STsite's Guide to Still Photography Camera

Digital Zoom vs Optical Zoom Lens

Some digital cameras actually don't have a zoom lens yet it is marketed as a camera with 3X digital zoom and sometimes just the misleading "3X Zoom". Digital zoom is not really zoom lens in the strictest definition of zoom lens - they are "faked" lens.

Before we go any further on the subject of digital zoom vs optical zoom, we should point out the difference between a digital zoom and a digital lens. Try not to get confused between these two. A digital zoom is a faked "lens" as it does not exist in physical form whereas a digital lens on high end digital camera is an optical lens designed specially for digital imaging.

Light strikes on film surface at an oblique angle.
Light strikes on image sensor at an oblique angle.

There are distinct physical differences between film and image sensor. Film has a flat 2 demensional surface whereas an image sensor has a 3 demensional surface formed by millions of tiny photo diodes or photo-cells. Traditional lenses were designed for optimun performance on film's 2 demensional surface so these lenses don't work equally well on image sensor's 3 demensional surface. Image sensor's performance is at its peak when light strikes on their photo-cells at a perpendicular angle to the sensor surface. Image sensor performs poorly when light strikes them at an angle. On the other hand films, with their flat surface, are effective and perform equally well no matter at whatever oblique angle the light hits them. When an optical lens designed for film is used with a digital camera it tends to have insufficient brightness, inaccurate colour reproduction and vignetting at the edge and corner of the images.

This calls for a new approach in lens design. Lenses with the new design concept dedicated for imaging with image sensor are called digital lens. They are actually optical lenses designed for optimum performance for digital cameras. There is nothing "digital" about the digital lens though digital lenses do have IC chips embedded inside them which is for information exchange between the camera body and lens about focal length, aperture setting, etc. However there is nothing new in having a cpu chip inside a digital lens as many 35mm SLR film cameras already have this kind of chip inside their optical lens for the past ten years. Maybe digital lens should rightfully be labelled as optical lens designed for enhanced digital imaging.

Let's get back to our topic on digital zoom. To make a long story short this is how digital zoom works: If you have used any imaging or graphics software on a computer you would have noticed how you can blow up or "zoom" in an image with the magnifier icon of that software. That is exactly what those digital zoom cameras do. You would also notice how an image degrades as you use the magnifier icon on the imaging software. The same thing happens when you use a digital zoom.

The digital zoom function actually crops out the outer part of the image on the image sensor according to the amount of digital zoom you apply. The image on the centre portion of the image sensor is then "enlarged" and the result is digitally "zoomed" and displayed on the LCD view finder. In actual fact the pixel on the centre portion of the image sensor is "duplicated" digitally. This process is called interpolation. All these "enlarging", "zooming" and cropping of the images are done digitally instead of actually zooming with an actual lens and hence called digital zoom.

3 times digital zoom Vs 3 X optical zoom
	left -- Original photo taken without zoom. The two below are comparison of digital vs optical zoom
3 X digital zoom	3 optical Zoom

With the same zoom ratio an image zoom with a digital zoom is lower in image quality and tends to be "pixelated" than one taken with the same zoom ratio optical zoom lens.

Every digital camera now comes with this digital zoom function. It costs next to nothing to the manufacturer to add a digital zoom to a digital camera as compared to an optical zoom. Yet the marketing department has a lot to sing and shout about on the product launch of the brand new "digital" zoom camera. Digital zoom function should be left out of the equation when comparing digital camera. In order to compare apple to apple so only use the optical zoom for comparison when comparing lenses of two cameras in your short list.

Even without using the digital zoom in the camera you can still reach the same result once you load the picture on your editing software in your computer. You will have more control doing it on your graphic software than using the digital zoom function of you camera.

Digital zoom is only practical if your digital camera has a resolution of 5 megapixels or higher. The faked "zoom" practically crops and throws image information away when you use the digital zoom. No image details are added to your enlarged image unlike the actual optical zoom. A digital zoom is fun to play with but we don't bother to use it and in our opinion digital zoom is more of a marketing gimmick. If you are concerned with image quality turn off the digital zoom function from your digital camera's menu system.