Focus
This subject is in sharp focus while the distant background is unfocused
Focus is the tendency for light rays to reach the same place on the image sensor or film, independent of where they pass through the lens. For clear pictures, the focus is adjusted for distance, because at a different object distance the rays reach different parts of the lens with different angles. In modern photography, focusing is often accomplished automatically.
The autofocus system in modern SLRs use a sensor in the mirrorbox to measure contrast. The sensor's signal is analyzed by an application-specific integrated circuit (ASIC), and the ASIC tries to maximize the contrast pattern by moving lens elements. The ASICs in modern cameras also have special algorithms for predicting motion, and other advanced features.
Aberration
Aberrations are the blurring and distorting properties of an optical system. A high quality lens will produce a smaller amount of aberrations.
Spherical aberration occurs due to the increased refraction of light rays that occurs when rays strike a lens, or a reflection of light rays that occurs when rays strike a mirror near its edge in comparison with those that strike nearer the center. This is dependent on the focal length of a spherical lens and the distance from its center. It is compensated by designing a multi-lens system or by using an aspheric lens.
Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light and the dependence of the optical properties on color. Blue light will generally bend more than red light. There are higher order chromatic aberrations, such as the dependence of magnification on color. Chromatic aberration is compensated by using a lens made out of materials carefully designed to cancel out chromatic aberrations.
Curved focal surface is the dependence of the first order focus on the position on the film or CCD. This can be compensated with a multiple lens optical design, but curving the film has also been used.
Spherical aberration occurs due to the increased refraction of light rays that occurs when rays strike a lens, or a reflection of light rays that occurs when rays strike a mirror near its edge in comparison with those that strike nearer the center. This is dependent on the focal length of a spherical lens and the distance from its center. It is compensated by designing a multi-lens system or by using an aspheric lens.
Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light and the dependence of the optical properties on color. Blue light will generally bend more than red light. There are higher order chromatic aberrations, such as the dependence of magnification on color. Chromatic aberration is compensated by using a lens made out of materials carefully designed to cancel out chromatic aberrations.
Curved focal surface is the dependence of the first order focus on the position on the film or CCD. This can be compensated with a multiple lens optical design, but curving the film has also been used.
Film grain resolution
Strong grain on ISO1600 negative
Black-and-white film has a "shiny" side and a "dull" side. The dull side is the emulsion, a gelatin that suspends an array of silver halide crystals. These crystals contain silver grains that determine how sensitive the film is to light exposure, and how fine or grainy the negative the print will look. Larger grains mean faster exposure but a grainier appearance; smaller grains are finer looking but take more exposure to activate. The graininess of film is represented by its ISO factor; generally a multiple of 10 or 100. Lower numbers produce finer grain but slower film, and vice versa.
Black-and-white film has a "shiny" side and a "dull" side. The dull side is the emulsion, a gelatin that suspends an array of silver halide crystals. These crystals contain silver grains that determine how sensitive the film is to light exposure, and how fine or grainy the negative the print will look. Larger grains mean faster exposure but a grainier appearance; smaller grains are finer looking but take more exposure to activate. The graininess of film is represented by its ISO factor; generally a multiple of 10 or 100. Lower numbers produce finer grain but slower film, and vice versa.
Diffraction limit
Since light propagates as waves, the patterns it produces on the film are subject to the wave phenomenon known as diffraction, which limits the image resolution to features on the order of several times the wavelength of light. Diffraction is the main effect limiting the sharpness of optical images from lenses that are stopped down to small apertures (high f-numbers), while aberrations are the limiting effect at large apertures (low f-numbers). Since diffraction cannot be eliminated, the best possible lens for a given operating condition (aperture setting) is one that produces an image whose quality is limited only by diffraction. Such a lens is said to be diffraction limited.
The diffraction-limited optical spot size on the CCD or film is proportional to the f-number (about equal to the f-number times the wavelength of light, which is near 0.0005 mm), making the overall detail in a photograph proportional to the size of the film, or CCD divided by the f-number. For a 35 mm camera with f/11, this limit corresponds to about 6,000 resolution elements across the width of the film (36 mm / (11 * 0.0005 mm) = 6,500.
