Not All Zooms Are Created Equal

Walking through the camera aisle of my local electronics retailer, I notice the features tag for a modest-looking digital camera on the shelf. The tag boasts that this camera has a “40x Zoom”.

40x zoom!? Wow! That sounds really great. With such incredible zooming ability, I should be able to get great close-ups on birds in flight. Or maybe I could freak my wife out with a tremendously detailed picture of a spider in all its hairy-legged glory. Right? Well no, not necessarily.

Upon further examination of the tag, I see that this 40x zoom is actually made up of a 4x optical zoom and a 10x digital zoom. True, the product of a 4x optical zoom and a 10x digital zoom does indeed yield a 40x total zoom, so why then am I making a fuss about how much of that zoom “optical” and how much is “digital”? The answer is that optical and digital zoom are two completely different animals and, more importantly, they have very different effects on the quality of your pictures.

To understand the difference between these zoom capabilities, let’s begin with a couple basics about how a digital camera works.

Digital Camera 101

For the purposes of our discussion, you can think of a digital camera as having two key components: a lens and a sensor chip. The lens is a collection of several pieces of glass that bend and manipulate light as it enters the camera body. The glass inside the lens moves to bring the image into focus. In a film camera, the light comes through the lens and hits a piece of film to capture the image. In a digital camera, the image coming through the lens is captured by a light-sensing electronic chip rather than film.

Sensor chips are made up of millions of individual light sensors called photosites. These photosites translate what they see into an individual dot or “pixel” in the picture. The number of dots that make up your picture define the picture’s resolution. More dots means higher resolution. In a high resolution picture, the dots are tiny and there are a lot of them, so your eye sees a nice smooth image rather than individual dots.

So the lens manipulates the light entering the camera and the sensor captures

Optical Zoom

Optical zoom makes images look closer or further away by adjusting the light coming through the lens. Individual pieces of glass inside the lens – called “elements” – move forward or backward to manipulate the image. If the lens is zoomed out, a wider perspective of the scene is brought through the lens. If the lens is zoomed in, the perspective narrows to pull in only a portion of the scene, making it appear closer. Again, when we’re dealing with optical zoom, this manipulation of the image is done completely by the lens.

So what’s going on inside your camera while the lens is doing all this zooming? Inside your camera, the sensor chip is blissfully unaware of how the lens has manipulated the image. The senor chip just continues to capture whatever image hits its millions of photosites. If the lens projects a wide scene on the chip, it captures the image using 100% of its photosites. If the lens projects a narrow, zoomed-in scene, the sensor chip again captures whatever image is projected across its entire collection of photosites.

The important point here is that the image, wide or narrow, is still being captured by the entire surface of the sensor chip. All the individual photosites on the chip are used to capture the image regardless of whether the lens is zoomed in or zoomed out. Because 100% of the photosites are being used to capture the image, your pictures will continue to be of the highest resolution your camera is capable of producing.

Digital Zoom

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