Virtually all digital cameras are sold with a digital zoom facility, whether or not they have an optical zoom lens fitted. I’m going to demonstrate why digital zoom is of much less value than optical zoom. This also applies to camcorders for exactly the same reasons. The functionality of digital zoom can be replicated on your computer after uploading, quite possibly with higher image quality.

What is digital zoom?

Zooming refers to changing the focal length (angle of view) of a lens. A zoom lens therefore has a variable focal length. Many people often think that a zoom lens is the same as a telephoto lens, but this is not correct: you can get wideangle zoom lenses and prime (fixed) telephoto lenses. The only defining characteristic of a zoom lens is a variable focal length.

Digital zoom simulates an increased focal length by looking only at the central pixels of its sensor, and using those to fill the image. You can only zoom in digitally: the camera cannot look outside the optical image to simulate a more wideangle view.

What are its limitations?

Let’s take a look at an example:

This is a moderate wideangle shot (0.88d, 38mm equivalent) of the main bridge of Port Grimaud on market day on 18 August 2002. It has been scaled down from the original 2-megapixel shot (933KB) by a factor of four in each direction, resulting in an image that you might get from a 0.125-megapixel camera, if such things were sold.

Let’s pretend that this shot was actually taken with our hypothetical 0.125-megapixel camera; and let’s also pretend this camera has 4× optical zoom and 4× digital zoom, which can be used independently of each other. Although the resulting images are smaller and less detailed that from real cameras, the effects shown here still apply.

Digital zoom example

Using the digital zoom simply takes the centre of the original image and blows it up. I’ve done the same on my computer to give this 3.46d (150mm equivalent) image:

Optical zoom example

To simulate using the 4× optical zoom instead, I have pulled the centre from the original 2-megapixel image without downscaling, which gives us the same coverage as the digital zoom but at the same quality as the original image:

Note that the optical zoom has captured more information in the centre of the original image by not capturing the wider view. All of our camera’s 125,000 (approx) CCD pixels have been used to capture this image. The digital zoom has simply discarded the outside of the image and tried to synthesize the telephoto lens view artifically from just the central 7,800 pixels, which leads to drastically lower quality. Sci-fi films’ endless capability to “zoom and enhance” is just that—science fiction.

What about using both together?

Of course, all real cameras equipped with both types of zoom are designed to use the full range of the optical zoom first, then apply the digital zoom on top of that: this maintains image quality for as long as possible before resorting to the artificial digital method. Our hypothetical camera has a total zoom range of 16× (4× multiplied by 4×). Let’s look at the 16× image, which is like using a 13.9d (600mm equivalent) lens:

Although the quality is poor, it looks as though we have an image we could not create any other way. Not so — this image is simply the centre section of the optical zoom image above, blown up by the camera. No more information has been added, and you could have done the digital zoom on the computer afterwards which would give you flexibility for re-composing the image.

Conclusion

Don’t think that digital zoom is equivalent to optical zoom. It always gives inferior results. It does have some uses: extreme close-ups for focusing and exposure checking; saving memory space on cameras that only save the centre section rather than blowing it up first (rare); and saving post-processing time where you are certain additional zoom is required beyond the optical limit of the camera.

By all means use it, but be aware of its limitations.