Lens, Shutter, Image Motion Compensation: Lens Properties and Distortions (Especially for GATE-Geospatial 2022)

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The lens gathers reflected light and focuses it on the focal plane to form an image. In its simplest form, a lens is a glass disk carefully ground into shape with nonparallel curved surfaces. The change in optical densities as light rays pass from the atmosphere to the lens and back to the atmosphere causes refraction of light rays; the sizes, shapes, arrangements, and compositions of lenses are carefully designed to control refraction of light to maintain colour balance and to minimize optical distortions.

Optical Characteristics and Distortions

  • Optical characteristics of lenses are determined largely by the refractive index of the glass and the degree of curvature. The quality of a lens is determined by the quality of its glass, the precision with which that glass is shaped, and the accuracy with which it is positioned within a camera.
  • Imperfections in lens shape contribute to spherical aberration, a source of error that distorts images and causes loss of image clarity. For modern aerial photography, spherical aberration is usually not a severe problem because most modern aerial cameras use lenses of very high quality.
Principal Plane

Working of Simple Positive Lens (Double Convex) : Focal Length

  • Above figure shows the simplest of all lenses: a simple positive lens. Such a lens is formed from a glass disk with equal curvature on both sides; light rays are refracted at both edges to form an image. Most aerial cameras use compound lenses, formed from many separate lenses of varied sizes, shapes, and optical properties. These components are designed to correct for errors that may be present in any single component, so the whole unit is much more accurate than any single element.
  • A simple lens differs greatly from those actually used in modern aerial cameras. The optical axis joins the centres of curvature of the two sides of the lens. Although refraction occurs throughout a lens, a plane passing through the centre of the lens, known as the image principal plane, is considered to be the centre of refraction within the lens. The image principal plane intersects the optical axis at the nodal point.
  • Parallel light rays reflected from an object at a great distance (at an “infinite” distance) pass through the lens and are brought to focus at the principal focal point — the point at which the lens forms an image of the distant object. The chief ray passes through the nodal point without changing direction; the paths of all other rays are deflected by the lens. A plane passing through the focal point parallel to the image principal plane is known as the focal plane.
  • For handheld cameras, the distance from the lens to the object is important because the image is brought into focus at distances that increase as the object is positioned closer to the lens. For such cameras, it is important to use lenses that can be adjusted to bring each object to a correct focus as the distance from the camera to the object changes.
  • For aerial cameras, the scene to be photographed is always at such large distances from the camera that the focus can be fixed at infinity, with no need to change the focus of the lens. In a simple positive lens, the focal length is defined as the distance from the centre of the lens to the focal point, usually measured in inches or millimetres.

Effect of Wavelength on Focal Length: Chromatic Aberration

  • For a given lens, the focal length is not identical for all wavelengths. Blue light is brought to a focal point at a shorter distance than are red or infrared wavelengths.
Effect is the Source of Chromatic Aberration
  • This effect is the source of chromatic aberration. Unless corrected by lens design, chromatic aberration would cause the individual colours of an image to be out of focus in the photograph. Chromatic aberration is corrected in high-quality aerial cameras to assure that the radiation used to form the image is brought to a common focal point.

Field of View of Lens and Aperture Stop

  • The field of view of a lens can be controlled by a field stop, a mask positioned just in front of the focal plane. An aperture stop is usually positioned near the centre of a compound lens; it consists of a mask with a circular opening of adjustable diameter. An aperture stop can control the intensity of light at the focal plane but does not influence the field of view or the size of the image.
  • Manipulation of the aperture stop controls only the brightness of the image without changing its size. Usually, aperture size is measured as the diameter of the adjustable opening that admits light to the camera.
Diaphragm Apertures Stop (A) Perspective View

Diaphragm apertures stop (a) Perspective view. (b) Narrow aperture. (c) Wide aperture. f stops represented below.

Relative Aperture

Relative aperture is defined as where,

  • Focal length and aperture are measured in the same units of length and f is the f-number, the relative aperture. A large f-number means that the aperture opening is small relative to focal length; a small f-number means that the opening is large relative to focal length.
  • The standard sequence of apertures is 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 64, and so forth. This sequence is designed to change the amount of light by a factor of 2 as the f-stop is changed by one position. For example, a change from f 2 to f 2.8 halves the amount of light entering the camera; a change from f 11 to f 8 doubles the amount of light.

Wide Field of Views: Vignetting and Anti-Vignetting Filter

Lenses for aerial cameras typically have rather wide fields of view. As a result, light reaching the focal plane from the edges of the field of view is typically dimmer than light reflected from objects positioned near the centre of the field of view. This effect creates a dark rim around the centre of the aerial photograph — an effect known as vignetting. It is possible to employ an anti-vignetting filter, darker at the centre and clearer at the periphery that can be partially effective in evening brightnesses across the photograph. Digital systems can also employ image processing algorithms, rather than physical filters, to compensate for vignetting.

The Shutter

  • The shutter controls the length of time that the film is exposed to light. The simplest shutters are often metal blades positioned between elements of the lens, forming “intralens,” or “between-the-lens,” shutters. An alternative form of the shutter is the focal plane shutter, consisting of a metal or fabric curtain positioned just in front of the detector array, near the focal plane.
  • The curtain is constructed with a number of slits; the choice of shutter speed by the operator selects the opening that produces the desired exposure. Although some analogue aerial cameras once used focal plane shutters, the between-the-lens shutter is preferred for most aerial cameras. The between-the-lens shutter subjects the entire focal plane to illumination simultaneously and presents a clearly defined perspective that permits the use of the image as the basis for precise measurements.

Image Motion Compensation

  • High-quality aerial cameras usually include a capability known as image motion compensation (or forward motion compensation) to acquire high-quality images. Depending on the sensitivity of the recording media (either analogue or digital) , the forward motion of the aircraft can subject the image to blur when the aircraft is operated at low altitudes and/or high speeds.
  • In the context of analogue cameras, image motion compensation is achieved by mechanically moving the film focal plane at a speed that compensates for the apparent motion of the image in the focal plane. In the context of digital systems, image motion compensation is achieved electronically. Use of image motion compensation widens the range of conditions (e. g. , lower altitudes and faster flight speeds) that can be used while preserving the detail and clarity of the image.

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