Technically, a camera does not need a lens: so why are there so many out there, and what are they all for? There are 6 broad categories:
general purpose
telephoto
wide angle
macro
niche
auxiliary
These are very arbitrary categories, and you could classify photographic lenses in may other ways. However, this is a starting point.
A “normal” lens is one whose focal length is equivalent to the diameter of the film or sensor. In the case of a full frame 35mm camera, that’s 45mm. It gives a perspective similar to that of the human eye. 50mm rather than 45mm is traditionally referred to as normal, since this was the focal length of the lens fitted to the very first “miniature” (ie 35mm) cameras made by Leica. For practical purposes, anything from about 38 to 60mm is a normal lens.
General purpose lenses are the kind of glass you tend to leave on your camera all the time. Obviously, any lens that is permanently attached to a retail camera has to fit this description, so general purpose lenses include zoom lenses with focal lengths from the equivalent of about 45mm on a 35mm camera, to about 180mm.
Zooms with a greater range than this, so called “super zooms” such as the 28-300mm lenses are also great general purpose tools on full frame 35mm cameras, although on small sensor digital cameras, where the conversion factor pushes them out to about 40-450mm, they are probably better thought of as telephoto lenses.
All these tend to be versatile and of adequate but perhaps not brilliant quality. Prime (ie non-zoom) lenses of normal focal length are also general purpose lenses. They tend to be of higher optical quality, partly because they are easier to manufacturer, partly because they have fewer surfaces and shorter bodies – extra glass surfaces and longer bodies degrade images due to light loss, glare and reflections. They are also faster – that is, they pass more light because the apertures can open wider. Where many zooms are limited to about f3.5 at the short end and f5.6 when extended, prime lenses rated at f2.8 and faster are relatively common.
Telephoto is a term originally applied to a lens whose focal length was longer than the film/sensor diagonal, while being physically shorter than its focal length. Now it refers to any lens with a significantly longer-than-normal focal length. Zooms from 70mm at the “short” end, and fixed focal length lenses of 70mm or longer are telephoto, although up to aout 90mm they are considered “short” telephoto lenses.
Short telephoto lenses are suitable (even ideal) for portrait photography and close-ups of people. They are also reasonable general purpose lenses if your main interest is in candids, street photography, and some stage and theatre work – they are useful for people at a distance of about twelve feet. Short teles are also useful to zero in on subjects where you want to crop and eliminate undesired elements from a scene.
Moderate-length telephoto lenses (85 mm to 130 mm lenses for 35 mm cameras) are ideal for portrait work, especially headshots and head and shoulder pictures. The 105 mm is considered to be the classic portrait lens as it does not distort features and does not crowd the subject.
Lenses in this range are fast enough to allow higher shutter speeds, so can be used without a tripod if necessary. Even a 200 mm lens can be easily hand-held in normal lighting condition.
These teles are very useful for bringing landscape and cityscape details closer, and parades, stage shows and even school pageants from the audience.
135 mm to 300 mm lenses are described as medium telephoto lenses, A fast 135 mm lens is a practical lens for candid and wedding photography and for action shots when the subject is not too distant, since it’s not too heavy, can easily be hand held and is great for isolating your subject from the background.
Longer focal lengths come into their own for nature, wildlife and animal photos. They are the obvious choice for sports, and bird photography without a long lens would have to rely on elaborate remote controlled systems.
With a 180 mm to 200 mm lens, individual and team sport photography is possible from the sidelines or the field, although you’ll need a more powerful lens if you are any further away. News photographers also value fast lenses in this range, but a slow moderate telephoto lens is an burden. Look for f3.5 or faster.
What a 200 mm lens does a 300 mm lens will do better, but most have a maximum aperture is ƒ/4, and zooms are slower still. This affects focusing, especially in low light as well as shutter speed. The ability of your SLR to capture low noise images at high ISO settings (800 ISO and above) will determine how useful such lenses are if you want to capture fast-moving images without subject blur. Fast 300 mm lens (ƒ/2.8 and above) are available, including some that are electronically stabilized, but at a cost, and to get the best out of all these lenses a tripod or monopod is a great asset.
