You've reached this post because you want to know what is focal length. Focal length determines how much of a scene in front of you will be included in your photo.
The definition of a focal length is the distance from the center of the lens to the focused image on the sensor. A longer length mean that it is more of a telephoto lens and has a narrow view of the scene. A shorter length means it's more of a wide angle lens and more of the scene is recorded on the camera sensor.
Here is a diagram to help you understand is focal length. An understanding of focal length and zoom range will help you when you are considering buying a camera or a lens.
In the diagram below, if you can imagine a longer lens (the space marked in red) with would give you a narrower angle of view of the subject. In other words a smaller amount of the subject would be focused on the sensor. Longer focal lengths are good for shooting sports, wildlife, and portraits of individual people.
Most cameras come with a zoom lens, which means you can essentially change the focal length of the lens. Zooming in narrows you angle of view and brings your subject closer. Zooming out widens the camera's view, includes more things in your viewfinder and make them smaller.
SHORT FOCAL LENGTH. A shorter lens would give you the wider view of the scene in front of you. Shorter focal length lenses are great for shooting landscapes, architecture, groups of people, and inside photos where space is limited. Because of the physical laws of optics, shorter focal lengths give you a deeper range of things in focus. To understand that more, see my post on deep depth of field.
LONG FOCAL LENGTH. A longer focal length on the same camera will give you a narrower view of the scene in front of you. Long focal length lenses are great for photographing birds, wildlife, and other distant subjects.
Longer lenses are also good for shooting individual portraits and headshots because they give a more attractive rendition of the human face. Long focal length lenses are great to use when you want to create an out-of-focus background to make the main subject stand out better. Here's more on shallow depth of field.
Long and short focal length lenses are both good. Shorter focal length lenses are better in some shooting situations and longer focal lengths are better for other situations. See the focal length examples below for a comparison.
In this case, neither photo is better. They're just different. The short focal length photo puts more of an emphasis on the curly and colorful leaves in the foreground. In the longer focal length photo the sun's starburst coming through the colorful trees is emphasized.
Here is a basic chart of a few typical zoom lenses and their range of focal lengths. The numbers in this chart are for a camera with a full-frame sensor. With smaller DSLRs and point and shoot cameras, the actual mm numbers will be smaller, but the equivalents can easily be calculated using the crop factor number which is one of the specs of the camera.
The chart below is for a full frame sensor cameras. Point and shoot cameras have smaller sensors and so the actual distances in "mm" millimeters is smaller. Usually the camera maker will give you the equivalent numbers for the zoom.
The actual numbers are not as important as knowing the best lens or zoom setting to use in certain photographic situations. These are the "common uses" in the chart below.
If it says it has a 4x zoom, it's range may be the 35mm equivalent of 25-100mm zoom. A 10x range might mean a 28-280mm zoom range of magnification.
Focal length is important for several reasons. This is what really matters over anything else you read about focal length numbers.
Another article that might deepen your understanding of what focal length numbers mean is here: Camera Lens Comparison. I included sample photos showing 4 different focal length settings while using a Canon 5d, a DSLR with a full-frame sensor.
These are not strict guidelines to follow, just typical uses for certain lens types. If you have any questions, feel free to ask and I'll write an article when I get a chance.
What is a Prime Lens. A zoom lens gives you many focal lengths to choose from while having to use only one lens. What is a prime lens and why would anyone want to use one .Have fun as you discover more digital photography tips on this site.
Keep shooting. Stay inspired!