How It Works - Zoom Lenses
2014-02-09 21:03:26


Nearly all digital cameras, including even the most basic entry-level models, are equipped with compact but powerful zoom lenses. We’re so used to using them every day that we seldom stop to consider just how remarkable they are or exactly how they work. We just push a button and the image on the screen gets larger or smaller at our command. Despite the advances in optical quality and miniaturisation in recent years, zoom lenses are not a new invention. In fact simple zoom lenses were used as early as 1834 in astronomical and naval telescopes to vary the magnification of the image. The first telephoto lenses also included moveable elements to change the focal length of the lens. The first practical zoom lens that corrected for optical aberrations was introduced in 1932, and the first production zoom lens for 35mm cameras was introduced in 1959. Since then, advances in optical design, particularly the use of computer-aided design, has made the development and construction of zoom lenses much easier, and they are now used widely in all types of photography. Modern zoom lenses are marvels of advanced optical engineering. The demand for ever more compact digital cameras has put increasingly difficult demands on the camera designers. Components such as batteries and LCD screens are large and take up a lot of space inside the camera body, so one of the only ways to save space is by making the lenses smaller. Today’s lenses are a fraction of the size of those of just few years ago, but thanks to advances in technology, they perform as well as or even better than older lenses many times their size.

There are many different designs for zoom lenses, but they all have some basic principals in common. They consist of a number of differently shaped individual lenses or ‘elements’, some of which move relative to one another to alter the magnification of the image without altering the focus. The diagrams below illustrate a very simplified design for a typical zoom lens. It consists of two distinct lens systems, the zoom system and the focusing elements. The key element of the zoom system is a concave lens which disperses the light path, and which can be moved relative to a convex lens behind it which gathers it in again. The function of the zoom system is simply to control the width or dispersal of the light rays entering the front of the lens, and therefore change the magnification. The zoom system does not focus the light. This job is done by the rear elements of the lens system which focus the rays onto the imaging sensor ensuring a sharp picture. In this first diagram, we can see the relative positions of the lens elements when the lens is set to wide angle. As you can see, the path of the light entering the front element of the lens is narrowed, producing a lower magnification. As a side effect, this also concentrates the light entering the lens, allowing it all to fall on the sensor, which is why wide-angle settings have a larger effective aperture value. In the second diagram the lens is set to telephoto. The concave element disperses the light path, so only the centre area of it is gathered by the rear element of the lens. This produces higher magnification, since only the centre potion of the image is captured by the sensor. The dispersal of some of the light entering the lens is why longer focal lengths have a narrower effective aperture value 

While optical design and the latest ultra-low dispersion glass can make for some very small lenses, a couple of manufacturers have added some clever tricks to make their lenses even more compact. One solution is Sliding Lens System first developed by Pentax and seen on many of the company’s Optio range of compact camera. This same design is used by several other manufacturers to produce zoom lenses for very slim cameras. When the lens is retracted, a block of lens elements moves out of the light path and slots into a space beside the rear element, allowing the whole lens to fit into a space of about 15mm.

Equally ingenious is the Folded Optics System, first seen on the Konica Minolta’s Dimage X series. In this system, the zoom lens is mounted vertically within the camera body, and looks out trough a prism angled at 90 degrees, like a submarine’s periscope. This has the advantage that the lens doesn’t need to be extended before it can be used, allowing extremely fast start-up times. Sony uses a similar system developed by Carl Zeiss on its T-series cameras, as does the Casio Exilim EX-V7.

Many camera brochures tout digital zoom as being a major selling point, and you’ll often see digital and optical zoom figures added together as a combined magnification as though they were exactly the same thing. They are not. Optical zoom is provided by the magnification of the image by the camera lens, so the image uses all of the camera’s CCD to maximum effect. Digital zoom simply takes the centre part of the image from the CCD and digitally enlarges it, so you end up with a much lower resolution image. It’s like throwing away half your CCD. I always recommend that as soon as you get a new digital camera, go into the set-up menu and turn off digital zoom. If you need to zoom in more than your lens allows, try moving closer to your subject instead. This picture was taken at the wide-angle setting of a 3x zoom lens. It shows plenty of detail, but what if we want to get a bit closer to the main subject, the cute thatched cottage? By using the optical zoom, we can increase the magnification of the image, so the cottage fills more of the frame. Because it still uses all of the sensor area there is even more detail visible. If we use digital zoom instead, all that happens is that a portion of the original image is enlarged, so instead of using the whole sensor area, only about a third of it is used, so although the cottage fills the same area of the frame, it is far less detailed than the optical zoom image. I’ve exaggerated the effect somewhat in this example, but if you use large amounts of digital zoom this is what you are doing to your pictures.