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Digital Camera Guide

What Is The Difference Between Optical Zoom and Digital Zoom? Which is Better?

Both optical zoom and digital zoom are means to magnify an image, but they work in radically different ways and yield radically different results. In general, optical zoom will always yield a far superior image than digital zoom.

How does optical zoom work? A digital camera that offers optical zoom works the same way as a zoom lens on a traditional camera. A traditional lens works by collecting light rays and projecting them onto a piece of film, or in the case of a digital camera, onto the sensor. The distance from the lens to the point where all of the light rays are in focus (focal point) is known as the focal length of the lens.

[Diagram illustrating focal length.]

Unlike a fixed lens, a zoom lens has several moveable glass elements inside. By adjusting these elements, the focal length of the lens can be changed. Adjusting the focal length both changes the view distance and reduces the field of view, causing the projected image to appear larger.

In a digital camera with optical zoom, the same number of pixels are captured when the image is magnified. The only thing that changes is which rays of light are projected onto the sensor to form those pixels.

How does digital zoom work? Unlike optical zoom, digital zoom works by scaling up the pixels in the final image after it is captured. This is very similar to cropping and resizing your picture in your favorite paint program, except that the camera does it for you.

[Illustration: The frame is captured, and the pixels inside the zoom region are selected. The selected pixels are expanded to fill the frame, which leaves gaps in the data. Based on its neighbors, the camera calculates what color the missing pixel might be. The combination of real and interpolated pixels creates the final zoomed image.]

Because simply doubling the size of the pixels would create a very "jaggy" or stair-stepped appearance, manufacturers use different techniques to fill in the missing pixels. The idea is to come up with a reasonable approximation of what color that pixel might have been had it been captured. Many algorithms involve looking at the pixel's nearest neighbors and coming up with some sort of an average; other algorithms are more sophisticated and take several different factors into account. While some algorithms are unquestionably better than others, the truth is that all of these algorithms are still only producing "educated guesses"... they are not able to determine what color the pixel is in reality. It is for this reason that digital zoom does not produce as crisp or clear an image as optical zoom, which has true color values for all of the pixels.

Is there ever a reason to use digital zoom? Many experts would say "no" as there is nothing that can be achieved with digital zoom that cannot be achieved after the fact - perhaps even with better results - with your photo editing software. However, in practical use many beginners still find digital zoom a nice feature versus no zoom at all.

If you are a novice with your computer, you may find it easier and more convenient to simply use your camera's digital zoom feature rather than to crop and resize the image on your PC. In addition, the growing number of docking stations, stand-alone print devices and photo services that will print directly from your camera's memory card are making it such that you don't necessarily have to own a PC to use a digital camera. If your camera doesn't have an optical zoom feature, then a digital zoom feature may still be useful in these cases.

Will digital zoom improve in the future? Maybe, but due to the nature of the differences it seems unlikely that digital zoom will ever replace optical zoom. Some advanced techniques such as fractal image restoration have the potential to yield serious improvements over the digital zoom methods commonly used today. However, general-case algorithms - especially ones that will work with the limited processing power available within a camera - still have a way to go before you are likely to see them widely used in this way.

At this time, optical zoom is the superior choice to digital zoom when selecting a digital camera.