A Beginner's Guide to Lenses
Your camera is a small, portable darkness, and the lens is its window on the world. The lens collects light from the outside universe and focusses it onto the film.
A simple lens is a polished piece of glass with concave or convex surfaces. Its function is to bend rays of light (by a physical process called refraction). A modern photographic lens, also called an “optic,” is a lot more complicated than a single piece of glass. It’s composed of an ordered series of specifically chosen simple lenses, called “elements,” encased within a barrel to form what’s known as a “compound lens.” No lens is perfect: lens designers use multiple elements in an attempt to correct aberrations that deteriorate the final image projected onto the film. A photographic optic will also usually contain the “iris,” which is the adjustable opening through which light enters the camera. The iris is typically located between two of the lens elements.
The schematic at right is a cross-sectional representation of Nikon’s 50 mm f/1.8 lens showing the compound lens construction. This optic has 6 elements in 5 groups.
Aperture and Focal Length
Lenses are identified by two primary parameters: focal length and maximum aperture. So, you might see something like “50 mm 1:1.8” inscribed on a lens barrel. What this is telling you is that the focal length of the lens is 50 mm, and the maximum aperture is f/1.8. Focal length is a measure of how strongly the lens bends light. It’s the distance over which light rays initially travelling in parallel lines are brought to a point focus. The shorter the focal length of the lens, the more it bends light, and the wider the section of the world the lens can “see.” The converse applies to longer focal length lenses. Focal length also determines magnification: at the same distance from the subject, a long focal length lens will make it appear closer, while a short focal length lens will make it appear further away.
The diagram at left shows the focal length of a simple, convex lens. Parallel rays of light entering the lens are brought to a point focus at F, and the focal length is given by f.
Aperture, or f-number, is the ratio of the diameter of the iris to the focal length of the lens. The smaller the ratio, the larger the iris. The smaller the f-number, the more light the lens allows into the camera. Typically, the longer the focal length of the lens, the more difficult and more expensive it will be for the lens designers to engineer large apertures. So, a 50 mm f/1.4 is relatively inexpensive and common (a few hundred dollars new), while a 600 mm f/4 is exotic and hideously expensive (close to $10,000 new). Lenses with wide maximum apertures relative to their focal lengths are called “fast” or “bright,” while those with narrow maximum apertures are “slow” or “dim.”
ASIDE: An introduction to aperture
If you need a refresher on aperture and f-stops, pause here to read this brief introduction.
Particularly on older lenses, you might also see crazy names like “Tessar,” “Planar,” and “Xenotar.” Don’t worry about these: they simply refer to the lens design. For example, the Tessar contains 4 elements in 3 groups, while the Planar has 6 elements in 6 groups. To this day, Leica insists on giving its lenses exotic, expensive-sounding names like “Noctilux” and “Summicron.” In Leica parlance, these names refer to the maximum aperture: all Noctiluxes are f/1.0 (think about that for a second!), while all Summicrons are f/2.0, regardless of focal length or lens design.
Classification by Focal Length
Broadly speaking, lenses can be broken down into three classes based on focal length: normal, wide and telephoto. A normal lens has a focal length approximately equal in length to the diagonal of a single frame of film — for 135 format film, this is 43 mm (usually approximated by either 45 or 50 mm), while for 120 format film and 6x6 cm square frames, it is 79.2 mm (approximated by 80 mm). Normal lenses render a perspective that looks “natural” to a human observer.
Lenses that have shorter focal lengths, typically 24, 28, and 35 mm for 135 format film are referred to as wide angle lenses. These lenses are characterized by having very short minimum focus distances (for example, 30 cm for a 28 mm lens), and typically give deep field of focus even at moderate aperture (for example, a 28 mm lens may give acceptable focus anywhere from 1.5 m to infinity at f/8).
Strictly speaking, a telephoto lens is one whose physical length is shorter than its focal length. However, the word “telephoto” has come to describe a lens whose focal length is significantly longer than normal, that is 105 mm to 600 mm on 35 mm cameras. These lenses typically have long minimum focus distances (for example, 1 m for a 105 mm lens) and narrow depth of field (for example, a 105 mm lens may give acceptable focus within only 10 cm of 1.5 m at f/8.)
Prime vs. Zoom Lenses
So far, we have been talking only about “prime” lenses, that is, lenses that have a fixed focal length. The other major classification of lenses is the “zoom.” Zoom lenses have a variable focal length that can be adjusted at will by the photographer. Just as primes are classified as wide, normal and telephoto, so zooms are also classified as wide-angle zooms (e.g., 14-24 mm and 16-35 mm), normal zooms (24-70 mm and 24-85 mm) and telephoto zooms (70-200 mm and 200-400 mm). In the past, it was generally true that zoom lenses had inferior image quality to prime lenses because zooms require more elaborate designs. These days, however, high-end zoom lenses are exceptionally good and produce images that rival those made by prime lenses. In addition to being larger and heavier than primes, the biggest disadvantage of zoom lenses is that they tend to be much “slower” — you simply will not find a telephoto lens with a maximum aperture at f/1.4. “Fast glass” in telephoto terms is in the region of f/2.8. Most consumer grade zoom lenses that typically come bundled with new camera bodies — so called “kit lenses” — are painfully slow, and their optical performance degrades as focal length is increased. For example, a 28-70 mm kit zoom may be f/3.3 at the wide end and a pitiful f/5.6 at the narrow end. (Remember that a lens at f/5.6 lets in 4 times less light than a lens at f/2.8.)
Fixed vs. Interchangeable Lenses
It’s also useful to consider a few other classifications of lenses. Lenses that can be removed from the camera body are called “interchangeable” lenses. Most single-lens reflex cameras (SLRs) and many rangefinders take interchangeable lenses. Interchangeable lenses are defined by their “mounts,” which are the interfaces between the lenses and the camera bodies that support them. Nikon bodies have so-called “F-mounts,” Canon bodies “EF-mounts,” and Pentax bodies “K-mounts.” F-mount lenses will not couple to EF- or K-mount bodies. Basically, they’re all mutually exclusive, but you will find third party lenses that are compatible with each of these mounts — and often a lot cheaper than the primary brands. For example, Tamron makes a wide variety of lenses for both F- and EF-mounts.
There are also many specialty lenses for specific purposes. High-end lenses for architectural photography tilt and shift to correct perspective distortions. Other lenses allow very, very close focussing and larger than life size magnification for photographing tiny objects (these are macrolenses — inexplicably, Nikon calls them micro). There are even specialty portrait lenses (Nikon dubs them “DC” for “defocus control”) which allow the photographer to manipulate out of focus areas to make them less distracting.
The biggest lesson about lenses is this: they are more important, more valuable, and last longer than cameras. Do not buy an expensive camera and handicap it with a cheap lens. It’s much, much better to do the opposite. It will pay you in the long run to invest in glass. Long after your camera’s resale value has plummeted to nothing, your good glass will probably be worth more than what you paid for it. My advice is to buy the best, most expensive, and fastest lenses that you can afford.