Did you know? The word lens comes from the Latin word of the same name meaning 'lentil', because the basic convex (curved) lens resembles a lentil.
As discussed in LESSON 1: Knowing Your Camera, a lens performs two jobs: a) to gather a large amount of light from the scene, and b) to focus the light from a plane of the scene to create a visibly sharp image. In fact, there is a third function that almost all lenses have. They take what arrives as a curved image from the scene and transform it into the flat image that is recorded by the camera. Lenses that do this are called rectilinear (rectus 'straight' + linea 'line') lenses. Here is an example of an image of how an image looks both with and without this rectilinear transformation:
A lens brings the light from a scene into focus onto the camera sensor perfectly, but only from the plane of focus. This is an imaginary geometric plane at a distance in front of the camera, perpendicular to the axis of the lens. The further away from this plane that we find elements in the scene, the more out of focus they become in the final image. This is particularly evident when the aperture is completely open when capturing an image. You can see that the term "fo•cal length" in the image below falls on the plane of focus, but the lines above and below this term become increasingly out of focus:
When one reduces the size of the aperture, the range of the image that we see as being in focus increases. This range is termed depth of field. Here is the same image shot with a very small aperture:
With this in mind, one can decide to use a larger aperture when one desire to have less of the scene in focus (for example, a portrait of a single person), or use a smaller aperture when one desires to have more of the scene in focus (for example, a large group of people).
Depth of field is also affected by the distance from the lens to the subject in focus. As one moves the lens closer to the subject, the depth of field decreases. As one moves further away from the subject, the depth of field increases.
The plane of focus (what you want to be in focus in the image) can be adjusted manually by rotating a ring on the lens called the focus ring. Cameras also have a technology called autofocus, a feature which automatically focuses the lens on a point in the scene. Autofocus is especially useful in capturing split-second moment where the photographer does not have much time to manually focus on the subject.
When the parallel light rays from a scene hit the lens, they meet at a point. The distance from the lens to this point is called the focal length and is measured in millimeters.
The focal length of a lens is important because determines two characteristics of a lens: a) the angle of view from which it can gather light, and b) the magnification of objects in the scene. The shorter the focal length, the wider the angle of view and the less magnification of objects. The longer the focal length, the narrower the angle of view and the greater the magnification of objects.
The Normal Lens
When the focal length of a lens is equal to the diagonal length of the camera's sensor, the image that is produced approximates that of human vision. If we take a picture of two objects which are at different distances from the lens, then we switch places with the camera and look at those same objects, their relative sizes will appear the same to us as they do in the picture.
Short Focal Length Lens
A short focal length lens captures light from the scene in a wider angle than a normal lens does. This is why it is commonly called a wide angle lens. I'm addition to getting a wider view of the scene, such a lens also has a lower magnification than a normal lens. The parts of the scene seen through a short focal length lens will be smaller than when seen through a normal lens. The relative sizes between two objects in a scene will be exaggerated: objects closer to the lens will appear larger in comparison to objects further away.
These characteristics are useful when you need to photograph a large group of people or when you want to emphasize an object in a scene by making it appear larger than it really is (for example, a bride's bouquet).
Long Focal Length Lens
A lens with a long focal length has a narrower angle of view than that of a normal lens. It also provides greater magnification of objects in a scene. These characteristics are useful when you want to get a large image of a subject that is small or far away, such as a bird on a tree branch.
Also the narrow angle of view allows the photographer to capture the subject in the foreground and eliminate much of the unwanted background. The red outline in the image below indicates the angle of view of a lens with a very long focal length:
A zoom lens is simply a lens that lets you change its focal length. Some zoom lenses have focal lengths ranging from short to very short, while others have focal lengths ranging from long to very long. And there are some with focal lengths ranging from wide to long.
A zoom lens allows one to change the magnification of a subject with a twist of the zoom ring without having to move closer to that subject. This feature is quite useful when moving around might cause a commotion, such as when photographing during a wedding ceremony.
Some lenses only focus light at a single focal length. These are called prime lenses. Since a zoom lens includes many focal lengths, you might wonder why someone would choose to use a prime lens over a zoom lens. While a zoom lens is versatile with regard to magnification and angle of view, prime lenses have a number of advantages over zoom lenses. They are smaller, lighter, can capture a larger amount of light and render a higher quality image than a zoom lens.
To increase the size of an object in a scene, you can either use a lens with a longer focal length or you can move the camera closer. However, most lenses cannot focus on objects that are too close to them. Special lenses, called macro (or sometimes micro) lenses, are able to focus on objects much closer than non-macro lenses. For example, trying to get close to a bowl of popcorn with a non-macro lens would result in an image with about five or six popcorn kernels across the frame of the image. With a macro lens, one could get close enough so that a single kernel occupied the entire image frame. Because of this special characteristic of macro lenses, they are best employed when photographing small objects, such as the bride and groom's wedding rings.