Shutter speeds are arranged in a similar scale, so that one step in the shutter speed scale corresponds to one stop in the aperture scale. It may simply be the opening of the aperture stop, or may be a magnified image of the aperture stop, formed by elements within the lens. The location of this virtual disk inside the lens depends on the optical design. For example, f/16 means that the pupil diameter is equal to the focal length divided by sixteen that is, if the camera has an 80 mm lens, all the light that reaches the film passes through a virtual disk known as the entrance pupil that is 5 mm (80 mm/16) in diameter. The values of the ratios are rounded off to these particular conventional numbers, to make them easy to remember and write down. Modern lenses use a standard f-stop scale, which is an approximately geometric sequence of numbers that corresponds to the sequence of the powers of √2 (1.414): f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128, etc. This corresponds to a decrease of the pupil and aperture diameters by a factor of √2 or about 1.414, and hence a halving of the area of the pupil. Each "stop" is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. On a camera, the f-number is usually adjusted in discrete steps, known as f-stops. The one-stop unit is also known as the EV (exposure value) unit. In photography, stops are also a unit used to quantify ratios of light or exposure, with one stop meaning a factor of two, or one-half. The aperture stop is the aperture that limits the brightness of the image by restricting the input pupil size, while a field stop is a stop intended to cut out light that would be outside the desired field of view and might cause flare or other problems if not stopped. A stop can be a physical object: an opaque part of an optical system that blocks certain rays. The term stop is sometimes confusing due to its multiple meanings. A focal ratio of f/16 tells us that the physical aperture inside the camera lens has a pupil diameter equal to one sixteenth of that lens' focal length and this applies to all lenses using this designation. The 135mm lens' f/4 opening is larger than that of the 100mm lens though both will transmit the same amount of light to the film or sensor. A 135mm lens with a setting of f/4 will have a pupil diameter of 33.8mm or 34mm. A 100mm lens with an aperture setting of f/4 will have a pupil diameter of 25mm. Note that the common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for all types of camera lens. Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image). In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The pupil diameter is proportional to the diameter of the aperture stop of the system. The literal interpretation of the f/N notation for f-number N is as an arithmetic expression for the effective aperture diameter (input pupil diameter), the focal length divided by the f-number: D = f / N. The greater the f-number, the less light per unit area reaches the image plane of the system. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. Where f is the focal length, and D is the diameter of the entrance pupil. The f-number f/#, often notated as N, is given by The actual size of the aperture will depend on the focal length of the lens. Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments each aperture has half the light gathering area of the previous one.
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