The “focal length” of a camera’s lens is a commonly used term (partly incorrect) to describe how far a lens can “zoom in”, or “zoom out”. This can be a complex term to understand if you do not have affinity with physics. Because of the complexity, this page is limited to the basics.
If you own a DSLR and have a lens attached to it, you’ll see your lens has numbers on the ring used for zooming in, or out. For example, on the Nikon 18-55 AF-S DX lens, these numbers are: 18, 24, 35, 45, and 55. These numbers are the millimetres we use to refer to as focal length.
Focal length is the distance between the optical centre of the lens and the camera’s sensor, that is required to create a sharp photo with the focal point on infinity. If you completely zoom in with a 18-55 mm lens, the focal length is 55 mm. This means that the distance between the camera’s sensor and the optical centre of the lens is 55 millimetres.
The focal point is the point where light rays converge and form a sharp image on the sensor. A fun example to learn how to converge light with optics, is using a magnifying glass to start a fire. In order for the light to become hot enough to burn an object, all light should be focused on one point, the focal point! The distance between the object you’re burning and the magnifying glass is the focal length!
Perhaps the horribly drawn image below can help you understand focal length. The “sticky doll” and the tree are far enough from the camera to create a sharp image when focused on infinity (set your lens to manual focus and turn the focus-ring until it reaches the infinity (∞) symbol).
The distance between the image (light) entering the lens and reaching your camera’s sensor is measured in millimetres. The result of the scene above would be a picture roughly like below.
In order to “zoom in” you need to move the lens further away from the sensor. Taking the scene above and doubling the distance between the sensor and the optical centre of the lens would result in a 100 mm lens.
The result is a much tighter angle of view, as seen below (again, rough sketch).
Focal length and “zooming”
Does this mean, that a 600 mm lens is approximately 60 centimetres (24 inch) long? No, it get’s more complicated. To avoid long lenses and make zoom-lenses possible, a camera’s lens is composed with multiple glass-elements that make it act longer. These multiple glass elements make it possible to focus your lens on objects close to you, by shifting the distance between the elements. Since this page is limited to the basics I refer to Pierre Toscani’s website where you can find a very detailed description on optics.
Crop and Full Frame
Most consumers use a so called “crop camera”. In the times when there were no digital camera’s, people used analog cameras with film. These films were made of a material that absorbed light creating a “negative photo”. In time the industry standard measure for a “roll film” became 35 mm wide. As technology developed, film was replaced by a sensor that digitally registered light. These sensors inherited the standard size of 35 mm (for technical reasons this became 36 mm). However, because producing such large sensors is expensive, the smart people in the photography/electronics industry found a way to create smaller sensors, which are used in consumers DSLR’s.
Today there are roughly three DSLR sensor sizes (excluding the small sensors in compact cameras, that can be ten times smaller than a Crop sensor):
- A Full Frame sensor is 36 mm wide
- A Nikon Crop sensor (1.5) has a width of app. 23,5 mm (36/1,5)
- Canon Crop sensors (1.6) have a width of app. 22,5 mm (36/1,6)
The size of the sensor changes the way light bends and therefore gives a different perspective. This is illustrated in the drawing below. The green rectangle represents a Full Frame (FF) sensor, while the black rectangle represents a Crop sensor. The green lines of the FF sensor bend differently from the red lines of the Crop sensor, creating a larger visible area.
To conclude: there are several factors influencing the angle of view of a photo. Not only focal length, but also sensor size. If you would like to read more about lenses this page of Cambridge in Colour is a great resource, or how about this fantastic page by Digital Camera World?