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F-number

F-number
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. In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.[2] It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. Notation[edit] The f-number N is given by where is the focal length, and is the diameter of the entrance pupil (effective aperture). A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A T-stop is an f-number adjusted to account for light transmission efficiency. Stops, f-stop conventions, and exposure[edit] A Canon 7 mounted with a 50 mm lens capable of an exceptional f/0.95 A 35 mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring. f/1 = , f/1.4 = , f/2 = , f/2.8 = (i.e . Related:  Camera Optics Theory

Lens speed A fast prime (fixed focal length) lens, the Canon 50mm f/1.4 (left), and a slower zoom lens, the Canon 18–55mm f/3.5–5.6 (right); this lens is faster at 18mm than it is at 55mm. Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower shutter speed. A lens may be referred to as "fast" or "slow" depending on its maximum aperture compared to other lenses of similar focal length designed for a similar film format. Lens speed given by the minimum f-number, or alternatively maximum aperture diameter or maximum numerical aperture, is a useful quantitative way to compare similar lenses. Fast lenses[edit]

Diaphragm (optics) A 35 mm lens set to f/8; the diameter of the seven-sided entrance pupil, the virtual image of the opening in the iris diaphragm, is 4.375 mm Most modern cameras use a type of adjustable diaphragm known as an iris diaphragm, and often referred to simply as an iris. See the articles on aperture and f-number for the photographic effect and system of quantification of varying the opening in the diaphragm. Nine-blade iris Pentacon 2.8/135 lens with 15-blade iris In the early years of photography, a lens could be fitted with one of a set of interchangeable diaphragms [1], often as brass strips known as Waterhouse stops or Waterhouse diaphragms. The diaphragm usually has two to eight blades, depending on price and quality of the device in which it is used. In case of an even number of blades, the two spikes per blade will overlap each other, so the number of spikes visible will be the number of blades in the diaphragm used. In 1867, Dr. * In optics, stop and diaphragm are synonyms.

Sunny 16 rule The basic rule is, "On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight."[1] For example: On a sunny day and with ISO 100 film / setting in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 second (on some cameras 1/125 second is the available setting nearest to 1/100 second).On a sunny day with ISO 200 film / setting and aperture at f/16, set shutter speed to 1/200 or 1/250.On a sunny day with ISO 400 film / setting and aperture at f/16, set shutter speed to 1/400 or 1/500. As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16. An elaborated form of the sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed / setting and f-number according to this table:[2][3] References[edit] External links[edit]

Entrance pupil A camera lens adjusted for large and small aperture. The entrance pupil is the image of the physical aperture, as seen through the front of the lens. The size and location may differ from those of the physical aperture, due to magnification by the lens. The apparent location of the anatomical pupil of a human eye (black circle) is the eye's entrance pupil. In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. The entrance pupil of the human eye, which is not quite the same as the physical pupil, is typically about 4 mm in diameter. See also[edit] References[edit] Jump up ^ Greivenkamp, John E. (2004). External links[edit] Stops and Pupils in Field Guide to Geometrical Optics Greivenkamp, John E, 2004

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks Diffraction is an optical effect which limits the total resolution of your photography — no matter how many megapixels your camera may have. It happens because light begins to disperse or "diffract" when passing through a small opening (such as your camera's aperture). This effect is normally negligible, since smaller apertures often improve sharpness by minimizing lens aberrations. Light rays passing through a small aperture will begin to diverge and interfere with one another. Large Aperture Small Aperture Since the divergent rays now travel different distances, some move out of phase and begin to interfere with each other — adding in some places and partially or completely canceling out in others. Diffraction Pattern For an ideal circular aperture, the 2-D diffraction pattern is called an "airy disk," after its discoverer George Airy. Airy Disk 3-D Visualization Barely Resolved No Longer Resolved The size of the airy disk is primarily useful in the context of pixel size.

Aperture A large (1) and a small (2) aperture Aperture mechanism of Canon 50mm f/1.8 II lens, with 5 blades Definitions of Aperture in the 1707 Glossographia Anglicana Nova[1] In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system. Application[edit] The aperture stop is an important element in most optical designs. The size of the stop is one factor that affects depth of field. In addition to an aperture stop, a photographic lens may have one or more field stops, which limit the system's field of view. The biological pupil of the eye is its aperture in optics nomenclature; the iris is the diaphragm that serves as the aperture stop. In photography[edit]

