To make smart lens choices, you should understand the basics of optical design and how focal length and lens speed affect the types of images you can successfully capture. You also need a translation of all of the intimidating acronyms tacked on to lens names. You don’t need a degree in optical engineering to take great images, but even casual photographers benefit from a better understanding of lens technology.
Focal Lengths. Lenses are broadly categorized as wide-angle, normal (or standard) and telephoto. A wide angle is typically any lens with a focal length less than 40mm (35mm equivalent), and they’re most useful for widening out the view in a confined space or for adding context to a scene. Standard lenses generally fall in the range of 40mm to 60mm and roughly approximate the viewing angle of the human eye. Telephoto lenses are great for portraits or close-ups of faraway subjects, and they range from 70mm all the way up to 800mm and more.
There are fixed-focal-length lenses, called “primes,” and zoom lenses with adjustable focal lengths. Zooms are more popular with most photographers because they allow photographers to carry the equivalent of multiple primes in a single package.
Maximum Apertures. A new photographer may hear the term “speed” in reference to a lens and assume it’s a description of how fast that lens focuses. The term actually refers to a lens’ maximum aperture—or how wide it can open to allow in more light. A wider maximum aperture correlates directly to the ability to use a faster shutter speed—therefore making a faster lens capable of faster shooting or working with less available light than a slower lens under the same conditions.
Some zoom lenses have maximum apertures (denoted by smaller ƒ-stop numbers) that are variable, meaning they decrease as your focal length increases. For example, a hypothetical 35-70mm ƒ/4-5.6 zoom lens has a maximum aperture of ƒ/4 when the zoom is at 35mm, but it becomes ƒ/5.6 as you zoom out to 70mm. Practically speaking, variable maximum apertures translate to more affordable and more compact zoom lenses for photographers who don’t need a constant fast aperture.
Digital Lenses. Film is flat, but digital sensors are not. Both CCDs and CMOS sensors are made up of tiny wells that register light as it hits the sensor. Think of the sensor as a collection of millions of tiny buckets. These buckets have depth, and you can’t fill them from an angle. It’s this depth of digital sensors that caused lens makers to reformulate how lenses work.
In order to register more of the light that enters the camera, designed-for-digital lenses are built to “collimate” the light more effectively. That means taking light that enters from the edges of the lens and moving it more to the center—striking the sensor from a more perpendicular angle. Another reason digital cameras do better with digital lenses is that they’re designed to provide better fidelity and sharpness. Digital captures are so unforgiving that every lens flaw would be amplified.
The easiest-to-see, digital-specific lens effect comes in terms of “crop factor.” Essentially, a number (such as 1.3x or 1.5x) and a separate set of “equivalent” millimeter measurements are used to represent a lens’ specifications when used on a smaller-than-35mm film-frame sensor. APS-C, Four Thirds and other popular sensor sizes are smaller than a frame of 35mm film. That means a lens measuring 100mm on a 35mm film camera would behave more like a 150mm lens on the smaller sensor.