Perhaps the most important concept to understand when purchasing a new camera lens is focal length — it has the single biggest impact on how your images will look.
This article will explain the definition of focal length, describe how it affects your photographs, and go into detail about what the different focal lengths are commonly used for.
What is focal length?
Contrary to what many believe, a lens’ focal length is not a number that describes its physical length. Instead, it is the distance in millimeters between the optical center of your lens and your camera’s sensor (or film, if you’re using a film camera).
Lenses are always named after their focal length. So, a 35mm lens has a focal length of 35mm, and that information can be found printed on the barrel of the lens.
More important to know than the technical aspect of focal length, however, is what that number tells us. The focal length of a lens describes the amount of the scene a lens can capture (the field of view) and how large the subject will appear in the frame.
Short lenses capture wide scenes but the elements appear small in the frame. Longer lenses capture a very narrow scene but subjects appear much larger than they do to our naked eye.
That said, focal length doesn’t just determine your field of view.
How does focal length affect your photos?
It should be noted, while focal length determines how much of your scene is in your frame, it also impacts the look of your images in several other ways:
- Depth of field. Lenses with longer focal lengths tend to give your images a shallow depth of field. On the other hand, shorter lenses have a larger depth of field, with more of the scene in sharp focus.
- Image shake. lenses with long focal lengths are highly sensitive to motion blur. The rule of thumb to avoid image shake is to always use a shutter speed as fast or faster than your length. For example, if you’re using a 300mm lens, your shutter speed should be set to 1/300th or faster. Or, use a tripod.
- Perspective. Focal length changes the appearance of space between the elements in your images. Short lenses expand the perspective between elements giving the appearance of more space. On the contrary, long lenses compress the perspective, making objects look much closer together than they really are.
Different focal lengths and what they’re used for:
Camera lenses can be classified into different categories based on their focal length. Each of those categories is best suited for a specific type of photography:
Ultra Wide Angle and Fisheye (8mm – 24mm)
Ultra wide angle and fisheye lenses capture a wider angle of view than you can even see with your naked eye. This unnaturally wide field of view causes distortion that makes them undesirable for portrait lenses.
But, they are often the lens of choice for landscape and architecture photographers. Their large depth of field allows for the entire scene to be in focus and the distortion makes for dramatic, visually interesting scenes.
Example at 11mm:
Standard Wide Angle (24mm – 35mm)
Most kit lenses start in this focal range because the wide-angle distortion is minimal, while still being wide enough to capture an entire scene. This range is perfect for photojournalists and nature photographers.
Example at 24mm:
Standard (35mm – 70mm)
Not to mention, you can easily adjust your depth of field using a wide or narrow aperture. It’s always good to carry a lens in this range in your camera bag.
Example at 50mm:
Telephoto (70mm – 300mm)
These focal lengths are used to get closer to your subject without getting physically close, much like a telescope. With this in mind, this is perfect for wildlife and sports photographers, who can’t get up close to their subjects.
That said, on the shorter end of this range, 85mm is extremely popular for portrait photography because the perspective of the lens is flattering to human faces. Also, many popular macro lenses fall into this range around 90mm-105mm.
Example at 300mm:
Super Telephoto (300mm and more)
Lenses with a focal length greater than 300mm fall into the Super Telephoto category and are used for photographing birds, wildlife, and other small objects that are very far away. They can be extremely heavy, large, and expensive!
Example at 500mm:
How does crop factor affect focal length?
If you shoot with a crop sensor camera, you’ve probably heard of the “crop factor” when it comes to a lens’ focal length. Crop factor describes the difference between your camera’s sensor size and a traditional 35mm film frame or a full-frame camera sensor.
Since they’re about 75% of the size of a full-frame sensor, crop sensor cameras usually have a crop factor of 1.5 meaning the same lens will produce a narrower field of view than it will on a full-frame camera.
This will give the appearance of a longer focal length, making it look zoomed in. This might be ideal if you’re looking for a super telephoto lens but if you want a super wide angle, be aware that you’re going to lose some of that width on a crop sensor camera.
To figure out the crop sensor equivalent of your focal length, simply multiply the focal length of your lens by 1.5. For example, a 50mm lens will be the equivalent of a 75mm lens on a crop sensor camera.
Zoom lens vs. Prime lens
Prime lenses have a single, fixed focal length while zoom lenses have focal lengths that can be changed. There are advantages and disadvantages to each.
Prime lenses are sharper than zooms. They are usually less expensive, too. And, they’re lighter, with wider maximum apertures. If you tend to always shoot in one particular focal length, a prime lens is the better option.
That said, zoom lenses are great for photographers who shoot with a wide range of focal lengths. You won’t have to constantly change your lens to change your field of view. You can also carry fewer lenses because you can cover a wider range with just one lens. Zoom lenses are perfect for photographers on the go.
Once you have a grasp on the concept of focal length, you can easily choose what lens you want to use based on what you plan to shoot. It would be highly unusual to choose an ultra wide angle lens to photograph a wild lion and you’d never select a super telephoto to photograph a person standing right next to you.
In fact, your lens probably wouldn’t be able to focus on them — but that’s an article for a different day! If you keep a large range of focal lengths in your camera bag, you’ll be ready for any shooting situation.