aperture size (the stop) and the focal length of a lens. So a large aperture and long focal length can transmit the same brightness of light as a small aperture and short focal length.

The scale photographers use to relate focal length and aperture size is called the f number system (f stands for the mathematical term, factor) and it is calculated by measuring the diameter of the aperture and dividing it into the focal length. An aperture of 25mm on a 100mm lens represents f4, but the same diameter aperture on a 50mm lens will represent f2. The smaller the f number, the larger the aperture and, therefore, the more light reaches the film.

The f numbers for any lens are calibrated by the maker on the basis that an aperture of f8, say, will always transmit the same brightness of light whatever the focal length. The series as a whole is arranged so that each f number lets in twice as much light as the number on one side of it and half as much as the number on the other. For example, f4 admits eight times more light than f11, but only half as much as f2•8.

You can see this relationship more easily on the chart on the right. Imagine that f16 allows through half a unit of light: as each f number becomes smaller, so the number of light units doubles.

The size of the aperture affects the image definition of a lens. At maximum aperture the sharpness of the image in the centre of the picture is nearly always greater than in the corners. Stopping down the lens improves central definition slightly and corner definition much more.

Lens construction differs so much that it is impossible to generalize but a sophisticated, large aperture lens of fl •4 probably performs best two or three stops up from its maximum aperture. At small stops such as flu there is usually slight fall-off in sharpness caused by diffraction, the scattering of light rays collected in the front of the lens as they pass the edge of the iris diaphragm. Both these points are really only important when a very high degree of definition is required—such as when copying documents. Other factors, such as camera shake, an incorrectly held camera or incorrect focusing, will have a far more noticeable effect on your pictures.

Depth of field

Depth of field describes the extent of the picture in focus at a given f number. The length of the zone on either side of the subject depends on the size of the aperture and the focal length of the lens. In theory, only the subject on which you focus is completely sharp but an area of acceptable sharpness lies in front of and behind it.

As the size of the aperture decreases, the depth of field lengthens, bringing more of the picture on either side of the subject into focus.

Most rangefinder and reflex cameras have depth of field scales on their lenses, which show the limits of the area in focus for any combination of aperture and distance. A study of this scale shows that the subject focused on is not at the centre of this sharp zone, which extends two-thirds beyond the subject, and one-third towards the camera—unless, of course, the subject is very close to you.

On cameras with separate viewfinders the scale on the lens is the only indication of depth of field, so you should check that the aperture size set will give you the depth you want, before pressing the shutter release.

On SLR cameras, which view through the lens, the photographer can immediately see how much of the finished picture will be in focus by using the depth of field preview button mounted on the camera body.

Focal length/depth of field

The focal length of the lens on a camera also affects depth of field. Using the same f number—f4—each time but progressively extending focal length by changing lenses alters the degree of sharpness surrounding the subject in each photograph. So if the same f number is used throughout, depth of field shortens as focal length becomes longer.

To sum up:

• The focal length of a lens determines the angle of view it takes in—that is, the ‘width’ of the scene.
• Depth of field describes how much of the scene, from front to back, is in focus.
• Depth of field shortens as focal length becomes longer, or as the aperture becomes larger.

Depth of field table

The chart on the right is a rough guide to depth of field at various apertures for each of four lenses, all focused on a subject four meters away. So, depending on the subject and how much of the scene you want to be in focus, you can use this chart to find the right aperture setting for each lens.

The numbers in the boxes give the distance in centimeters that will be in focus (00 = infinity). For example, 275765 means that the nearest point in focus is 275cm away from the camera, while the furthest point in focus is 765cm away. Depth of field therefore extends over almost five meters.

Now take, for example, a normal 50mm lens on an SLR camera. Focus on a subject four meters away and look through the camera. Close the aperture progressively from f2.8 to f22. At f4 everything just over three meters to just over six meters will be sharp. At fl 1 everything from two meters to infinity will be sharp.

Reading down the columns, note how depth of field increases as the aperture becomes smaller—represented by larger value f number.

Reading across, depth of field for given aperture decreases as the lens exchanged for one of longer focal length.

The colour coded diagonals pick o some of the constant depths of fiel^d,this way you can choose the right aperture to maintain the same depth of fie with different lenses.

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