COURSE NAME: PHOTOGRAPHY AND AUDIO VISUAL PRODUCTION (VOCATIONAL) FOR UNDER GRADUATE (FIRST YEAR)

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1 COURSE NAME: PHOTOGRAPHY AND AUDIO VISUAL PRODUCTION (VOCATIONAL) FOR UNDER GRADUATE (FIRST YEAR) PAPER TITLE: BASIC PHOTOGRAPHIC UNIT - 3 : SIMPLE LENS TOPIC: LENS PROPERTIES AND DEFECTS OBJECTIVES By the end of the session, students will be able to; 1. Understand the simple lens, 2. Know the properties of a lens, 3. Understand the image formation and magnification, 4. Know the lens defects and methods of correcting these defects. Introduction A camera lens is an optical lens used in conjunction with a camera body. It is a mechanism which is used to store an image on a photographic film or any other material chemically or electronically. The classic definition of a lens is that it is an optical system consisting of at least two refracting surfaces out of which one is curved and other can be any non linear surface like parabolic or electrical. In terms of photography the understanding of a lens is quiet clear. That is to say we have a normal lens that is a lens which gives a normal view to the human eye. We also have a wide angle lens which gives a wider view to the subject and third we have telephoto lens which gives a closer view of the subject. Besides this we also have macro lenses, we have fish eye lenses and even pc lenses. To elaborate further a normal lens is the one, the focal length of which is equivalent to the diagonal of the negative. On the other hand a wide angle lens is the one whose focal length is less than that of the normal lens. And quite to the other side the telephoto lens is one whose focal length is more than that of a normal lens. Lens Properties Let us now see lens properties in relation to photography.

2 Aperture: We have the aperture for example; the aperture is the iris of the opening of the lens which is marked in terms of f1, f2.8, f4.5, f5.6 and so on this aperture works out the amount of light of light that has to go through the lens on to the film or the recording material. Aperture when it varies the amount of light also goes on varying. Depth of Field: Second point to note is that it has a depth of field, that is to say depth of field is the area within which a picture is within the acceptable sharpness. Even if we may focus on a particular object very sharply we will find that some area in front of it which is 1/3 and 2/3 behind is generally in acceptable sharpness. Focus: Now third factor is that with a lens we can focus sharply on any object. Distance Scale: We also have a distance scale that is to say we can measure the distance between camera and the object. Field Of View: Every lens has a field of view that is to say if you look through the view finder we see a particular area which is visible through the lens. In the case of normal lens we get a normal field of view, in the case of a wide angle lens we get a wider field of view and in the case of telephoto lens we can home on to a particular subject that is we can go closer to it or we can isolate it. Similarly we have a zoom lens in which case we can vary the field of view depending upon the composition and our requirement of the final picture. Image Formation: we know that there are basically two kinds of lenses, the concave lens and the convex lens which are shown in the diagram besides this we can also have a Plano convex lens, we can have a Plano concave lens, we can have a concave convex lens, we can have a convex concave lens and there are permutations and combinations. Let s now see image formation as I mentioned earlier we have a convex lens and we have a concave lens, let us see how each one of them behave. In the case of a convex lens we have an image forming across the lens or beyond the lens which is a real image which can be recorded as shown in the diagram. On the other hand the concave lens forms virtual image on the lens as shown in the diagram. Image Magnification: Let s see another aspect which is really important and it is known as image magnification. It is the ratio of the size of an object with relation to the image. Let us say if the distance between the lens and the object is u and the distance between the lens and the image formed is v then we have the magnification that is v/u, if we is longer and u is smaller we will get a magnification of a higher size on the other hand if the u is larger and v is shorter then we will get a image of a smaller size. This is applied in terms of photography that is to say when the subject and the lens distance is smaller that is u is smaller and v is larger we get a telephoto, we get a enlargement we get a bigger image on the other hand

