Aperture

In the study of optics, an aperture is an opening that limits the light passing through an optical system.

This opening defines a bundle of light rays from every point on an object. These rays then come together to form a focus on an image plane. An optical system often contains several structures that limit these bundles of light. These structures are known as stops. The most important one is called the aperture stop. It determines the specific cone of rays that the system accepts. This stop also controls the brightness at the image point and the ray cone angle.

Designing an optical system requires careful management of these stops. The aperture stop is not always the smallest stop in a system. Magnification or demagnification by lenses can make a large stop function as the aperture stop. Sometimes, light rays from objects away from the center are blocked by other surfaces. This effect is known as vignetting.

If an aperture is too large, the image may suffer from optical aberrations. These are distortions that occur when light reaches the edges of the optics. Sophisticated designs can use larger apertures while still managing these distortions.

In photography, the aperture stop is often adjusted using a device called a diaphragm. This diaphragm acts very much like the iris in a human eye.

The diaphragm changes the effective diameter of the lens opening. This diameter is called the pupil in biological terms. In humans, the pupil can constrict to 2 mm or dilate to over 8 mm.

By changing the aperture size, a photographer can control the exposure. A large aperture allows more light to reach the sensor. This is why lenses with large openings are called "fast" lenses.

Photographers use a specific measurement called an f-number to describe the aperture. The f-number is a ratio between the focal length and the effective aperture diameter. A lower f-number indicates a larger aperture. A higher f-number indicates a smaller aperture. For example, an aperture of f/2.8 is larger than f/8. One "f-stop" represents a change in diameter by a factor of about 1.41. This change results in a factor of two change in light intensity.

Many cameras offer a mode called aperture priority. This allows the user to select the aperture while the camera sets the shutter speed.

The size of the aperture also affects the depth of field. Depth of field describes how much of an image appears in focus. A smaller aperture, or a higher f-number, produces a longer depth of field. This happens because a smaller aperture limits the angle of the light cone. This reduction in angle allows objects at many different distances to stay in focus. Conversely, a large aperture creates a shallow depth of field. This is useful when a photographer wants to isolate a specific subject.

In astronomy, the aperture is a critical design parameter for telescopes. The aperture is typically the diameter of the objective lens or mirror.

Astronomers generally want the largest aperture possible. A larger opening collects more light from distant, faint objects. However, making very large apertures is difficult. They can be very heavy, expensive, and time-consuming to manufacture. In 1896, a massive 40-inch lens was polished for the Yerkes Observatory to help with these tasks.

Beyond photography and telescopes, apertures are used in many scientific fields. In light microscopy, apertures can refer to the condenser or a field iris. In laser technology, apertures are used for beam cleaning and energy control. They are also used in a technique called close aperture z-scan. In astronomy, researchers use a photometric aperture around a star. This is a circular window used to measure light intensity. These various uses show how controlling light through an opening is essential to modern science.