Telephoto lenses from 400 mm to 800 mm and up are expensive, and here we are beginning to move into the “niche” category of professional sports and wildlife photography. Just one slower lens can cost more than any camera you will ever own. A 600 mm ƒ/4 lens is even more exotic while an 800 mm ƒ/5.6 lens can serve astrophotography and give sharp, colorful images of many stellar bodies.
Wide-angle lenses capture a wider angle of view than a normal lens does. Some may be described as super or ultra wide-angle lenses; the widest of all are the “fish-eye” lenses.
Wide-angle on a 35 mm camera start at about 35mm, although many photographers consider 28 mm to be the “standard” wide-angle lens. 24mm is still basic wide angle, and 20 mm to about 13 mm are the “super wide angle” lenses.
Wide-angle lenses provide large depth of field, making it simple to have foreground and background in focus. Their wide angle of view makes them excellent for working in tight spaces, such as building interiors, or close-up group pictures.
Above 28mm, wide angles begin to distort the image, especially subjects photographed close-up. This can be exploited to give unusual views of common objects, or corrected in software post shooting…but there are limits as to how much correction ca be applied. Because they take in the whole scene and most of it will be in focus, they are very forgiving of focus and composition. Photojournalist and people wanting candid street scenes in which people are not aware of being photographed (and therefore self-conscious) know that wide angle lenses can capture people in their shots even though the camera is not pointing at them.
They are also ideal for landscapes, taking in large areas of ground and sky, and travel photography, but are impractical in photographing close-up portraits or small subjects up close unless extreme distortion is what you are looking for.
[b[Macro[/b} lenses are normal or short to medium telephoto lenses optimized for close focusing. They are good general purpose lenses except for one fact: the “throw” from close focusing to normal distance subjects is very long, resulting in slow focusing and some “hunting” while they lock on to more distant subjects. They are necessarily longer than their normal cousins, which also makes them less acceptable for day to day use, and many find them too sharp for portraits…they show every blemish!
Niche Lenses are those that serve very special areas of photography or that only get attached to the camera on very rare occasions. The super-telephoto and fish-eye lenses mentioned above are examples, as are special lenses designed for hidden camera surveillance work. Scientific applications like those designed to properly focus light or energy waves outside the visible spectrum are others.
I also classify “trick” lenses here, like the flexible barrel lenses that allow you to choose different sections of the image to be in focus – a selectable “sweet spot” that draws the viewer’s eye (“Lens Babies” are a classic example). These are quite inexpensive relatives of the pricey “Tilt and Shift” lenses beloved of architectural photographers.
Tilt-and-Shift lenses might actually be the most versatile and useful of all prime lenses, but their bulk and high price are barriers to wider use. Price of course is related to the size of the market, but they are complex to make, the market for casual photographers and snapshooters is very small, and the evolution of perspective control in software suggests that this is not going to change.
What makes them so useful?
Firstly, perspective can be corrected - perfect perspective is only possible when the planes of the lens, the image sensor, and the object being imaged are parallel. Shift and tilt lenses let you achieve this.
Secondly, Shift lenses let you control where you place the centre line if your image, without changing the angle of your camera which in turn, lets you control converging verticals in buildings, window frames and other square subjects.
Thirdly, you can easily exclude unwanted areas of the foreground by lowering the lens (less grass/roadway/old sheds) or raising it (less unwanted sky).
That’s just a start, and I will provide separate Intel in this lens type, but to whet your appetite: you can achieve virtually infinite depth of field, or minimise it at any plane within the photo; you can add distortion for creative effects and correct exposure.
Auxiliary Lenses fit between the camera and lens or attach to the filter ring of the lens. They extend the capacity of the lens, often at some cost in quality or, with the rear fitted lenses, light loss. Rear fitted lenses are used to magnify the central part of the image, giving a telephoto effect. They come in 1.4x, 2x and 3x magnifications and generally the loss in light reaching the sensor/film limits their practical use: for instance, above 1.4x the light loss makes auto focus impractical.
Front element auxiliary lenses are the only option for cameras without interchangeable lenses. They are available in wide angle and telephoto versions, and offer moderate extensions to the original lens capacity, although the quality is generally average at best. Many cameras have a menu option which has to be set to allow the camera to operate correctly when an auxiliary lens is fitted.
Close-up filters also fall into this classification. Their simple construction means that image degradation in slight, and the more expensive ones are excellent.
