A lens is probably the most important element in a camera, because the image quality is greatly affected by the quality of the lens. Compact cameras have fixed lenses, however SLR cameras use interchangeable lenses.
Different types of lenses are available for various applications. These lenses are used for photographing different subjects from long to close distance or to cover a large or small area.
The focal length of the lens is usually expressed in millimeters and is engraved on each lens very prominently. It is the distance between the centre of the lens and the film when the lens is focused at infinity. It determines the image size on the negative, irrespective of the size of the negative. The longer the focal length, the larger is the image. Focal length and image size are directly proportional. If we wish to increase the size of the image on negative, we have to use a lens of higher focal length.
For single lens reflex type of cameras a variety of lenses are available in the market like wide angle, telephoto, zoom, macro etc. these lenses are used for photographing different subjects from long to close distance or to cover a large or a small area.
The aperture is the size of the hole through which the light passes. The size of the hole dictates the depth of field.
The aperture is described in ‘f’ stops, inversely in proportion to its size. A small or narrow aperture will give an image sharp focus from foreground to background, and is represented as a high number.
The relative aperture or speed of a lens is a measure of the light transmission of a lens. It is expressed in the form of the ratio of the focal length to the diameter of lens. For example, if the focal length of a lens is 50 mm (standard for all 35 mm cameras) and effective diameter is of 25 mm, its relative aperture would be 50:25, that is, 2. In actual practice the relative aperture is expressed in f-numbers. The amount of light passing through the camera lens is controlled by the diaphragm, which can be closed down or opened up. The f-number on a lens denotes the size of the opening through which light is allowed to pass. On any lens the f-numbers are designated generally as f-1.4, 2, 2.8, 4.5.6, 8, 11, 16, 22, 32.45. The higher the f-number, the smaller is the diaphragm opening. Each consecutive f-number reduces the amount of light falling on the film by half and so requires twice the exposure of the preceding larger diaphragm opening.
The covering power of lens depends on its focal length and the negative size. The focal length should be on e which should cover the specific negatives size. If it can cover the negative evenly and sharply from corner to corner, then the lens is said to have adequate covering power. A lens with a long focal length may have very little covering power, and vice versa. For example, a 135 mm lens of a 35 mm camera will cover negative size 1 X 1 ½ inches whereas 90 mm lens of a 4 X 5 inch camera will cover a negative size 4 X 5 inch.
Unless they are very expensive, lenses used in photography suffer from certain flaws which create distortions in the image. Distortion comes in two basic forms: pincushion and barrel.
Less expensive are the point and shoot cameras with focus-free lenses in which the focus is permanently set. Lenses for better cameras come in two types: manual focus and auto-focus. With a manual focus camera one has to turn the focusing ring to focus the lens; the subject appears sharp in the viewfinder. With auto-focus lenses, one has just to point the auto-focus frame in the viewfinder at the subject and lightly press the shutter button. The lens automatically focuses on the subject.
The depth of field is a creative tool in the hands of an imaginative photographer. The depth of field of a lens is the zone of sharpness. This zone extends in the front and back of the subject in which the image is sharp. The depth of field of any lens depends upon the focal length of the lens, the relative aperture and the distance between the object and the lens. It increases when: (1) The distance between the object and the lens increases; (2) The focal length of the lens decreases; and (3) The aperture is stepped down.
In most of the cameras (not Box) a depth of field scale is given for each aperture setting. The depth of field plays a very important part in picture-making. By increasing or decreasing it, one can entirely change the character of a picture. The depth of field is maximum when the aperture is minimum, and it is minimum or nil when the aperture is full open. So, by decreasing the depth of field (larger aperture) unwanted things in the background or foreground can be thrown out of focus. Isolation of any subject is possible. A three-dimensional effect can be created; illusion of different plains can be achieved and above all the picture can be softened. On the other hand, if the depth of field is increased (smaller aperture) everything in the foreground and background including the main subject will be sharp and in focus. The overall effect of the picture will be harsh. Therefore, before you take the picture, decide about the effect you want to achieve and then accordingly select the aperture.
