F-numbers and aperture
F-numbers and aperture | ||
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Inside most photographic lenses you will see a circular hole or aperture located about midway between front and back elements. Usually a series of overlapping black metal blades called an iris diaphragm allows the size of this aperture to be narrowed continuously from full lens diameter to just the centre part of the lens. It is adjusted with a setting ring or lever outside the lens barrel. Most smaller-format cameras control the aperture size automatically, at the moment of exposure. On single-lens reflex cameras you may not see the aperture actually alter when you turn the ring, unless you first detach the lens from the camera. A series of relative aperture settings can be felt by 'click' and are shown on a scale of figures known as f-numbers. Notice that the smaller the relative aperture the higher the f-number. They typically run: f/2; 2.8; 4; 5.6; 8; 11; 16; etc. The f-numbers follow what is an internationally agreed sequence relating to the brightness of the image. It is like operating a 'light tap'; each change to the next highest number halves the amount of light passing through your lens. And because the aperture is positioned in the lens centre it dims or brightens the entire image evenly. The f-number system means that every lens set to the same number and focused on the same (distant) subject gives matching image brightness, irrespective of its focal length or the camera size. You can change lenses or cameras, but as long as you set the same f-number the image brightness remains constant. | ||