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Criminalistics & Forensic Physics MODULE No. 27

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Criminalistics & Forensic Physics MODULE No. 27 SUBJECT FORENSIC SCIENCE Paper No. and Title PAPER No.7: Criminalistics & Forensic Physics Module No. and Title MODULE No.27: Photographic Lenses, Filters and Artificial Light Module Tag FSC_P7_M27 FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction- Camera Lenses i) Convex Lens ii) Concave Lens 3. Useful terms of the lens 4. Types of Photographic Lens 5. Defects of Lens 6. Filters for Photography 7. Film Sensitivity 8. Colour of Light 9. Summary FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light 1. Learning Outcomes After studying this module, you shall be able to know – What are Camera Lenses and their types Various terms of the Lens Various types of Filters used in Photography 2. Introduction – Camera Lenses Camera lens is a transparent medium (usually glass) bounded by one or more curved surfaces (spherical, cylindrical or parabolic) all of whose centers are on a common axis. For photographic lens the sides should be of spherical type. A simple or thin lens is a single piece of glass whose axial thickness is less compared to its diameter whereas a compound lens consists of several components or group of components, some of which may comprise of several elements cemented together. Lenses are mainly divided into two types, viz. i) Convex Lens ii) Concave Lens FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light i) Convex lens: This type of lens is thicker at the central portion and thinner at the peripheral portion. It casts real image and so can be used to take photographs. Convex lens can be divided into three types- (i) Bi-Convex or Double Convex (ii) Plano Convex (iii) Concave Convex or Meniscus ii) Concave Lens: This type of lens is thicker at the peripheral portion and thinner at the centre. It cannot cast real image, so a single concave lens cannot serve the purpose of photography. Like convex lens, this type can be subdivided into three types- (i) Bi-Concave or Double Concave (ii) Plano Concave (iii) Convexo Concave FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light In modern cameras, both negative and positive lenses are used but the net effect of the combination should be positive. 3. Useful Terms of the Lens 1) Optical Centre It is a point within the lens through which light passes undeviated or without changing the travelling path. 2) Principle Axis The axis passes through the optical centre of the lens which is horizontal when the lens is placed vertical to the horizon. It is a stroke linking the centers of curvature of its surfaces. 3) Focus or Focal Point When analogous beam of light passes through the positive lens (analogous to the principle axis) then it converges at a point and it seems to diverge from a point in case of a negative type lens. This point is known as focus. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light 4) Focal Length The space between optical centre and focus is known as Focal Length. Focal length commonly measured in millimeters (mm) and is the basic explanation of a photographic lens. Though it is not a dimension of the actual length of a lens, but a calculation of an optical distance from the point where light rays meet to form a sharp image of an object on the digital sensor or 35mm film at the focal plane in the camera. The focal length of a lens is determined when the lens is focused at infinity. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light 5) Focal Plane The imaginary plane passing through the focus and perpendicular to the principle axis is known as focal plane. 6) f-Number It is defined as the ratio between the focal length and the diameter of the diaphragm. “f” number = F/D where F= Focal Length and D=Diameter of the slit or diaphragm. The intensity of a lens is given by an arrangement of focal length and its diameter. In case, if the focal length of any two lenses is found to be identical then the lens having greater diameter will be brighter. In an instance, the focal length is 50mm and the lens diameter is 17.8mm then, focal length divided by lens diameter gives the lens a maximum F-stop of 2.8. 4. Types of Photographic Lens Photographic lenses are divided into various categories according to their focal length, speciality and use. Although modern compact cameras come with integrated lens with variable focal length however, only SLR cameras have the advantage where one can change and use almost any type of lens according to their requirement. The diagram below shows how change in focal length determines the angle of view of the lens. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light i. Normal Lens The standard lens has a stable focal length (50mm, 85mm) and imitates exactly what the human eye sees in terms of viewpoint and angle of view. For a 35mm film camera or a full-frame DSLR, the 50mm lens is considered as standard. ii. Wide Angle Lens A wide-angle general has less focal length (10 mm 42mm) in relation to a standard lens. This enables us to seize a relatively wider angle of view. A wide-angle lens is a natural choice for capturing outdoor landscapes and group portraits. In fact, wide angle can be the only way to capture the complete setting without omitting any important elements in the image. In this manner, we can use wide-angle lenses to capture a deep Depth of Field. iii. Telephoto Lens Telephoto lenses (100mm - 800mm) provide a constricted field of view. These extended lenses allow compression of distance (and constricting the sense of depth, also) and take view of a particular object from distant. They have a good resolving power as well as an inherent shallow DOF, where even the minor lateral movement can take a subject out of focus. iv. Zoom Lens Zoom lenses are beneficial because they allow for an array of different focal lengths lacking the necessity to transfer various prime (fixed focal length) lenses. This allows the photographer to quickly zoom in and capture the shot, then zoom back for a wider angle. While this is a wonderful advantage, there are optical limitations that should be understood when using a zoom lens. Each lens possesses a maximum aperture or lens opening used for capturing the light. On most zoom lenses, the maximum aperture can change as we zoom and the optics move to focus at the fresh zoom setting. These zoom lenses are said to possess a "variable" aperture. To attain the widest probable aperture, we are required to use widest zoom setting. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light v. Close UP or Macro Lens Close-up or Macro lenses are used for close-up or “macro” photography. The focal length ranges between 50-200mm. These lenses accomplish razor-sharp focus for objects in the macro focus distance, although they lose their capacity for sharp focus at far distance objects. These lenses permit the photographer to achieve life-size or larger images of subjects like wasps, butterflies, and flowers. vi. Fish Eye Lens A fisheye lens is a specialized, wide-angle lens that provides extremely wide images by altering straight lines into curves. It occasionally forms circular, convex, or oval pictures by altering the viewpoint and forming a 180° image. The range of focal length differs from 7~16mm in a fish-eye lens. vii. Tilt-Shift Lens The Tilt-Shift lens permits us fluctuate the vanishing points. If you are firing buildings, you can modify the perspective of an image so the parallel lines don’t converge, thus eliminating the distorting quality of the lens. The tilt-shift lens also allows us to desirably focus on an image where only particular parts of the image are in and out of focus inside the same plane. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light 5. Defects of Lens i. Spherical Aberration Spherical Aberration is an optical complexity which arises when all inward light rays terminate focusing at diverse points later passing through a spherical surface. Light rays passing through a lens near its horizontal axis are refracted lesser than the rays nearer to the edge or “periphery” of the lens and as a result, end up in different spots across the optical axis. ii. Chromatic Aberration Chromatic Aberration, also known as “color fringing” or “purple fringing”, is a common optical problem that occurs when a lens is either unable to bring all wavelengths of color to the same focal plane, and/or when wavelengths of color are focused at different positions in the focal plane. Chromatic aberration is caused by lens dispersion, with different colors of light travelling at different speeds while passing through a lens. FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light iii. Coma Coma is an aberration which causes rays from an off-axis point of light in the object plane to create a trailing "comet-like" blur directed away from the optic axis.
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