Lens types

Key characteristics of lenses

Most camera systems offer a variety of lenses to suit the needs of individual photographers. This variety falls into general categories - normal, wide-angle and telephoto lenses, for example. These three categories are based on the different angles of view, or perspectives, provided by each group, and the differences are due to the focal lengths of the lenses. Understanding the key characteristics of these different types of lenses is important.

You should also know something about the various options available within the general categories. If you are planning on purchasing a new lens (or even renting or borrowing someone else’s lens), this information will be helpful in confirming that the lens will meet your needs.

KEY CHARACTERISTICS OF LENSES

The key characteristics of lenses (other than sharpness) are:
1. Aperture and Depth of field;
2. Focal length; and
3. Focusing.

APERTURE AND DEPTH OF FIELD

Aperture, or ƒ-stop or ƒ-number, refers to the size of the opening in the lens. An aperture is in fact another word for “opening,” but when a photographer refers to the aperture that is being used (or the ƒ-stop), he or she is referring to the size of the opening.

Light passes through the aperture. Therefore, the bigger the aperture, the more light will pass through the lens. A lens that has a very wide maximum aperture is said to be a “fast” lens because it permits fast shutter speeds in dim light. Generally, faster lenses are expensive. (We have yet to meet a photographer who regrets spending the extra amount for a fast lens.) A lens that can be set to a very small aperture provides a great deal of depth of field (permitting more to be in focus).

Depth of field is the foreground to background zone of an image that is in acceptably-sharp focus.

Depth of field increases with smaller lens apertures, and decreases with wider apertures. A small aperture, such as ƒ/22 provides a greater zone of focus (more depth of field) than a medium aperture of, say, ƒ/8. But ƒ/8 provides more depth of field than the lens’s widest aperture, which may be ƒ/3.5, ƒ/2.8, ƒ/2 or wider.

The maximum aperture of the Nikkor 45 mm lens at left can be seen on the aperture ring. It is ƒ/2.8 and would provide very shallow depth of field, indeed.

FOCAL LENGTH

Focal length tells the photographer what a lens’s angle of view is, and therefore determines the scale of an image at a given subject distance. If you take a picture of the same subject from the same camera position using lenses of different focal lengths, the lens with the shortest focal length shows the view on the smallest scale, but also covers the widest angle of view. A lens of longer focal length covers a smaller angle of view, but records the subject on a larger scale. A wide-angle lens has a short focal length, whereas a telephoto lens has a long focal length.

It is sometimes easier to think of focal length as a measure of a lens's magnification, with a long focal length magnifying a portion of a scene more than a short focal length does.

MEASURING FOCAL LENGTH

Focal length is a measurement that is taken when the lens is focused at infinity. It is the distance between the lens’s optical center and the film plane (or the sensor in a digital camera), or more technically, the distance from the rear nodal point of the lens to the point where light travelling through the lens is focused on the image plane. Focal length is expressed in millimeters - e.g. a 24 mm lens, or a 300 mm lens.

FOCAL LENGTH NUMBERS

The significance of specific focal length numbers changes with film format or the size of a digital camera's sensor - See Using 35mm SLR lenses on dSLR camera bodies. A normal lens for a camera that uses 35 mm film has a focal length of about 50 mm, whereas a normal lens for a medium-format camera that uses 2-1/4" by 2-1/4" film has a focal length of about 80 mm. (A "normal lens" is one that is neither wide angle nor telephoto, but shows perspective about the same as normally seen with the naked eye. Although a 50 mm lens on a 35 mm camera is generally accepted as having this perspective, in reality a 43 mm lens on 35 mm camera shows a scene closer to the way we see things.)

FOCUS

Our eyes adjust automatically for distance, focusing our vision with no conscious effort so that objects we see appear sharp. A camera's lens must also be focused to get a sharp image of an object. This is accomplished by moving the lens closer to or further from the film or a digital camera's sensor. With many lenses, this is accomplished by turning the focusing ring that is on the barrel of the lens, thereby extending the barrel and moving the lens. (Lenses equipped with internal focusing - known as “IF” lenses - focus without the barrel of the lens extending.) For distant objects, the lens is brought closer; for objects that are near, the lens is moved away from the film/sensor.

Different lenses have different focusing characteristics. Some focus automatically while others require manual focusing. Some sophisticated autofocus lenses are capable of tracking a moving subject, automatically keeping the subject in focus wherever it goes. This is known as “dynamic autofocus.” Some basic point and shoot cameras cannot be focused at all; their focusing is preset for a specific minimum distance. (The camera’s manual explains its focus limitations.)

LENS SELECTION

Various lens types for 35 mm cameras are described in the following subsection, entitled "Lens selection." Please bear in mind that other film formats and digital cameras may have a similar selection of lenses to achieve identical purposes, but their range of focal lengths to provide similar views is different.