As the diagram below shows, however, digital zooms work by simply blowing up the central part of the image captured by the sensor. It looks like you've zoomed in further, but in fact all that's happened is that the middle part of the picture has been cut out and blown up:
1. This is the original image, and the red rectangle in the centre indicates the much smaller area in the centre which is used when the digital zoom is activated.
2. Sometimes this is saved exactly as it is, so that you get an image with much smaller dimensions, just as if you'd cropped the original in an image-editing program. Some makers call this 'precision zoom' or 'extra optical zoom' because the image isn't being reprocessed or blown up in any way.
3. Usually, though, the camera will blow up this centre section by interpolating (resampling) the pixels, so that you end up with an image with the same number of pixels as you would normally. The quality is much lower, though, and the illustration above gives a pretty good idea of just how much softer and less detailed photos are when they're taken with a digital zoom.
Makers will often introduce impressive-sounding technologies which make it harder to figure out exactly what's going on, but they still boil down to the same thing. For example, this is what Panasonic says on its website about the Lumix TZ10:
"Furthermore, the Extra Optical Zoom function that extends zoom power to 23.4x (at 3-megapixel resolution) by using the center part of the large CCD to bring subjects even closer."
Now if it's using the centre part of the CCD, and the resolution is reduced, that sounds like a perfectly standard digital zoom cropping. So is it really reasonable to call it an 'Extra Optical Zoom function'?
Digital zooms are worth using only if you're prepared to accept the lower resolution in exchange for a tighter crop on your main subject. But if you find you're using the digital zoom often, you'd be better off swapping your camera for one which has a longer optical zoom range.
