Today, we continue our educational series on the Basics of Photography with the letter O for Optics. We already covered a number of topics pertaining to lenses in previous articles, but there is a lot more to optics than meets the eye (pun intended). In this article, we’re going to take a look at some of the most important aspects of optics, and explain some of the most common terms and concepts.
This is what you call the often undesired optical phenomena that can make an image appear blurred, distorted, or sporting false colors. Aberrations are intrinsic to the design of a lens, that is they stem from the shape and combination of the various glass elements inside a lens. By refining the optical formula, aberrations can be eliminated to a certain degree. Those with an eye for the unconventional often like to employ aberrations in an artistic way.
The aperture is the iris-like ring of metal blades inside a lens that is used primarily for two things. By either closing or opening it, it can be used to control how much light enters through the lens and hits the sensor. On a sunny day, it may need to be closed, while in the dark you’ll want it to be opened wide. Closing the aperture also increases the depth-of-field, that is, how much of what you photography will be in focus. With an open aperture, you get a blurred background which is nice in portraiture and close-ups.
Also read: The Basics of Photography: A for Aperture
Typically, the surface of a lens element is spherical–that is, it has the shape of a section of a circle. In the past, all lenses were sporting exclusively spherical optical elements, as these are much simpler to manufacture. These designs often employ severe aberrations, though, which can only be fixed by using aspherical lenses. As the name gives away, an aspherical lens has a surface that is not (Greek a- = “un-, not”) the shape of a section of a circle. By using aspherical surfaces, the path that light rays take through a lens and onto a sensor can be better controlled for optimal image quality.
The term ‘bokeh’ comes from the Japanese language and originally described the aesthetic quality of the blurred and out-of-focus parts of an image. Today, it is mostly used to refer to the out-of-focus blur itself, which you can achieve by using a lens with a wide aperture. Bokeh works nice for portaiture and close-up photography like that of flowers of animals, but it can also be distracting. Depending on the optical formula of a lens, bokeh can be ‘harsh’, that is with lots of artifacts, or ‘creamy.’ It can also be used artistically, especially in combination with certain kinds of aberrations.
Also read: The Basics of Photography: B for Bokeh
Today, most lenses employ a special coating on their surfaces that help minimize certain kinds of optical aberrations such as flare. Flare is a phenomenon that appears when a bright light source is either within or in just outside the field of view of a lens. A special coating, can help minimize this effect. Other advantages that coatings provide is better light transmission, which means that less light gets bounced back and forth inside of the lens, and more light actually hits the imaging area of your camera. There are numerous kinds of coatings that are being employed these days, and every lens manufacturer has their own formulas and designations. Very old lenses often don’t sport any coatings at all.
Focal Length and Angle of View
The focal length of a lens is the distance at which the incoming light rays converge into a single point, the focal point. As a general rule, the longer the focal length of a lens, the smaller its angle of view. The focal length of a lens is always measured in millimeters. Take for example a 14mm lens, which on a full-frame camera has an approximate diagonal angle of view of 114°. Doubling the focal length will lead to only half the angle of view, and thus a 28mm lens has an a-o-v of only 72°. This is still very wide, though, compared to the even more restricted angle of view of a “normal” 50mm lens, which is 47°. Depending on the size of the sensor inside your camera, the actual angle of view of the resulting image can be even smaller–this is called the “crop factor”.
Focus Point and Focal Plane
The focus point is the one single point in which convergent rays of light meet. When focusing a lens, this point should ideally coincide with the plane of the camera’s sensor–or the film if you use a film camera. When the focus point is in front of or behind the imaging device of your camera, your images appear to be out-of-focus, that is blurred instead of sharp. The focal plane is the ideally two-dimensional field in which all convergent light rays build their focal points. We say ideally because in reality the focal plane is mostly three-dimensional. This has the effect that an image appears, for example, to be sharp in the center, but blurred in the corners. Closing down the aperture of your lens often helps in achieving a flatter focal plane.
Focusing – Manual vs. Autofocus
Most cameras today offer both autofocus and manual focus. As explained above, focusing is the act of making the incoming light hit the imaging device in your camera in such a way that your subject appears sharp in the photo–i.e. that it is in-focus. Most DSLRs, mirrorless cameras and compacts use autofocus lenses. These sport a motor that moves the optical elements inside the lens, while at the same time the camera assesses whether the desired subject is actually in focus. When focusing manually, the photographer’s eye has to assess whether the desired subject is in focus, while turning a focusing ring around the lens barrel.
Also read: The Pros and Cons of Manual Focusing
Lens vs. Lens Element vs. Optical Construction
In the English language, the term lens describes two different things. It originally referred to what today the term lens element is used for, that is a single piece of glass that has been grounded or molded in such a way that it refracts the incoming light. In the early days of photography, single-element lenses weren’t too uncommon. Today, the optical construction of a lens, that is the amount, shape and configuration of the lens elements that it is made up of, may contain over a dozen individual lens elements. The shape, number and form of the individual elements in a lens depends on its purpose: wide-angle and zoom lenses usually sport more elements than normal or telephoto primes.
A prime lens, also called single-focal-length lens, is a lens that does not offer any magnification capabilities. This is what sets it apart from a zoom lens. A prime lens sports a single focal length, that is its angle of view cannot be changed. Prime lenses are available as wide-angle lenses, as normal lenses (such as Canon’s “nifty fifty”, the 50mm f1.8 lens) or as telephoto lenses. Their optical formulas are typically less sophisticated than those of zoom lenses, and their optical qualities and aperture ratings are usually higher than those of zoom lenses.
A zoom lens is a lens that lets you change the degree of magnification in an image; that is, its angle of view is variable. A typical zoom lens of the kind that is sold with DSLRs or mirrorless cameras features an approximate 3x range, from a wide-angle view up to a moderate telephoto setting. However, there are also those with a shorter reach, and the so-called superzooms with magnification factors of 10x or higher. Zoom lenses are typically much larger and heavier than prime lenses because they employ a more complicated optical construction. Also, they are usually not as fast (in terms of their widest aperture opening) as prime lenses. On the upside, they offer much greater versatility.
Be sure to check out more in the series