Whereas an optical zoom uses the optics (lens) of the digital camera to move you closer to your subject, a digital zoom simply uses a portion of the existing image and enlarges it digitally.
Enlarging the image digitally reduces picture quality, and should therefore usually be avoided. However, a judicious use of digital zoom may sometimes yield images that are of quite acceptable quality, especially in small prints. So, use with caution.
Some manufacturers label their lenses with the “total zoom” by multiplying the optical with the digital. Ignore total zoom claims because you can use any multiplier digital zoom you want in an image editing software.
What is important when comparing digital cameras is the optical zoom. Digital zoom can always be achieved later in an image editing software, such as Photoshop, so should not really be a determining factor when choosing a digital camera.
In the picture above, we used optical zoom to imply motion while zooming. This technique is only possible if your camera has a manual zoom ring. You need to set a slow shutter speed (so that your camera will record the zooming process), focus on the subject in the middle at wide-angle, then as you trip the shutter, zoom in at the same time. It’s best if your camera is on a tripod so that the only blurriness recorded is the result of the zoom and not of camera shake. The central part of the image should remain sharp. This technique is very effectively used in sports to accentuate and dramatize the action.
Read our Optical vs. Digital Zoom tutorial.