Within most photographic lenses, you will observe an approximately circular opening situated midway between the front and back elements. This opening, known as the aperture, is controlled by a series of overlapping black metal blades called iris diaphragms. These blades adjust the aperture size continuously from the full lens diameter to just the central part of the lens. This adjustment is typically made using a ring or lever on the outside of the lens barrel.
For single-lens reflex (SLR) cameras, you won’t see the aperture change when you turn the ring unless you remove the lens from the camera body. In these cameras, the aperture generally remains wide open until the moment of exposure, at which point it closes to the pre-set value. Some cameras or lenses may feature a preview button that allows you to close the aperture ahead of time to check the effect.
A series of relative aperture values, known as f-numbers, are indicated on a scale and can be felt as clicks when adjusting the aperture. It’s important to note that smaller aperture openings correspond to larger f-number values. The standard sequence of f-numbers typically runs: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.
This sequence follows an internationally accepted standard concerning image luminosity. In this sequence, f/1.4 represents the widest aperture, allowing the maximum amount of light to pass through for a bright image, while f/22 represents the smallest aperture, permitting only a fraction of the light and providing greater depth of field, which is particularly useful in bright conditions. The aperture scale can extend beyond these values for specific purposes, with apertures wider than f/1.4 and smaller than f/22.
Each step to the next higher f-number halves the amount of light passing through the lens. Because the aperture is centrally located within the lens, it ensures uniform illumination or attenuation across the entire image.
The f-number system ensures that any lens set to the same f-number will provide consistent image brightness, regardless of the camera’s focal length or sensor size. Thus, you can switch lenses or cameras and, as long as the same f-number is set, the image luminosity will remain constant.
