A Short Course Book
Using Your Digital Camera
A Guide To Great Photographs

Understanding Focal Lengths

 
The focal length of a lens determines its angle of view.
 
 
Click to explore how the focal length of a lens determines its angle of view.
 
 
Click to explore how the size of an image sensor determines the focal length of a lens.
 
 
 
Sounds like science fiction but liquid lenses that focus by changing shape are now being used in some camera phones. Courtesy of Varioptics.
 
 
Click to explore the sizes of image sensors.
The focal length of a lens has a huge impact on your images and is one of the most important tools in your creative toolbox. On fixed lens cameras you change the focal length by zooming the lens. On SLRs you can also do so by changing lenses. These differences create some synonyms that can be confusing at first.
  • Wide-angle, short focal length, short lens and zoom out refer to the same thing lenses that capture a wide expanse of a scene.
  • Telephoto, long focal length, long lens and zoom in refer to the same thing lenses that bring distant subjects closer.

    The focal length you choose is a creative choice because it has two effects on your images:

  • Angle of view refers to how much of a scene the lens covers. Fisheye lenses, the widest available, can capture 180-degrees. As you zoom in or change lenses to increase the focal length, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.
  • Magnification is related to the lens' angle of view. Since using a short focal length lens or zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in or using a longer focal length lens gives a much narrower angle of view, so objects in a scene appear larger.
Lens focal lengths are based on the physical characteristics of the lens so they are absolute values. However, a given focal length lens may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "telephoto" depend on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies more. There are currently a number of differently sized image sensors used in digital cameras. For that reason, different focal lengths are needed to give the same image coverage on different cameras. Because of the confusion this causes, most digital camera companies give the actual focal length of their lenses and then an equivalent focal length were the lenses to be adapted to a 35mm camera. For example, a camera may list its lens as 7.5mm (equivalent to 50mm on a 35mm camera). Because digital equivalents vary widely, we often use the more familiar 35mm focal lengths in this book.

The impact of sensor size on focal length isn't limited to fixed lens cameras. Digital SLRs are often adapted from 35mm film cameras and use lenses designed to project an image circle large enough to cover a frame of 35mm film. When these lenses are used on a digital camera, the angle of view captured in the image depends on the size of the sensor placed within this image circle.

  • When the image sensor is the same size as a frame of 35mm film, called a full-frame sensor, then a lens' angle of view, and hence the focal length, is the same as it is on a film version of the camera.
  • When the image sensor is smaller than a frame of film, as many are, it captures a smaller area of the image circle, effectively increasing the lens' focal length by a factor of 1.5 x or so compared to the focal length indicated on the lens. Therefore, a lens that is 100mm on a film camera will be 150mm or 160mm on the digital version of the camera. This multiple works across the entire family of lenses that work with the camera, making wide-angle lens less so on a digital SLR, and making telephoto lenses more so.

A lens projects the image as a circle and the size of the film or image sensor determines what area of the circle is captured. Here the frames (from largest to smallest) show the areas captured by 35 mm film or a full-size sensor, an APS-H sensor, and a APS-C sensor.

A smaller sensor penalizes you when used with shorter focal length lenses (top). Its smaller sensor captures a smaller part of the image circle (the white outline) than a camera using a full frame sensor or film so it has a longer effective focal length

A smaller sensor gives you a bonus when used with long focal length lenses or macro lenses (bottom). Its smaller image sensor captures a smaller part of the image circle (the white outline), increasing magnification.


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