In its most basic form, a lens is a piece of glass or plastic with curved surfaces. Lenses are used to bend rays of light by exploiting a physical phenomenon known as refraction.
Light travels at different speeds in substances with different densities: it moves more quickly through air than glass or plastic, for example.
Therefore, when light crosses the boundary between air and the glass of a lens, it slows down. If the light enters the lens at an angle, the decrease in speed will also cause the rays of light to change direction – this is refraction. To see why that happens, picture a bulldozer: if the tracks move more slowly on one side than the other the bulldozer will turn. Similarly, when a ray of light enters a lens at an angle the part that enters first slows down and so the path of the light is bent.
A lens with surfaces that curve outwards will cause light rays to converge and is called a convex lens. It is this process that is used to focus light from far away on the to image sensor in a camera. The distance from the front element of the lens to the place where that lens will project a focused image is known as the focal length. Focal length is measured in millimetres and the longer it is, the more magnified the image will be and the narrower the angle of view.
Inside a Modern Lens
While in science a single piece of curved glass is considered a lens, the lens attached to your camera is much more complex. Within the body of a modern camera lens, you might find as many as 20 or more lens elements, all of which can be considered lenses in their own right.
Within the body of a modern camera lens, you might find as many as 20 or more lens elements, all of which can be considered lenses in their own right.
Lens elements are most commonly made from glass, though some high-end lenses, such as Canon’s L series, incorporate elements made from fluorite. Budget lenses, on the other hand, may have elements made from optical plastic.
Combinations of lens elements are used to ensure that the images captured by a camera are free from optical imperfections. A single lens may cause different wavelengths of light to be refracted to a different degree, which can lead to coloured fringes around the edges of objects known as chromatic aberration. By using combinations of lens elements, these issues can be reduced.
Focus control is another motivator for using more than one element. A single lens can only focus the light from a set distance away. By incorporating a mechanism which allows the user to move some of the lens elements, it becomes possible to change the distance at which a subject is in focus.
A lens also needs a mechanism to regulate the amount of light that reaches the film or image sensor to achieve correct exposure. This is achieved through the use of a diaphragm comprised of a number of movable blades, which allows the aperture to be adjusted in size, much like the iris of a human eye.
Stray light bouncing around inside a lens is another consideration, since it will cause lens flare and affect the contrast in an image. Therefore, lens elements have coatings designed to reduce reflections. Other coatings may be used to correct the color balance of the image, for example by blocking UV light.
While older lenses were purely mechanical, modern lenses contain a wealth of electronics. Motors allow the camera to automatically set focus and control exposure. Servo-powered zoom lenses for video cameras allow adjustment of the focal length with the press of a button. Some advanced lenses have in-built motion sensors to detect camera shake, then use motors to adjust groups of lens elements to stabilize the image when shooting handheld.
Types of Lenses
This basic technology can be adapted in a number of ways, depending on the purpose of the lens. Differences in construction will make some lenses more suited to certain kinds of work and will also affect their cost.
Prime lenses have a fixed focal length and so a fixed angle of view – if you want to change the size of the image you are capturing, you need to move the camera. A zoom lens has lens elements that can be moved in relation to one another to change the focal length and as a result, the angle of view. External zooming lenses achieve this by extending the body of the lens, which can cause issues if you are using a camera rig with a matte box. Internal zooming lenses, which move elements within the body of the lens to change focal length, are therefore more suited to filmmaking.
While most still photography lenses can be used equally for filmmaking, there are certain features unique to lenses designed for cinematography. If you change the focal length of most still photography lenses, you need to refocus the image which makes a zoom in or out impractical when filmmaking. Cinema zoom lenses are parfocal which means that they maintain focus when changing focal length.
The aperture settings on a still photography lens is marked in f-stops which derives from a simple ratio calculated from the focal length of the lens and the diameter of the aperture. However, factors such as the number of lens elements in a lens can affect the amount of light which reaches the image sensor in your camera, so you may get differences in exposure between lenses set at the same f-stop.
While this is not an issue for still photography, filmmaking requires a camera operator to be able to match the exposure between shots in a sequence. Cinema lenses have their aperture settings marked in T-stops (transmission stops) which indicate the level of light arriving at the image sensor so that two lenses set at the same T-stop will have matching levels of exposure.
Now that you have a better understanding of how lenses work, go to Videomaker’s Ultimate Guide for more advice on choosing the right lens for your next film project.
Pete Tomkies is a freelance cinematographer and camera operator from Manchester, UK. He also produces and directs short films as Duck66 Films.