3 when the u is larger and v is shorter we get a wide angle image of the subject that is to say if the lens moves closer to the object the image becomes larger and vice versa. Lens Defects: Let us see another aspects of lenses which is known as lens defects. The falling aberrations or the aberration which follow directly lower the sharpness of the image, they are chromatic aberration, spherical aberration, coma and astigmatism. We will see them one by one. Chromatic Aberration: Chrome means color, colors have different wavelength and behave differently. Violet which have a shortest wavelength moves closer to the length while the others move away from the lens accordingly. Similarly chromatic aberration is a type of distortion in which there is a failure of a lens to focus all the colors of the spectrum at a same converging point. It occurs because lenses have a different refractive index for different wavelength of light the refractive index decreases when they increase in wavelength. Chromatic aberration is noticed on the fringes of color along boundaries that separates the dark and bright parts of the image. Because each color in the optical spectrum cannot be focused on the same point. Chromatic aberration can be corrected by using a combination of a convex lens and a concave lens because concave lens behave quite the opposite of the convex lens and therefore they neutralize the effect. This is known as achromatism or minimization of chromatic aberration. Alternatively chromatic aberration can be controlled by monochromatic light which means light of a single color, however in terms of photography this is rarely used. Spherical Aberration: Let s see spherical aberration we know that lens is a part of circular ball which can be a perfect circle or a parabola or electrical, that is to say that radius of curvature of the lens is higher at the centre and lower on the periphery. It is an optical problem that happens when all the incoming rays end up focusing at different parts after passing through the spherical surface and end up in different parts or a spots across lens axis. In other words the parallel rays of incoming light do not converge at the same point because of this spherical aberration can affect resolution and clarity. There by making it hard to get a sharp image. Spherical aberration is not just caused by the lens design but also by the quality of material which is used in making this lens. Lenses are made up of poor quality materials and large bubbles can impact the light refraction. A perfect lens will have all the rays converge on to a single point, As illustrated in the diagram. Spherical aberration is most pronounced when the diaphragm of a lens is wide open that is fully open, it can be reduced by stopping down the lens. Stopping down the lens even by a single stop can dramatically use spherical aberration. Removal of spherical aberration is possible by using a Plano convex lens, or by stopping down the lens, or by using blocks on the periphery of the lens, or by

4 using a combination of convex and concave lens in contact, or by using two plano-convex lens of a same material at a suitable distance. Coma: Let us see yet another lens defect and that is coma, coma is an aberration in which light rays pass through off axis point causing the lens to focus at different point resulting in a blur. It is a lens aberration producing a symmetrical distortion of points in an image. In coma the image of a point source of a light is prevented from being broad into focus. It is a lens defect which results in points of light appearing in image not as points but as disks with comet light tails this refers to the blurring of the objects at the edge of field of view. The term has been taken from the shape of the image which is like a comet. Coma can be reduced by stopping down the lens. Astigmatism: We will now take another defect which is known as astigmatism. This defect arises due to development of different curvatures along different planes of a lens. This lens aberration makes a single point lighting source impossible to focus as a true point. It is imaged instead either as of a two shot lines at right angles at different distances from the lens. This is real problem because light rays passing through the lens at different planes get focused at different points. This occurs when a subject is much of the axis or when inclination of light is much higher, to remove this we block the rays coming from higher inclination and are cut off or we have an appropriate combination of convex and concave lens. Flare: Flare is yet another defect which occurs due to the various elements forming part of a lens. Now each elements of lens reflects certain amount of light back and some of it may go to the film there by causing a ghost image. Flare is much reduced when the glass air surfaces within the lens are coated with metallic fluorides and the interior of the lens is coated matte black. Flare has greatest effect on image shadows which integrates there by lowering contrast. We now know that flare is caused by the partial reflection of light from the lens. Mirror has 100% reflection while a lens does not have a 100% reflection. In the case of lens there is partial reflection and partial refraction. Let s take the case of a real view mirror in the car; what we see is our own reflection which is 100% while that of the interior of the car is basically refraction. Conclusion We can conclude this topic by saying that lens defect occurs due to various reasons. Primarily they affect sharpness of the image. They can be spherical aberration, chromatic aberration, coma or astigmatism. Flare is also one of the defects that can also be included in

5 this. Through various permutation and combinations it is possible to minimize these defects and thereby get an image which is acceptably sharp.

Ch 24. Geometric Optics

text concept Ch 24. Geometric Optics Fig. 24 3 A point source of light P and its image P, in a plane mirror. Angle of incidence =angle of reflection. text. Fig. 24 4 The blue dashed line through object