The main identifying feature of a lens is its focal length. Lenses with a single fixed focal length are known as prime lenses.
The focal length of a lens is a measure of how strongly it converges or diverges light. A lens with a short focal length is stronger than one with a long focal length. In other words, short focal lengths bends the rays more strongly, bringing them to focus in a shorter distance. Short focal length lenses have a wider angle of view. Conversely, a lens with a long focal length is weaker, and bends the rays more feebly, bringing them to a focus in a greater distance. Long focal length lenses have a narrow angle of view.
A lens with a focal length about equal to the diagonal size of the film format is known as a normal lens. For 35 mm film format cameras, the diagonal is 43 mm. While 45 mm was once a common normal lens focal length, 50 mm or 55 mm is more typical (and I have no idea why). A lens with a shorter focal length is often referred to as a wide-angle (typically 35 mm and less). A lens with a significantly longer focal length may be referred to as a telephoto (typically 85 mm and more).
There is much more to wide-angle and telephoto lenses than simply making a subject bigger or smaller (closer or further): they should be used to control perspective. Wide-angle lens exaggerate or stretch perspective. Near objects appear closer, while far objects appear further away. Telephoto lenses have the opposite effect and compress or flatten perspective.
Perspective control can be a powerful compositional tool in photography, and often determines choice of focal length lenses used. I would say more, but this article is an overview of lens options, and not about composition.
Most lenses have an adjustable iris, which is made from a number of overlapping/interlocking blades (typically between five and eight) that open and close to adjust the amount of light passing through the lens. This structure is more commonly known as a diaphragm. Higher numbers of blades are generally better, since they create a rounder hole for light to pass through.
The diaphragm is used to set the lens aperture (literally the size of the hole through which light passes). Lenses with large apertures are said to be fast, because they can permit enough light passage to enable the use of a faster shutter speed. Conversely, lenses with a smaller maximum aperture are slow, because less light is transmitted, and a slower shutter speed is required.
Aperture sizes are referred to as “f-stops”. The numerical value of the f-stop is the result of the lens’s focal length (numerator) divided by the diameter of the aperture (denominator). In this equation, if a lens has a fixed focal length, as the aperture gets smaller, the denominator also gets smaller, so the f-stop number gets bigger as aperture become smaller (e.g. 50 cm/10 cm = 5. 50 cm/5 cm = 10). The results of these calculations are a common set of f-stop values: typically f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, where f/1.4 is the widest aperture and f/22 the smallest.
Each of these f-stop (as written) permits twice and much light to pass as the next f-stop to the right, and half as much light to pass as the f-stop to the left.
Lens aperture setting rings are commonly click-stopped for these aperture values. Some lenses have fractional stops. For example f/1.8 lenses are common, and of course, fractional stops can be set deliberately by ignoring click-stops.
The maximum aperture size, or speed of the lens, is the important factor, and usually emblazoned on the front on the lens along with the focal length and manufacture’s name. Not all aperture setting perform equally well, and generally, the best (more optically perfect and aberration free) overall aperture is somewhere around the middle of the range.
Depth of Field
There’s more to aperture size selection than simply controlling the amount of light entering the lens. Different aperture sizes have differing “depths of field”.
Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance. Depth of Field (DOF) is the distance between the nearest and farthest objects that appear acceptably sharp.
Large apertures (such as f/2) have a shallow depth of field, while small apertures (like f/16) have a deeper depth of field. In many instances, it can be desirable control the depth of field. Sometimes, its good to have the entire image sharp, and in other instances, a small depth of field will emphasise the subject while de-emphasizing the foreground or background. In other words, these components can be blurred and out of focus.
Most lenses feature depth of field markings that show the depth of field for each aperture setting against the lens’s distance scale. They literally indicate the limits of acceptably sharp focus either side of the precise distance at which the lens has been focused.
In the world of 35 mm photography, zoom lenses are relative newcomers. They are optically more complex, and it wasn’t until the late 1970s that zoom lenses achieved sufficient quality to become commonplace.
As explained in the section about aperture sizes, changes in the focal length of the lens alter the value of aperture sizes. This is why zoom lenses are normally identified by two maximum aperture values (for example, f/4 ~ f/4.5).
The point of a zoom lens is to combine the performance characteristics of equivalent prime lenses within its range, to offer move flexible control perspective. It’s an easier option than carrying and changing lenses, but isn’t a device to save the photographers from moving to the correct position to compose a shot. Sadly, this is often the way zooms are used.