Photography Tech Tip : Focal length and optical zoom

This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.

FOCAL LENGTH AND OPTICAL ZOOM
After megapixels, the second most talked about feature of compact digital cameras in particular is the zoom. Two types of 'zoom' are available in digital photography: optical zoom and digital zoom. All references to zoom within this article refer to optical zoom (see Tech Tip: Digital zoom and cropping).

Optical zoom is a measure of the ratio of the minimum and maximum focal lengths of a lens. E.g. a lens with focal range 50mm to 150mm has an optical zoom of 3x; a lens with focal range 75mm to 300mm has an optical zoom of 4x; and so on.

The image below shows two objects in frame with a short focal length used:

This shows the subject (blue arrow) on the left, the lens in the middle and the camera sensor on the right. You can see the projection of the subject onto the sensor (the image is upside-down because of the optical characteristics of the lens).

Now compare this to the image below. The subject is the same size, the distance from the lens to the subject is the same, and the sensor size is the same, but the focal length (the distance from the lens to the sensor) has been increased:

Increasing the focal length means the subject appears much larger in the frame. We've 'zoomed in' on the subject.

When you zoom in using a compact camera or zoom lens, you'll often seen the lens barrel move towards the subject. The camera is moving the lens away from the sensor, exactly as in in the example above, thereby increasing focal length.

At maximum focal length, the image appears as large as possible on the sensor.