Understanding Your Digital Camera
Lesson 6 - how the zoom lens works

How many times have you wanted to take a picture of a far away object or scene only to realize your regular lens just runs short of the picture in your mind's eye? Or you might want to take a picture of a flower but can't since your lens won't focus to a close enough range. These are common and exasperating scenarios.

Many of the lens properties of point and shoot cameras offer limited telephoto ranges, some more than others, which is often a selling point of the better camera... zoom lens range. Often, the telephoto range of a zoom is rated in powers of X, such as a lens offering 3x, 4x or even 10x zoom capabilities. The higher the X power digit, the larger the zoom range ability. This usually goes hand in hand with price and performance.

So what makes zoom lenses so special?

Well, the first and obvious reason is the ability to go from wide to telephoto with one lens. No longer do you have to carry around a bag full of expensive lenses, not to mention the weight of those lenses while trying to hike somewhere.

Another reason zooms are popular is the ability to zoom close from far away. The image will look different than if you walked up to the subject and used a wide angle setting to capture the same picture (more on the reason for that in another lesson). Angularity and depth of field play a big part in why a zoomed in image and a wide angle image of the same scene look so different.

In the illustration at right, the front lens element, L1, is stationary, as is L3, the rear lens element. L2 is the zooming lens element that makes the image appear closer when you zoom in.

Notice in the wide angle view with L2 closest to L1, the red arrows indicate a wider angle of view toward L3. As the user zooms the lens elements, the red arrows appear to almost stay straight. This would be closer to a normal lens of around 50 mm. As the user applies more zoom, the red arrows start to come closer together, indicating a tighter, more telephoto lens aspect.

Lastly, notice that the tele or zoomed-in portion uses the center-most portions of L2 and L3. Because these portions are more similar to each than different, compared to the image passing through L1, the less distorted the image becomes. But the biggest thing to understand is the amount of light or luminance that is cut off by zooming in.

Usually, when you look at the information about your zoom lens, you will find that the manufacturer may give two minimum F/stop values. One will be for the wide angle or zoomed-out aspect and the other for the tele or zoomed-in aspect. The amount of light cut off by zooming in affects the performance of the lens to give the same exposure in either zoom setting.

What does this all mean? Well, it means that you or your camera will have to use a slower shutter speed for a zoomed-in picture. This is why some cameras manufacturers offer vibration reduction or anti-shake features to make up for slower lens performance.

Zoom affect during exposure

Zoom lens elements moving from wide to tele

So, the next time you use your zoom lens on your digital camera, remember that it is doing a lot more than just giving you great pictures. The optics involved in the design of the lens have been carefully calculated and produced to provide the clearest image with the least light loss for the money you invested in this particular camera. Are there better zoom lenses? Of course, but if you paid $200, $300, $500, or more for your digital camera or just the lens itself, the lens quality should be comparable with the price you paid.

As always, there are better manufacturers with better glass than others. It is always better to buy a brand you know or have done research into. If you are considering a different camera with a non-removable lens than your current one, the lens characteristics are a major factor in the purchase. And if you are buying a better zoom for your digital than what came with it, always try to look at original equipment from the same camera manufacturer first to see what they offer. More often than not, the lens will be more expensive than third-party providers, but will generally contain higher quality glass, better glass grinding methods and stricter quality control than third-party lenses. They also are generally more expensive but worth it if you resell them later.