Photography Tech Tip : Digital zoom and cropping

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.

DIGITAL ZOOM AND CROPPING
When the optical limits of a lens are reached (you're zoomed right out), in order to increase the subject size further it is necessary to enlarge the print size, creating a physically larger representation of the subject, or crop the image, which increases the relative size of the subject to the print area.

Increasing the print size will rarely be practical or desired so cropping the image is the most likely option to be used, however, both options have drawbacks as they are equivalent to 'zooming in' on the sensor and this is how digital zooming works. I.e. by increasing the size of the subject in this way you're also increasing the size of each sensor photosite (see Tech Tip: Sensors) that makes up the image, and this may cause pixelation (visible pixels) in the final product. For larger print sizes, which are normally intended to be viewed from a greater distance anyway, this effect may not be an issue, but this is a consideration when extensive cropping is required.

The basic rule with cropping is that if you half the source image size (the one captured by the sensor), then you half the resolution of the image when printed. In practical terms, if you start with a 6 megapixel image and crop to half the size, you have the equivalent of a 3 megapixel image at the end. This is the same as using a 2x digital zoom.

Digital zoom should always be used with caution and please remember that what looks good on a 3-inch LCD viewer (where the pixels aren't visible) may not look so good when enlarged to printable size. If in doubt, take the best possible shot without digital zoom and use an image manipulation application (e.g. Adobe Photoshop) to crop the image at a later date.