Resolving power is the capability of a lens and of a photographically emulsion to reproduce sharply and accurately all the fine details in a picture. This resolving power is measured by the maximum number of the alternate lines of equal width of black and white, generally measured across a width of 1mm of the image and expressed as so many lines per mm.
The lens shows a somewhat irregular pattern of resolving power across the image field due to its aberrations. But in general it is the best at the centre and becomes less and less towards the edges.
Performance of any lens depends on five factors: Sharpness, Color correction, Flare, Evenness of light distribution and Distortion.
ultimately determines the performance of a lens and is determined by the lens manufacturer, depending on the money he is going to spend in solving the physical problems involved in lens manufacturing. Faults like aberrations, curvature of field, coma and astigmatism have to be solved by the lens manufacturer and they ultimately determine the quality and performance of a lens. Sharpness is essentially incompatible with high speed lens and large covering power. Sharpest lenses are almost characterized by limited covering power and relatively slow speed. Sharpness of any lens improves as the diaphragm is stopped down.
Light of different wavelengths (color) is not uniformly bent (refracted) by glass. If a simple and ordinary glass element is used, the result would be hazy as different colors will not focus at the same point. To correct this lacuna, all photographic lenses are color-corrected to some extent, the degree varying with the design of the lens. Today, sophisticated computers design the construction of lenses for use in top quality color reproduction and to make separations for color prints.
A part of light passing through a lens does not reach the film but gets reflected by the surfaces of the lens elements, inside of the lens mount or inside of the camera and eventually falls on the film in the form of flare and fog. This flare appears as light spots on the negative which are generally circular or oval.
Evenness of light distribution:
Almost all lenses deliver proportionately less light at the edges of the negative than at the centre. As a result of this uneven distribution of light, the edges and corners often turn out darker than the centre. More expensive lenses take care of this technical problem.
This lens fault makes straight lines appear as curves. This defect occurs more with wide-angle lenses.
Different types of lenses are given below:
A standard lens is one, which has a focal length equivalent to the diagonal of negative size. For 35 mm cameras (negative size 36X24 mm), a lens of focal length 50 mm is considered as a standard lens.
A standard lens offers an angle of view, which is approximately same as that of human eye.
A standard lens can be used for shooting head to shoulder shots, but it should not be used for full-face close-ups, otherwise a noticeable distortion in facial features is resulted.
A lens of focal length shorter than the standard lens is called a wide-angle lens. (This is because; it offers a wider angle of view than the standard lens.)
Wide-angle lenses (28 mm) are widely used for landscape photography, since they offer wider view, and a good depth of field.
Extreme wide-angle lens of smaller focal lengths (17 to 28 mm) offer unusual angle of view, and are used for their dramatic effects.
A lens of longer focal length is called a telephoto lens. A telephoto lens has a narrow angle of view, and hence distant objects appear bigger and hence nearer!
A lens of 85 or 105 mm focal length is called a medium telephoto lens and is widely used in portrait photography. Long telephoto lenses (300-600 mm) are used in wildlife and sports photography.
A telephoto lens offers a small depth of field, such that the subject can be isolated from the background.
Zoom lens offers a range of focal length which is variable, and enables change of focal length without the need to change lenses.
A 28-70 mm lens is called a standard-zoom, whereas 70-300 mm lens is called a telephoto-zoom lens. Nowadays, lenses of 28-300mm zoom are also available, which are called super-zoom lenses.
Macro lenses are specially designed to give optimum performance at high magnifications. They also have wide aperture to allow precision focus of dim areas at close distance.
A macro lens allows you to focus while standing very close to your subject in order to take close-up shots. It is designed to maintain superior sharpness and contrast when focused on a subject that is very near the camera. Most macro lenses are made in a single focal length. Although some have wide-angle or telephoto focal lengths, typically a macro lens has a normal field of view.
