FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
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. A lens with considerable coma may produce a sharp image in the center of the field, but become increasingly blurred toward the edges.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
iv. Astigmatism
Astigmatism (pointlessness) is a refractive error which occurs when a point sending light through a lens cannot be projected as one point. It appears as a line on the focal plane. Another explanation is that astigmatic lenses fail to represent horizontal lines in the vertical line form into the same image plane. . This effect mainly appears when biconvex or biconcave lens elements are used.
It has been observed that all lens defects can be minimized by combination of positive and negative lens and modification of design and coating.
6. Filters for Photography
Filter is a device of glass or other material interposed between the scenes being photographed for the purpose of deducting or eliminating certain colours, generally to which the film is most sensitive also called a colour filter or optical filter. Generally, two types of filters available are as follows:
i. Coloured Glass Filter
ii. Gelatin Dye – Sandwich filter.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
There are five main types of filters as per their usage:-
a) Correction Filter:
This is used to correct the imperfect colour sensitivity of the film and make it translate the subject into tones of grey, and the same brightness as the colours appear to the eye. It is also used in the colour to achieve desired colour temperature.
Filters do not really enhance color, but only absorb various wavelengths to upsurge the relative proportion. So the initial light source must have the colors in it which we want to start with. Some sources which lack various wavelengths, will not be added back using only filters. This is particularly true of many types of metal halide lighting. With other lighting categories, such as fluorescent, color temperature quantities will not make available the correct filter necessities subsequently color temperature theory is based on having a continuous spectrum, meaning light at all wavelengths.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
No Filter 85B = Cokin P30
b) Contrast Filter:
This is used to darken the reproduction of a certain colour for special effect. There are many circumstances, such as bright sunlit exteriors, where proper contrast is difficult to maintain. Exposing to highlights or darkness will leave the other under or over exposed. Low Contrast filters create a less amount of "localized" flare near highlight areas within the image. This reduces contrast by lightening close shadow parts, parting highlights almost unaffected. Soft Contrast filters comprise a light absorbing component in the filter which, when deprived of exposure compensation, will decrease contrast by darkening highlights too. Use this latter filter when lighter shadows are not according to the need. In both cases, the mild flare created from bright highlights is mostly used as a lighting effect.
c) Red Filters
Red filters forms strong effect and significantly enhanced contrast. They are generally considered as too "harsh" for various types of photography, however can be used to create remarkable creative effects.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
d) Orange Filters
Orange filters falls between red and yellow filters. It forms a nice balance of each one's properties.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
e) Yellow Filters
Yellow filters produce the most subtle effect of the 5 coloured filters. In most of the cases, difference is hardly evident, although it can help us to lift a photo.
f) Colour Compensation Filter:
Color compensating filter is used to make adjustments to the red, blue or green characteristics of light. These are applied in correcting for color balance, light source variations, different reversal film batches, and other color effects. They are available in density variations of Cyan, Magenta, Yellow, as well as Red, Blue, and Green filters.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
g) Special Purpose Filter:
It is used to take picture by the light of a single colour only. These are mainly used for scientific, aerial and other special photography (as telephotography in infra-red).
h) Infra-Red Filters :
In special circumstances, we use black-and-white or color infra-red sensitive films. For aerial haze penetration, recording heat effects, and forensic photography, they are invaluable.
Their color and tonal renditions are very diverse, though; from other film types (refer film producers for further details). Various filters are used to diminish undesirable visible light. Red, orange, and yellow filters used for panchromatic black-and-white film, can enhance contrast and alter color.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
i) Neutral Density Filters
When it is intended to uphold a specific lens opening for sharpness or depth-of-field purposes, or merely to get appropriate exposure when exposed to high light intensity, neutral density (ND) filters are used. This will absorb light consistently all over the visible spectrum, efficiently fluctuating exposure without needing a change in lens opening and without presenting a color shift.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
j) Polarizing Filter:
Polarizers allow color and contrast enhancement, as well as reflection control using optical principles different from any other filter types. Most light that we see is reflected light that will take up its color and amount from the matters we are looking at. White light, as from the sun, reflecting off a blue object, appears blue because all other colors are absorbed by that object. A small part of the reflected light reflect back the object without being absorbed and highlighted, holding the original (generally white) color of its source.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
k) Filter Factor
This is the relative increase in exposure required when an optical filter is used. The filter factors will not always be a persistent number. It depends on the colour of the filter as well as the hue &
saturation, the colour sensitivity of the film, the colour of the light and the colour of the subject.
Generally deeper the colour of the filter, higher the filter factor since it restricts more amount of light. Manufacturers’ advice should be followed to get desired exposure. Exposure value compensation is usually given in half or whole number such as ½ x, 2x, 3x etc.
7. Film Sensitivity
Orthochromatic films are sensitive to U.V., Blue-Green and slightly towards green of the spectrum. Fast orthochromatic films are sensitive to Yellow-Green and slightly towards yellow as well, but to a great extent to violet and blue. Panchromatic films are sensitive to all colours.
8. Colors of Light
Daylight is more blue and has higher colour temperature than the tungsten lamp (blue filter can reduce the tungsten lamp effect). It should be noted that colour temperature of daylight depends on the time of the day, weather and season. All films are over sensitive to blue, so subject like landscape, blue sky is much too light(white) in the final print. The tone and the details between sky and clouds are almost lost. This difference can be reduced by using a yellow filter. Orange green filter often is used in daylight for proper tonal compensation.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
a) Artificial Light
The most common lighting tools used today are tungsten light, fluorescent light, HMI light and electronic flash. Each technology has distinct advantages and disadvantages. While I will discuss each in detail, all have one key drawback in common, they are essentially fixed in terms of color temperature. Because the color temperature is fixed, these light sources are not able to emulate the ever-changing nuances of natural light. Digital cameras typically have presets for daylight, flash, tungsten and fluorescent light sources in an effort to arrive at pleasing results under these traditional illuminants.
b) Electronic Flash :
Originally, all flashes were manual in operation. That is, depending on the power of the flash, the distance from the subject, and the film speed, we had to find the proper f-stop to use. If we moved a few feet, we had to recalculate a new f-stop. In the 1960's, Honeywell pioneered the automatic flash. This was quite an improvement, for now as you moved around; the flash automatically provided the proper amount of light. This was accomplished by adding a sensor to the flash unit. As light travels to the subject and bounces back toward the flash, the sensor measures the light and quenches the flash when the subject has received enough exposure. Automatic flashes are highly recommended. Most modern flashes have three or four different automatic ranges, allowing different f-stops and working distances.
They can also be placed into manual mode for certain effects.
FORENSIC SCIENCE PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 27: Photographic Lenses, Filters and Artificial Light
9. Summary
1. 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.
2. The distance between optical centre and focus is called Focal Length. Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens.
3. If the focal length of two lenses is the same, the lens with the larger diameter will be brighter.
4. When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, neutral density (ND) filters are used.
5. All films are over sensitive to blue, so subject like landscape, blue sky is too light(white) in the final print.
