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 that light as individual dots or “pixels”. Now let’s take a look at how our two zoom types, optical and digital, use the camera components to bring images closer. We’ll begin with optical zoom.
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
For example, let’s assume that you’re pointing your camera at the outside of your house. Now let’s say that you want to zoom in on one of the house’s windows using digital zoom. As you zoom in, the image inside the camera is completely unchanged. Your sensor chip still sees the entire house because the lens hasn’t changed the image. However, the sensor chip creates a zoom effect by discarding sensor information that doesn’t pertain to the window. If the window makes up 5% of the total house image, then the sensor chip will only use 5% of its individual photosites to capture the image, ignoring the other 95%. The result is a zoomed in image that is much lower resolution because it’s using only a small area of the sensor to capture the image.
A Question of Quality
Notice that the image is still pretty clear and smooth. You can’t really see the individual dots that make up this image.
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Notice how blocky the picture is? Can you see the individual pixels? We’re getting this lousy quality because the camera is using only a portion of its sensor. My sensor still sees the entire school, but it has discarded all the parts of the picture that aren’t the flag To be fair, I should confess that I’ve deliberately simulated a worst-case, very low resolution situation to more clearly illustrate the difference. Digital zoom will always degrade the quality of the image, but it may not always be as obnoxiously bad as I’ve illustrated here.
A Word about Camcorders
I’ve spoken mostly in terms of digital still cameras thus far, but the optical vs. digital zoom distinction holds true for digital camcorders as well. Just like digital still cameras, digital camcorders have lens components that can be zoomed in or out to control an optical zoom. And, just like digital still cameras, many digital camcorders can also selectively scale down which parts of their sensor chips are being used to create a digital zoom effect. Some digital camcorders even take this digital zoom to extreme levels. Here’s the marketing label from my camcorder boasting a whopping 700x digital zoom capability.
Now that you understand how digital zoom works, I hope you can envision what a 700x digital zoom would do to the picture quality of a camcorder if we chose to zoom all the way in. We’re talking about a seriously blocky, low-quality video image at 700x.
Recommendations
Here are a few recommendations to help you deal with zooming:
- When selecting a digital camera, compare just the optical zoom capabilities of the different models you’re considering. As we’ve seen, optical zoom can get you closer to your subjects with much less loss in image quality. Most consumer digital cameras these days come with a 3x or 4x optical zoom, though some go up to 6x or even 10x.
- If your camera doesn’t have digital zoom capability, don’t worry. In fact, most high-end digital cameras do not come with any digital zoom capability whatsoever. Any decent photo editing program on a computer can achieve the same effect by cropping. Cropping allows you to draw a box around only the part of the picture you want to keep and throw the rest away. The effect works exactly the same way digital zooming does but it gives you more control.
- Most digital cameras will use their optical zoom capability first and then, if you continue to zoom in, they’ll go the rest of the way using digital zoom. If you want to avoid blocky, pixilated pictures, most digital cameras give you the option to shut off digital zooming. This will limit your zooming range to just your camera’s optical zoom capability, but it will guarantee that you’ll never ruin a picture with digital zoom.
- Digital zoom isn’t necessarily a bad thing. If you like the convenience of zooming way in on your subject and you don’t mind some loss in image quality, then by all means use your camera’s digital zooming capability. It can be a very convenient feature and, if you don’t go too far with the digital zoom, the image quality may still be good enough for your needs.
