In a broad sense a lens is a piece of glass or groups of pieces of glass called elements that gathers light reflected off of an object and focuses & projects it onto an image sensor forming a photograph.
An example of a simple lens would be a common magnifying glass and we are all familiar with one of those. The lens of your camera works pretty much the same way but in a much more refined and complicated fashion.
When selecting a lens one has to consider the type of photography one intends to be shooting, for example a wide angle lens may work better for architecture or landscape photography but not as good for portraits or sports photography.
There are basically two types of lenses, Primes & Zooms.
A prime lens has a set focal length.
A zoom lens covers a range of focal lengths.
Prime lenses are generally faster (have a wider aperture) and are sharper and less expensive than zooms. However, zooms can cover a range of focal lengths and are pretty sharp as well which makes them very convenient in varying situations. Don’t fall into the which is better debate. Learning how to use both types will help you evolve into a much better photographer.
The two main characteristics of a lens is it’s Focal Length and it’s Aperture.
Focal length determines the field of view and the degree of magnification of what’s projected onto the sensor while aperture governs the amount of light and the Depth of Field of what is projected onto the sensor.
The focal length of a lens is the distance between the lenses convergence point and the image sensor when the subject is in focus. It is measured in millimeters. Aperture is the adjustable hole or iris in the lens that the light goes thru. The bigger the iris, the more light is able to reach the sensor but the tradeoff is less depth of field. That makes lenses with larger apertures highly desirable and highly expensive as well.
Aperture is measured in a sequence of f-stops, each one lets in half as much light as the previous one. A lens with a maximum aperture of f/2.8 will have the following sequence of f-stops: f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc. depending on the lens. These properties and more are indicated by the numbers on the lens.
A Canon 18-55mm kit lens
EF/EF-S is the mount type.
18-55mm is the variable focal length of the lens.
1:3.5-5.6 is the maximum aperture of your lens across the zoom range. With this lens the more you zoom, the smaller the aperture becomes. Wide open at 18mm, the maximum aperture is f/3.5, but when you zoom in toward 55mm, the aperture will slowly close until it reaches a point where the maximum aperture now becomes f/5.6.
Ø58mm indicates the filter size.
IS means Image Stabilization which helps reduce camera shake which results in sharper images shot at slower shutter speeds.
AF/MF is the auto focus to manual focus switch.
I hope this helps you better understand lenses as you evolve with your photography.
Remember to consider the types of images you will be creating and above all else…
Have Fun Shooting!