Most macro lenses can focus from infinity right up to a distance that gives an image with magnification up to half life size.
Supplementary close-up lenses are used in close-up photography, which is normally not possible with a standard lens. These can be screwed onto the standard lens, and are available in various strengths, such as-Dioptre +1, +2, +3 etc.
Many zoom lenses also have a macro mode; however their performance is not a s good as true macro lenses.
Apart from the above, macro-lenses are available, which are specially designed for small camera to subject distances.
Here are the accessories for lens:
Filters are used over the lens for several purposes. In B&W photography, filters are used to improve contrast. In color photography, filters are used to correct any color cast. In addition to above, some filters are used for their creative effects.
Several types of filters are used in color photography, such as: UV filters, Polarizing filters, Graduated filter, Neutral density filters, creative-effect filters, etc.
Color correction filters:
These filters are primarily used to correct any color cast produced by light, so that the color rendering is correct.
UV filters are used to absorb Ultraviolet rays, which produce a bluish cast in daylight. They are especially useful as high altitudes and beaches, where the effect of UV rays is more pronounced. A ‘skylight filter’ is a UV filter with a pink tint, which provides a little warming effect to the photographs. A good quality UV filter can be permanently left on the camera lens, as it protects the lens from scratches and physical damage.
Polarizing filters are used to reduce the reflections from shiny surfaces, such as glass, metal, water etc. These are also useful in taking landscape pictures, as it reduces reflections from the foliage, and thus helps improving the color saturation. Polarizing filters are also used to take photographs of the subjects beneath the surface of water, such as in beaches.
Neutral density Filters:
ND filters are gray colored filters, and are used to reduce the light entering the lens, and thus they help to increase the exposure time. These are available in various strengths. These are especially useful in photography of waterfalls, where slow shutter speeds are desirable, to give water its flowing character.
Also known as sun-shade, it is a must for all camera users. The function of a lens hood is to shield the extra light, other than that coming from the subject itself. Further, it makes the image look brighter in the view-finder. Lens hoods are available in square, conical, rectangular and circular shapes and in various sizes to suit different lenses. Fix the hood on the camera lens for all types of photography and especially for against the-light pictures.
Sometimes you may want to take close-up pictures of a small object, and then a supplementary lens or lenses are very useful. These are extra lenses of various focal lengths, which change or shorten the focal length of the camera lens. They are fitted in mounts and you can fix them over the camera lens just like a filter. These lenses can be purchased in various focal lengths and sizes to suit your cameras. Supplementary lenses are useful for all types of close-up photography and for copy work.
Caring of the lenses is not a difficult task and it must be religiously done to ensure their long lives. Care should be taken at the time of fixing and removing the lens from the body of SLR camera. No force should be used.
Do not store your camera and lens in leather or raxine cases that are hygroscopic.
Use your equipment regularly (especially in the rainy season) instead of packing it for long periods.
Expose the lenses to sunlight. The UV rays present in the sunlight kill the fungi spores.
Fungus mostly starts at the lens mount and it thrives on dust/dirt therefore it is necessary to clean the equipment regularly.
Remove your lens periodically from the camera mount. Examine it against the light to see any invasion of fungus. If you notice the fungus (or if you are in doubt), get it cleaned immediately.
All interchangeable lenses should have caps (at rear and front) when not in use. Lenses should be kept in padded compartment of the bag when not in use. It is advisable to keep a UV filter on each lens permanently, which takes up the dirt and dust that may otherwise damage the front lens element. From time to time, front and back of the lens need cleaning with a soft brush blower. If available, one may use a few drops of a lens-cleaning solution (available from any optician’s shop) on the lens element, gently cleaning the lens by using a circular motion, starting at the centre of the glass and working out to the edges. A lens tissue should be carried in the camera bag and on an outdoor assignment the lens should be cleaned on a daily basis.