In other words, for two distant point to appear as separate objects on the film or sensor, the distance from each to the two sides of the open lens aperture must differ by at least a half wavelength; otherwise they will be both in phase at the same point on the film or CCD and won't be distinguished from each other.
Since light propagates as waves, the patterns it produces on the film are subject to the wave phenomenon known as diffraction, which limits the image resolution to features on the order of several times the wavelength of light. Diffraction is the main effect limiting the sharpness of optical images from lenses that are stopped down to small apertures (high f-numbers), while aberrations are the limiting effect at large apertures (low f-numbers). Since diffraction cannot be eliminated, the best possible lens for a given operating condition (aperture setting) is one that produces an image whose quality is limited only by diffraction. Such a lens is said to be diffraction limited.
The diffraction-limited optical spot size on the CCD or film is proportional to the f-number (about equal to the f-number times the wavelength of light, which is near 0.0005 mm), making the overall detail in a photograph proportional to the size of the film, or CCD divided by the f-number. For a 35 mm camera with f/11, this limit corresponds to about 6,000 resolution elements across the width of the film (36 mm / (11 * 0.0005 mm) = 6,500.
In other words, for two distant point to appear as separate objects on the film or sensor, the distance from each to the two sides of the open lens aperture must differ by at least a half wavelength; otherwise they will be both in phase at the same point on the film or CCD and won't be distinguished from each other.
Quantum efficiency
Light comes in particles and the energy of a light-particle (the photon) is the frequency of the light times Planck's constant. A fundamental property of any photographic method is how it collects the light on its photographic plate or electronic detector.
Light comes in particles and the energy of a light-particle (the photon) is the frequency of the light times Planck's constant. A fundamental property of any photographic method is how it collects the light on its photographic plate or electronic detector.
CCDs and other photodiodes
Photodiodes are back-biased semiconductor diodes, in which an intrinsic layer with very few charge carriers prevents electric currents from flowing. Depending on the material, photons have enough energy to raise one electron from the upper full band to the lowest empty band. The electron and the "hole", or empty space where it was, are then free to move in the electric field and carry current, which can be measured. The fraction of incident photons that produce carrier pairs depends largely on the semiconductor material.
Photodiodes are back-biased semiconductor diodes, in which an intrinsic layer with very few charge carriers prevents electric currents from flowing. Depending on the material, photons have enough energy to raise one electron from the upper full band to the lowest empty band. The electron and the "hole", or empty space where it was, are then free to move in the electric field and carry current, which can be measured. The fraction of incident photons that produce carrier pairs depends largely on the semiconductor material.
Photomultiplier tubes
Photomultiplier tubes are vacuum phototubes that amplify light by accelerating the photoelectrons to knock more electrons free from a series of electrodes. They are among the most sensitive light detectors but are not well suited to photography.
Photomultiplier tubes are vacuum phototubes that amplify light by accelerating the photoelectrons to knock more electrons free from a series of electrodes. They are among the most sensitive light detectors but are not well suited to photography.
Aliasing
Aliasing can occur in optical and chemical processing, but it is more common and easily understood in digital processing. It occurs whenever an optical or digital image is sampled or re-sampled at a rate which is too low for its resolution. Some digital cameras and scanners have anti-aliasing filters to reduce aliasing by intentionally blurring the image to match the sampling rate. It is common for film developing equipment used to make prints of different sizes to increase the graininess of the smaller size prints by aliasing.
It is usually desirable to suppress both noise such as grain and detail of the real object that are too small to be represented at the sampling rate
Aliasing can occur in optical and chemical processing, but it is more common and easily understood in digital processing. It occurs whenever an optical or digital image is sampled or re-sampled at a rate which is too low for its resolution. Some digital cameras and scanners have anti-aliasing filters to reduce aliasing by intentionally blurring the image to match the sampling rate. It is common for film developing equipment used to make prints of different sizes to increase the graininess of the smaller size prints by aliasing.
It is usually desirable to suppress both noise such as grain and detail of the real object that are too small to be represented at the sampling rate
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