In photography, an objective lens is simply another word for the lens, or rather “lens” is short for “objective lens.” The lens' function in a camera is similar to that of a telescope or microscope; it allows for magnification of a given object as well as the gathering of all light bounced off of the object in order to focus it into a single image. As such, both telescopes and microscopes contain objective lenses, however, they vary greatly from that of a camera.
When speaking of objective lenses in general, all types of lenses for cameras, microscopes, telescopes and binoculars are included. An objective lens is a series of glass lenses (or one single lens at the front of the viewing object) through which light that is bounced off of an object is collected and concentrated into one specific image. In all forms, photographic, microscopic and telescopic, objective lenses are the closest part of the viewing apparatus (camera, telescope, etc.) to the object and the farthest item from the viewer's eye.
Photographic lenses contain a series of optical elements (curved glass plates) whose purpose is to gather and refract the light bounced off of an object into a concentrated single image. Each element layer is responsible for correcting the rays (bending them properly) so they reach the sensor or film inside of the camera as true to life as possible.
Lenses are divided into categories of focal length or how far the camera can “zoom” in to magnify an image. The greater the focal length, the greater the magnification. Focal length is measured on a lens in terms of millimeters (mm) and is displayed on the outside body of the lens itself. Lens focal lengths can range from as low as 21 mm to above 300 mm.
The other element, aside from focal length, that the photographer must be aware of (and set manually) is the aperture. The aperture is the size of the opening at the back of the lens that controls how much light is let through to the film or sensor. Apertures are measured on a scale known as f-stop; the smaller the number, the larger the opening and the more light allowed in. An aperture of f 1.0 is greater than f 5.6 or f 9, meaning more light is let in. Think of the aperture as the iris of a human eye: when the pupil is large, more light is let in; when it is small, less light is let though.
Types of Lenses
There are a wide variety of camera lenses, and all are objective. Lenses are generally labeled in categories of wide angle, normal and telephoto. The only difference among these categories is the measure of focal length: wide angle lenses are 35 mm or lower, normal between 35 and 70 mm, and medium telephoto to full telephoto are between 70 mm and 300 plus. The length, size and weight of the lens correspond directly with these categories of focal length; the greater the focal length, the longer and heavier the lens. Aside from these three categories, there are a variety of purpose-specific lenses that provide the photographer with specific effects; these lenses are less common in amateur photography and include fisheye, soft-focus, swivel, tilt-shift and infrared lenses.