A photographic lens (or objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct the many optical aberrations that arise.
There is no difference in principle between a lens used for a camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.

A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths and other properties.
A practical camera lens will often incorporate an aperture adjustment mechanism, often an iris diaphragm, to regulate the amount of light that may pass. A shutter, to regulate the time during which light may pass, may be incorporated within the lens assembly, or may be within the camera, or even, rarely, in front of the lens.
The lens may usually be focused by adjusting the distance from the lens assembly to the image-forming surface, or by moving elements within the lens assembly.

The lens elements are made of transparent materials. Glass is the most widely used material due to its good optical properties and resistance to scratching. Various plastics, such as acrylic (or PMMA), the material of Plexiglas, can also be used. Plastics allow the manufacture of strongly aspherical lens elements which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacture and performance. Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Moulded plastic lenses have been used for the cheapest disposable cameras for many years, and have acquired a bad reputation: manufacturers of quality optics tend to use euphemisms such as "optical resin".

The maximum usable aperture of a lens is usually specified, as the focal ratio or f-number, which is proportional to the focal length divided by the actual aperture in the same units. The lower the number, the more light is admitted through the lens. Practical lens assemblies may also contain mechanisms to do with measuring light, to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a bright image, etc.
The two main optical parameters of a photographic lens are the focal length and the maximum aperture. The focal length determines the angle of view, the size of the image relative to that of the object, and the perspective; the maximum aperture limits the brightness of the image and the fastest shutter speed usable.

Focal lengths are usually specifed in millimeters (mm), but older lenses marked in centimeter (cm) and inches are still to be found. For a given film or sensor size, specifed by the length of the diagonal, a lens may be classified as

·Normal lens: angle of view of the diagonal about 50°, the same as the human eye: a focal length approximately equal to the diagonal produces this angle.

·Wide-angle lens: focal length shorter than normal, and angle of view wider.

·Long-focus or telephoto lens: focal length longer than normal, and angle of view narrower. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens is designed to be physically shorter than its focal length.

The 35mm film format is so prevalent that a 90mm lens, for example, is always assumed to be a moderate telephoto; but for the 7x5cm format it is normal, while on the large 5x4 inch format it is a wide-angle.

The real difference between lenses of different focal length is not the image size, but the perspective. You can take photographs of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing your distance from the subject. But the perspective will be different. With the wideangle, the hand will be exaggeratedly large relative to the head; as the focal length increases, the emphasis on the outstretched hand decreases. However, if you take pictures from the same distance, and enlarge and crop them to contain the same view, the pictures will be truly identical. A moderate long-focus (telephoto) lens is often recommended for portraiture because the flatter perspective is considered to look more realistic.

Some lenses, called zoom lenses, have a focal length which varies as internal elements are moved, typically by rotating the barrel or pressing a button which activates an electric motor. The lens may zoom from moderate wide-angle, through normal, to moderate telephoto; or from normal to extreme telephoto. The zoom range is limited by manufacturing constraints; the ideal of a lens of large maximum aperture which will zoom from extreme wideangle to extreme telephoto is not attainable. Zoom lenses are widely used for small-format cameras of all types: still and cine cameras with fixed or interchangeable lenses. Bulk and price limit their use for larger film sizes.
The complexity of a lens—the number of elements and their degree of asphericity—depends upon the angle of view and the maximum aperture. An extreme wideangle lens of large aperture must be of very complex construction to correct for optical aberrations, which are worse at the edge of the field and when the edge of a large lens is used for image-forming. A long-focus lens of small aperture can be of very simple construction to attain comparable image quality; a doublet (with two elements) will often suffice. Some older cameras were fitted with "convertible" lenses of normal focal length; the front element could be unscrewed, leaving a lens of twice the focal length and angle of view, and half the aperture. The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture. Obviously the bellows had to extend to twice the normal length.
Good-quality lenses with maximum aperture no greater than f/2.8 and fixed, normal, focal length need three (triplet) or four elements. The widest-range zooms often have fifteen or more. The reflection of light at each of the many interfaces between different optical media (air, glass, plastic) seriously degraded the contrast and color saturation of early lenses, zoom lenses in particular, especially where the lens was directly illuminated by a light source. The introduction many years ago of optical coatings, and advances in coating technology over the years, have resulted in major improvements, and modern high-quality zoom lenses give images of quite acceptable contrast.
Source: various internet articles



Additional:
An aspheric lens or asphere is a lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.
The asphere's more complex surface profile can eliminate spherical aberration and reduce other optical aberrations compared to a simple lens. A single aspheric lens can often replace a much more complex multi-lens system. The resulting device is smaller and lighter, and possibly cheaper than the multi-lens design. Small glass or plastic aspheric lenses can be made by molding, which allows cheap mass production. Due to their low cost and good performance, molded aspheres are commonly used in inexpensive consumer cameras and camera phones

Film photography is anticipated to endure for some time, as dedicated amateurs and skilled artists often prefer the use of traditional and familiar materials and techniques. The comparison of resolution between film and digital photography is complex. While the resolution of commercial 35 mm color film is estimated to be 19 megapixel, this measure may be misleading.

When considered in the context of film and lens in a camera, typical film achieves a resolution of about 40 line pairs per mm, or 80 dots per mm. This is equivalent of about 5.5 megapixels in the image of 35 mm film. In contrast to that, advertised pixel counts on digital cameras do not account for the actual number of pixels used to store the image, nor the effect of the Bayer pattern of sensor filters on the digital sensor, nor the image processing algorithm used to interpolate sensor pixels to image pixels. In addition, digital sensors are generally arranged in a rectangular pattern, making images susceptible to moire pattern artifacts, whereas film is immune to such effects due to the random orientation of grains.

The resolution of modern black and white slow speed film, exposed through a high quality prime lens working at its optimum aperture yields usable detail at a scanned file size of greater than 30 megapixels, with consumer 35mm color negative film an effective resolution of over 12 megapixels is achievable and in an inexpensive 35mm point and shoot camera a resolution of over 8 megapixels may be achieved. Film also offers ease of processing with drop off services for processing available in many locations, in contrast to digital photography where the process of printing can require the time and effort of the user in areas where commercial digital to print services are not yet readily available. A market of online printing of digital images has developed in response to this demand.

One major advantage of Digital photography is the ability to manipulate images given access to a personal computer. Software bundled with nearly all digital cameras today allows the user to modify the characteristics of the recorded image to produce a more desirable final image. More sophisticated users may choose to manipulate or alter the actual content of the recorded image. Another important advantage is the ability to evaluate a shot in real time, to decide if you wish to take another shot of the subject, in a different manner or with different settings. Images that are not appealing may be erased, freeing up storage space.

Nonetheless film still has advantages over digital, at least with current technology. One of the main advantages is its latitude, that is, the ability to produce a good image from over or under exposed negatives. Digital images which are slightly overexposed can lose all data in the highlights, and underexposed digital will lose significant shadow detail. Film, on the other hand, can be greatly over or underexposed and still be able to produce a normal image. This is particularly true with black and white film.

One useful approach to deciding between a film or digital camera is to consider the ultimate medium of display for your photographs. If your pictures are for display on computer or television screens, small format prints such as snapshot or 8x10" photo paper, then the resolution provided by a 5 to 9 megapixal digital camera may be adequate. If your final medium is magazine or poster images, or slides for projection, then you may need the resolution provided by film in order to get a satisfying product. Lower resolution images will look soft and/or noisy when printed or projected in large format.

The Oxford Companion to the Photograph, ed. by Robin Lenman