DSLR Camera Basics: Understanding Focal Length

In its simplest definition, focal length is the field of view for a particular camera lens.

Focal length determines how much of a scene the camera sees, and it varies with the lens. A wide-angle lens can take in an entire landscape; a telephoto lens zooms in on a small subject in the distance.

Focal length is important to understand, particularly if you're shooting with a DSLR camera. Some basic knowledge of the concept can help you choose the right lens for a particular subject and know what to expect even before you look through the viewfinder.

Technical Definition of Focal Length

The scientific definition of focal length goes like this: When parallel rays of light hit a lens focused at infinity, they converge to form a focal point. The focal length of the lens is the distance from the middle of the lens to this focal point.

Another way to understand focal length is simply the distance from the center of your lens to the subject it's focused on.

The focal length of a lens is displayed on the barrel of the lens. 

Types of Lenses

Lenses are usually categorized as wide-angle, standard (or normal), or telephoto. The focal length of a lens determines the angle of view, so a wide-angle lens has a small focal length, and a telephoto lens has a large focal length.

Here are the accepted focal length definitions for each category of lens:

• Less than 21mm: Super wide-angle lens
• 21-35mm: Wide-angle lens
• 35-70mm: Standard / Normal lens
• 70-135mm: Standard Telephoto
• 135-300mm (or more): Telephoto

Zoom and Prime Lenses

There are two types of lenses: prime (or fixed) and zoom.

• A prime lens has only one focal length (e.g., 50mm).
• A zoom lens covers a range of focal lengths (e.g., 17-40mm). 

Zoom Lens Advantages

Zoom lenses let you change focal lengths quickly while looking through the viewfinder, so you don't have to carry a camera bag full of lenses around. Most amateur digital photographers can get by with one or two zoom lenses that cover the full range of focal lengths.

One thing to consider is how big of a range you want in a single zoom lens. Many lenses go from 24mm to 300mm (and anywhere in between), and these are very convenient.

The issue is often the quality of the glass in these lenses; that's because, the wider the range, the more elements the light has to travel through. If you are interested in one of these dynamic-range lenses and want the best picture quality, it would be best to splurge on a top-quality lens.

Prime Lens Advantages

Prime lenses have two main advantages: quality and speed.

Speed relates to the widest aperture (f/stop) built into the lens. At a low aperture (small number, wide opening), you can photograph in low light and use a fast shutter speed that will stop action. This is why f/1.8 is a highly preferred aperture in lenses. Zoom lenses rarely get this fast, and if they do, they are very expensive.

The prime lens is also much simpler in construction than a zoom lens because fewer glass elements are inside the barrel, and it does not need to move to adjust focal length. Less glass to travel through means less chance for distortion; this often yields a much sharper, clearer photograph.

Focal Length Magnifier

The focal length of lenses was set back in the days of film photography and relates to the focal length of a lens on a 35mm camera.

If you are lucky enough to own a professional full-frame DSLR, then your focal length is unaffected. If, however, you use a crop-frame (APS-C) camera, then your focal lengths will be affected. Because crop-frame sensors are smaller than a 35mm strip of film, magnification needs to be applied. The magnification varies slightly among manufacturers, but the standard is x1.6. Canon uses this magnification, but Nikon uses x1.5 and Olympus uses x2.

For example, on a Canon crop-frame camera, a standard 50mm lens becomes a standard telephoto 80mm lens (50mm multiplied by a factor of 1.6, to result in 80mm).

Most manufacturers now make lenses that allow for this magnification, and they work only on crop-frame cameras. This is particularly useful at the wide-angle end of things, where magnification can turn these lenses into standard ones!