The Aperture

The aperture of a lens is a variable size opening which controls how much light enters the camera. It is one of two elements of exposure, the other being shutter speed. But the aperture doesn’t only affect the exposure; it also affects sharpness and depth of field, two important aspects of creative control within the scope of photography.

Apertures are designated by “f-stops”, and written as a ratio (1:2.8) or as an f-number (f/2.8) The smaller the number, the larger the opening and the more light enters the camera. The larger the number, the smaller the opening and the less light enters the camera. Starting from the widest aperture, each successive full stop lets in half the light of the previous stop. Some lenses have half-stop clicks, others only full stops. Digital cameras can typically select between half stops and one third stops through the camera’s software.

Apertures can be set with a variety of different controls. On most cameras, a ring or lever is located on the lens that rotates or moves to increase or decrease the aperture size mechanically. In the case of a digital SLR, a dial or thumbwheel is used to set the aperture electronically, while compact digital cameras may do this through a system of menus.

Every lens has a range of apertures, from one (for a simple, fixed-lens camera) to many (seven or more for a typical interchangeable SLR lens). Normal lenses typically have apertures ranging from f/1.4 to f/22. However, a compact digital camera may have a range of only f/2.8 to f/5.6. Simple or “toy” cameras might have two or three apertures, while “fixed-focus” cameras typically use smaller apertures for greater depth of field.

Depth of field is the width of the plane of sharp focus, from front to back. Narrow or shallow depth of field means that if you focus on the eyes, the nose and ears might be out of focus. Wide or deep depth of field means that not only will the nose and ears be in sharp focus, but elements both in front of and behind the face, within a reasonable proximity. Control over depth of field means that when taking a portrait, a distracting background can be thrown out of focus. It gives you the ability to isolate or separate your subject from its surroundings.

Sharpness is the maximum resolving power of the lens. With a truly sharp image, the greater you magnify the film negative or digital image, the more detail you will see, up to the limit of the pixels or film grain. Maximum sharpness is limited by the design and quality of the optics, and also by the effects of diffraction.

Diffraction is the tendency of parallel light rays to disperse when passing through an aperture. Normally the amount of dispersion is negligible. But beyond a certain point, the amount of diffraction exceeds the circle of light projected on the film or sensor, softening the image. The only way to overcome this is to increase the size of the film or sensor. Diffraction is the reason compact digital cameras with smaller sensors are limited to about f/5.6. Smaller than that, and image resolution would suffer greatly.

Different apertures produce different effects, based on degrees of depth of field and sharpness, and it’s important to know what these effects are. Which of the available apertures the photographer can use depends on several other factors; the shutter speed range of the camera, the ISO setting of the camera, and the available light.

• f/1.2, f/1.4, f/1.8 — These apertures allow you to take photos in existing light without the use of flash, while minimizing motion blur. However, while they may not yield the sharpest images, their narrow depth of field affords you the greatest ability to isolate your subject from the foreground and background.

• f/2.0, f/2.8, f/4.0 — These apertures provide a good combination of sharpness and speed, with the ability to isolate your subject within the narrow plane of sharp focus. They require precise focusing.

• f/5.6 — An aperture available on most cameras and lenses, offering a good combination of speed and depth of field. Often the aperture at which most lenses are their sharpest. Focusing is less critical.

• f/8.0, f/11 — When there is adequate light, these are ideal apertures to use, giving high degrees of both sharpness and depth of field. f/11 is about the smallest aperture you can use before the effects of diffraction begin to degrade sharpness. A single focus setting will cover a wide range of distances. Many fixed-focus cameras use f/11.

• f/16, f/22 — When extreme sharpness is not a priority, but the need to avoid critical focusing is, these are the apertures to use. Everything from about 4ft. to infinity will be in focus, although not at maximum sharpness.

The aperture plays a major role in flash photography. Apart from the flash’s intensity and duration, the aperture is the sole means of controlling flash exposure.

The shape of the aperture is a consideration, as it has a direct affect on the bokeh. Some apertures use an iris with rounded blades which create a more circular bokeh. Others use a higher number of blades to create a smoother circle. Lenses with apertures consisting of “waterhouse stops” (interchangeable disks with varying-sized openings) can use custom shapes which will determine the shape of the highlights.

Automatic exposure cameras that have an Aperture Priority mode allow the user to select an aperture, while the camera selects the corresponding shutter speed for the proper or desired exposure. Aperture priority is the most popular auto exposure mode due to its ability to allow the user to control depth of field.