Photon Nomenclature[edit] In 1900, Max Planck was working on black-body radiation and suggested that the energy in electromagnetic waves could only be released in "packets" of energy. In his 1901 article [4] in Annalen der Physik he called these packets "energy elements". The word quanta (singular quantum) was used even before 1900 to mean particles or amounts of different quantities, including electricity. Physical properties[edit] The cone shows possible values of wave 4-vector of a photon. A photon is massless,[Note 2] has no electric charge,[13] and is stable. Photons are emitted in many natural processes. The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ): where k is the wave vector (where the wave number k = |k| = 2π/λ), ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant.[17] Since p points in the direction of the photon's propagation, the magnitude of the momentum is Experimental checks on photon mass[edit]

Neutral density filter Demonstration of the effect of a neutral density filter In photography and optics, a neutral density filter or ND filter is a filter that reduces or modifies the intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter. For example, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion blur effect. Mechanism[edit] For an ND filter with optical density d the amount of optical power transmitted through the filter, which can be calculated from the logarithm of the ratio of the measurable intensity (I) after the filter to the incident intensity (I0),[1] shown as the following: Fractional Transmittance (I⁄I0) = 10-d, or Uses[edit] Comparison of two pictures showing the result of using a ND-filter at a landscape. Neutral density filters are often used to achieve motion blur effects with slow shutter speeds Examples of this use include: Varieties[edit] See also[edit]

Lens Genealogy LENS GENEALOGY Part 1by Roger Cicala Where do new lens designs come from?I knew that today’s lenses are all designed using computer programs, but I was surprised to find new lenses aren’t designed from scratch. Designers start with an existing lens design and modify it. So camera lenses, like Darwin’s finches, obey a very strict “survival of the fittest” law. Even knowing this, when I wrote a series of articles on the development of camera lenses, I was amazed to find that virtually every camera lens in use today can trace its heritage back to one of five lenses, four of which were developed by 1900. Does the pedigree of a lens matter? Knowing the ancestry of a lens can be interesting from a historical standpoint. Early Lenses DesignsThe first lenses were rather simple things. A mensicus lens (left) and an achromatic doublet After the invention of the Daguerrotype camera lens design improved rapidly and literally hundreds of photographic lenses had been marketed by the early 1900s.

DSLR Magnification By: Nick Rains We live in ‘interesting times’. Not since colour film was introduced has so much controversy raged about photography. Problem: How to sift the simple facts and truths from the myths and rumours? Answer: With a basic knowledge of certain aspects of photography, especially basic lens theory. Rainbow Lorikeet Canon D60 with Canon 300/2.8L IS lens and 2x Extender (960mm Equivalent Focal Length). The ‘Focal Length Multiplier’ is one of the most easily misunderstood characteristics of the some of the new breeds of DSLR and so I would like to offer a brief look at this aspect of digital imaging and attempt to lay to rest some of the myths. I own a Canon D60 and, like many other photographers, I was initially concerned about the ‘Focal Length Multiplier’ and the way it limits my use of wide lenses. OK, firstly, when you put a 300mm lens on a D60 you do NOT get a 480mm lens – it is still a 300mm lens. Think in terms of Medium Format, say, 6x7 on a Mamiya RZ. Here is the rub.

Circle of confusion In photography, the circle of confusion (“CoC”) is used to determine the depth of field, the part of an image that is acceptably sharp. A standard value of CoC is often associated with each image format, but the most appropriate value depends on visual acuity, viewing conditions, and the amount of enlargement. Properly, this is the maximum permissible circle of confusion, the circle of confusion diameter limit, or the circle of confusion criterion, but is often informally called simply the circle of confusion. Real lenses do not focus all rays perfectly, so that even at best focus, a point is imaged as a spot rather than a point. The smallest such spot that a lens can produce is often referred to as the circle of least confusion. The depth of field is the region where the CoC is less than the resolution of the human eye (or of the display medium). Two uses[edit] Two important uses of this term and concept need to be distinguished: Circle of confusion diameter limit in photography[edit]

Depth of field and your digital camera What is depth of field? A photographic lens renders a sharp image of points at one given distance, measured along the lens axis. This distance can be adjusted (the process of focusing). Any points at a different distance will be rendered more or less unsharp, and this unsharpness increases gradually as we move away from the "sharp" focus plane. Within some limits it will be small enough to consider the image of our point "sharp enough" for a given purpose. We are talking here only about the unsharpness due to the subject being out of the focused distance. The term depth of field (DOF) is often used to refer to the fact that points not exactly in focus are rendered acceptably sharp in the image. Is it good or bad for your pictures? For many types of photography we would like to have everything in the frame as sharp as possible. What is "acceptably sharp" — circle of confusion The extent of the depth of field depends on what we understand as "acceptably sharp" in the definition above.

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