f-stop – What it is and how it works

The pupil of the eye becomes bigger or smaller in order to control the amount of light it allows through. When a person is standing in a dark room, the pupil of his eye will be large to allow more light to enter. The camera also has a mechanism which controls the amount of light moving through the lens – the aperture.

The aperture controls not only the amount of light moving through the lens but also the depth of field. The f-stop is the unit in which the opening of the lens is measured. The f-stop number is inversely proportional to the size of the lens opening. This means f22 is a very small lens opening, with a large depth of field, and f4.5 has a wide lens opening with a shallow depth of field. With a wide lens opening and shallow depth of field, more light will reach the film. Thus a large lens opening is more suitable for photography in bad lighting conditions, but unfortunately with a loss in depth of field. Also see illustrations below. The f-stop number is equivalent to the diameter of the opening of the lens relative to the diameter of the front lens unit. An f-stop of f16 will therefore be 1/16 (one sixteenth) of the diameter of the front lens unit.

Apperture size

Apperture size

Tip: If you have a SLR camera with removable lenses remove the lens from your camera. Hold it in a position to enable you to look through it from the back. Now turn the f-stop ring on the lens from side to side. You will see the opening of the lens changing as you turn the ring.


You will notice that the f-stop numbers are usually about twice the value of the previous number (f4, f8, f16, f32). These are called full-stop settings. When your lens has settings in between, these are called half-stop settings (f4.5, f5.6, f8, f11, f16, f22, f32). It may happen that the values of the half-stop settings are not indicated, but there may be a hitch for the values on the ring. These types of lenses are usually a lot more accurate than full-stop lenses and are therefore a better option when buying a lens. On older cameras the f-stop values will appear on the f-stop ring attached to the camera. More modern cameras will also have the f-stop value on the lens and displayed on the LCD screen. In this screen it will be shown as Av or A (“A” for aperture).

f-stop

f-stop

Quality of image

The general quality of the image can also be controlled by the f-stop when a standard lens is used. Since the lens forms a minuscus, the image can appear distorted around the edges. A better quality of the image can be obtained by using a smaller aperture, forcing the image not to go through the edge of the lens. It is also not advisable to use the minimum aperture setting, since problems with too little light can be experienced. A good guideline is to use the f-stop in the middle of the lens. If the minimum aperture of the lens is f32, use f16 to ensure that you have a sharp image. More expensive lenses are cut more precisely and this problem may not be visible in pictures shot with these lenses.

Quality of your image

Quality of your image

Depth of field

Another alien term, but a very handy aid. When a lens is focused at a certain point, details in front of and beyond that point of focus will also be acceptably sharp. This range of sharpness is called the depth of field. When a portrait of a person is shot, and only the eyes and nose are in focus, with the rest of the face being out of focus, we have a small depth of field. See the figure below:

Depth of field

Depth of field

What is depth of field used for?

As mentioned earlier, the aperture controls the amount of light which passes through the lens. The difference in depth of field can also be used creatively. Here are two extreme examples:

1. When you shoot a landscape, you wish to have everything in the frame in focus to show maximum detail. In this instance the f-stop will be set on the maximum number (the smallest aperture) and the camera should preferably be mounted on a tripod.

2. You want to photograph an object which you cannot move but a disturbing background prevents you from taking the picture. In this case you will set the aperture on a suitable setting. The f-stop depends on the type of lens used and the distance you are from the object and the background. It will help a lot if your camera has a depth of field preview function. If it does not have this function, you will have to shoot more than one picture. When you want to photograph a person with a standard 50mm lens at about 10 metres from the background, you will use f5.6. By using a very large aperture, you will blur the background to such an extent that it will not be a disturbance any more.

Depth of field

Depth of field small to focus on one object

Tip: Remember that the shutter speed has to be amended with the f-stop. When you have determined the correct reading for a specific image and you have set the f-stop number smaller (one stop more light), the shutter speed must be set one stop higher (faster). If you have an f8 and 1/250 setting and you change it to f5.6, then the shutter speed should be adjusted accordingly to 1/500.

Maximum depth of field

From the diagram on the left, it is clear that a larger part at the back of the object will be in focus than in front. This division is usually one third in front and two thirds behind the object. This means that when you photograph an object and want maximum depth of field, you should focus on a fictitious point. To determine this fictitious point, try the following:

Focus on the point closest to you and mark the position of the focus ring. Then focus on the point of the object furthest from you. Again mark the position of the focus ring. Now turn back the focus ring about two thirds of the distance. If your camera does not have a depth of field preview function, the object will usually appear out of focus at this stage. Select your maximum aperture (f22 or f32) and shoot the picture. Although the image may appear out of focus through the view finder, the image should in most instances be in focus. This problem escalates the closer you move to the object and when you use a macro lens.

Remember that the image seen through the view finder never represents the results. When you look through the view finder and focus, the diaphragm in your lens will always be on a maximum opening. When you change the f-stop, the diaphragm won’t change accordingly. The lens has a function which will change the diaphragm to the correct size as the picture is shot. The reason for this is that you need as much available light as possible to ease the focusing and composition process. The only time you can really see what the picture will look like, is if your camera has the depth of field preview function. Don’t get discouraged when your depth of field seems too shallow through the view finder.

All in focus

All in focus

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