Even if cell phone cameras keep improving and reach unimaginable levels of image quality, I’ll always carry around a larger, heavier, and more complex camera. Why is that? One word: lenses! An interchangeable lens camera opens up a vast world of photographic visions through a huge variety of optics. Choosing among the dozens and even hundreds of lenses can be confusing and intimidating, so in this beginner’s guide, I’ll explain the types of lenses available and what you should buy.

What Does a Lens Do?

A lens focuses light to form an image on the camera’s digital sensor or film plane, much the way our eye works. And importantly, a lens determines how much of the subject is seen and captured, from the broad sweeping view of a wide angle lens to the narrow, selective view of a telephoto lens. We call this the angle of view.

Lenses are classified by their specific focal length in millimeters. At the simplest level, this millimeter marking corresponds to the distance between the lens’s optical center and the camera’s image sensor when focused. From that focal length designation, we know how an image will look – in particular, the angle of view – on a given camera. Focal length is the most important factor to determine which lens to use for a given photo.

Note that the images above are taken with a full-frame camera – i.e., a camera with a sensor that’s about 24×36mm in size. If your camera has a smaller sensor like aps-c or Micro Four Thirds, it will act as a “crop” of the images above and give a more zoomed-in appearance at each focal length. To be specific, aps-c cameras have about a 1.5× crop factor, and Micro Four Thirds cameras have about a 2× crop factor. (For the remainder of this article, any time I mention specific focal lengths, I’ll be doing so in terms of a full frame camera. To get the equivalent number on your camera, it’s as easy as dividing by your crop factor.)

Along with focal length, a lens also has a diaphragm that can change size – commonly called aperture – which controls how much light is let through the lens (part of how we control exposure). As your aperture changes, it looks like this:

Every lens lets you change the aperture size, so you’re not stuck at one aperture. However, lenses are usually named by their maximum aperture because it’s so important – for example, the Nikon 28mm f/2.8 has a 28mm focal length and a maximum aperture of f/2.8. Other lenses have maximum apertures of f/4 or f/5.6 (which don’t let in as much light), and some go the other direction to f/1.4 or f/2 (capable of capturing much more light).

Aperture doesn’t just change how much light you capture. It also determines how much of our subject is in focus from front to back – what we call depth of field. As the aperture narrows, depth of field increases, which is why landscape photographers often use apertures like f/8, f/11, or f/16 to get sharp focus from front to back.
Combined, these two factors – focal length and aperture – are the most important features of a lens. If you know a lens’s focal length(s) and maximum aperture, you already know a great deal about what subjects it’s intended to capture. I’ll cover more about those intended subjects next.

The Normal Lens

Lenses with a “middle” focal length – not super wide, not super telephoto – are known as normal lenses or standard lenses. Many photographers swear by the normal lens as their main tool because it does not exaggerate perspective and can be pressed into service for a wide variety of photographic needs. Photos taken with a normal lens feel like looking at the world with your eyes, not a camera.
The normal lens for a given camera system has a focal length similar to the diagonal length of that camera’s sensor or film. Full-frame cameras (again, with a roughly 24×36mm sensor) have about a 43mm diagonal. The classic normal lens on full-frame is a 50mm, which is a bit longer than 43mm but pretty similar.

Normal/standard lenses were almost always sold with the camera as a kit in the film days of decades past. Today, there are still plenty of 50mm lenses available from each manufacturer (or equivalents for smaller sensors like 35mm and 24mm lenses).
Everything from family candids, low-light street scenes, wedding group photos, and even landscapes look natural with a normal lens. It’s a flexible tool.

Wide Angle Lens Drama

Wide angle lenses can be exciting to look through, as they take in a much more expansive view than the normal lens and can be used to exaggerate perspective in pleasing ways.
A typical use for a wide angle is in a dramatic landscape, where the wide field of view allows you to get close to an interesting foreground such as a field of wildflowers, while still capturing a sweeping view of the mountains in the background. Wide angles are also commonly used in architectural photography, such as including all of the grand interior of a cathedral in the photograph.
On full frame, wide angle focal lengths range from about 10mm (uncommon and excessively wide for many uses) to 35mm (which is long enough that some photographers consider it a normal lens rather than a wide-angle).

Telephoto Lens Power

A telephoto lens is like looking through binoculars – it has the power to bring your subject up close and personal. A telephoto has a selective angle of view and is commonly used to photograph more distant subjects such as wildlife or sports. It can also make pleasing head and shoulders portraits of people from a relaxed and comfortable distance.
If you can’t get close to your subject, chances are you will want a telephoto lens. They are my personal favorite lens type for landscapes, where I can compose a picture of a photogenic section of a forest rather than taking in the entire hillside. Telephoto focal lengths begin at 70mm and continue up to about 800mm.

Specialty Lenses

Within the broad categories of wide angle, normal, and telephoto lenses, there are also more specialized optics. For example, a macro lens is designed to focus very close so that tiny objects such as insects, flowers, or jewelry can fill the frame. Another specialty lens is the high-speed (or fast) lens, which has larger lens elements and a wider maximum aperture – great for letting in more light and capturing very shallow depth of field photos, where not much in the image is in focus.
Other speciality lenses included fisheye with its extreme and distorted field of view, tilt/shift lenses which are used by some architectural, studio, and landscape photographers to more precisely control perspective and focus, and the huge, exotic super-telephotos seen on the sidelines of major sporting events.