Aperture

Aperture is arguably one of the most confusing and misunderstood topics in photography. It's the one that requires a certain degree of memorization and a lot of abstract thinking to understand fully.

In its most basic terms, aperture simply refers to the size of the opening in the lens. Also called "f-stop," the aperture is usually written as f/number. The number assigned to it is relative to the lens focal length. This allows the number to correspond to the amount of light entering the camera more than the actual size of the opening. An aperture of f/2.8 on one lens illuminates the sensor or film the same amount as f/2.8 on another lens. Sounds pretty straightforward right? Guess again.

Because aperture is relative and amounts of light are usually expressed in logarithmic terms, the numbers don't line up the way they do with other exposure controls. With shutter speed, a shutter speed of 1/30th of a second lets in twice as much light as 1/60th of a second. With aperture, each successive number denoting half as much light is multiplied by 1.4. Rather than doing the math, photographers usually memorize the number sequence. The "whole stop" sequence is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44, and 64, where 1 is the widest and 64 is the smallest. Most modern cameras have half and one-third stop increments in between these numbers to fine tune the settings.

Are you with me so far?

All lenses have a maximum aperture and this aperture is usually listed in the lens name. Very few lenses can open up all the way, or past, f/1. Most fixed-focal length (also called prime) lenses start somewhere between f/1.4 and f/2.8. The best zoom lenses start at f/2.8 while consumer zoom lenses may start as small as f/5.6. Maximum aperture can also be affected by zoom range and focusing distance. Most consumer zoom lenses do not compensate optically for the enlargement at their maximum focal length, so the effective aperture will suffer, usually by a full stop or more. That's why you might see a lens described as a 70-300mm f/4-5.6, since the maximum aperture is f/4 at 70mm, but f/5.6 at 300mm. Most macro lenses have this problem too when close focusing. A macro lens might be f/2.8 at infinity, but drop to f/5.6 at its minimum focusing distance.

Now that we've covered what aperture is, let's look how it affects our images. The first, and obvious, change is exposure. A larger aperture (lower number) will let in more light where a smaller aperture (higher number) will let in less. I will discuss this in greater detail when I go over exposure. There is another way aperture affects images though, and that is with depth of field.

To understand this concept, one must understand how a lens works. The first cameras were pinhole cameras. They did not use a lens but rather used a tiny hole in a box to let in light. Since this forced light rays entering the box to be confined to a narrow trajectory, it formed an upside-down and backwards image on the opposing end of the box. The sharpness of this image was directly affected by the size of the hole since it determined how precisely the light was allowed to enter. This approach obviously had its drawbacks. While it affected light regardless of distance travelled, in effect allowing the whole image to be sharp, it also meant that the entire image was also out of focus by however large the hole was.

Lenses changed this completely. Rather than relying on the size of the opening to determine sharpness, a lens bends incoming light to converge at a single point. This allows the opening to be much larger while maintaining a sharp plane of focus. This approach has its drawbacks too. If that plane of focus wis not on the subject, the subject would be out of focus. This meant that the photographer now had to adjust the camera to compensate for lens-to-subject distance.

A single point of light entering a lens will refract, disperse, and converge in a predictable fashion. If the point of convergence is on the film or sensor, it will be recorded as a point of light just like it was originally. However, if that point is out of focus, it will spread out and take the shape of the opening in the lens. If that shape is a large circle, the point will render as a large circle. If the shape is a small octagon, then the point will render as a small octagon. Some lens makers such as Lens Babies capitalize on this by allowing shaped openings like stars to create effects in the out of focus portions of an image.

Theses out-of-focus shapes are referred to as "circles of confusion." The larger the opening, the larger the circles of confusion, and the more out of focus the background will appear. The smaller the opening, the smaller the circles of confusion, the sharper the background will be. Once these circles of confusion become too small to see with the naked eye, the background is said to be "in focus" or have a "wide depth of field." This is actually how the focus plane looks sharp too. The lens will cause the light to converge at such a tiny area that the naked eye can no longer see the circle of confusion. The smaller the aperture, the sharper it will be, while at larger apertures, the lens has to be more precise to continue looking sharp.

There is a counter agent to this rule though. When light hits the edge of the opening, it scatters slightly. This is not noticeable at large apertures since the circles of confusion are very bright, but at smaller apertures, this effect becomes more and more prevalent. If the small aperture has hard edges (non-rounded diaphragm blades), this will cause points of bright light to burst outward into star shapes. In most cameras, this will soften images starting at about f/11.

So, what does all this mean? Most lenses have a "sweet spot" of optimum sharpness usually somewhere around f/4-5.6. Wider (lower number) apertures will have a shorter depth of field and will be slightly softer, while smaller apertures will have a wider depth of field, but will start to suffer from softness caused by diffraction. Higher quality lenses will expand this sweet spot. The general rule of thumb is that a lens is at its sharpest two stops below (higher number) its maximum aperture while diffraction remains a constant. This means a 50mm f/1.4 lens will have a sweet spot from f/2.8-8 while a pro zoom lens at f/2.8 will be at its sharpest at f/5.6-8. Consumer lenses that start at f/4 or f/5.6 will be sharpest only at f/8.

As you can see, aperture plays an enormous role in how a photograph will look. Lenses with large apertures will drive up the price of a lens because of the technical prowess required to make it work as well as the demand photographers have for them. A 50mm lens with an f/1 or f/0.95 aperture can cost as much as $10,000. With aperture playing such a large role, many photographers will want to maintain direct control over their aperture by using manual or aperture priority exposure modes on their camera. I personally shoot in aperture priority mode (denoted by A on the mode dial) the most.