|MadSci Network: Computer Science|
Hi, David – you ask a simple question which underlies a complex subject. I’ll try to keep it as uncomplicated as I can, but I need to explain a bit of background first. Cameras are designed to make a sharp image (focus) on a plane where the image is formed. A pinhole would do the job - causing a ray of light from the subject to fall only on one part of the image sensor (film or digital chip), but pinholes are impractical for photography because they let so little light through, and exposures would be intolerably long for most subjects. Handholding the camera would not be possible. There are other optical problems, too. So, lenses are designed to allow lots of light to get through to the image sensor, shortening exposure times and overcoming the optical problems you get with pinholes. In most cameras, the image size is fixed – for example in a 35mm film camera it is a rectangle 24mm high and 36mm wide. The lens is round, and causes a circular image to fall on the imaging plane. The diameter of the circle has always to be bigger than the diagonal of the image rectangle. In most cameras, the lens mount is fixed. This means the mount to image plane is also fixed although with interchangeable lens cameras extension tubes and bellows do allow this to vary. A key parameter of the lens is the focal length. Essentially this is the distance between the lens and the image plane such that light from very distant objects are brought into focus at the image plane. This is easy to understand when a lens is a simple single element, but most camera lenses are compound – which means they are made of lots of separate individual lenses. Compound lenses have an effective distance from the image plane, somewhere amongst all the elements and groups, and the further away from the image plane that is – the longer the focal length. (Long focal length lenses are long, and short focal length lenses are short - for exactly this reason.) So what does focal length do? The effect of different focal lengths can be thought of in two ways. One is magnification. Binoculars are almost always marked in terms of magnification – e,g, 8x. This means that the image seen by the eye – or the film - is magnified so we see it 8 times larger than normal. The other is field of view – in other words, how much of the scene is included. The two go together – but inversely – the bigger the magnification, the smaller the field of view. So with that background – what is a zoom lens and how do they work? Well another name for a zoom lens is a varifocal lens. The name tells you what’s special – the focal length can be varied. This means the field or angle of view is also varied. A zoom lens also has the property that these things are varied while the plane of focus remains the same. In a viewfinder, zooming has the effect of showing a smaller or larger field of view with a more or less magnified part of the image. How this is done is by using compound lenses in which the separate elements or groups of elements of the lens can be moved relative to each other. (The optical design calculations are frighteningly complex – so much so that many designs would never have been made without the help of computers.) Recently, with digital cameras, one often sees “extra zoom” capability advertised. For example, “3x optical zoom” and “4x digital zoom” – giving a zoom total of 12x. “Digital zoom” is really a trick – and not zoom at all. It is simply a crop of a small part of the image and the extra magnification is done in printing. The problem with this is that in essence it is an artificial way of changing (diminishing) the size of the sensor. (Digital zoom throws away the information collected around the outside area of the sensor.) The result is a loss of image quality because all the magnification is done in printing and not in capturing the image, unlike the true magnification you get with optical zoom. So why would anyone want to have “digital zoom”? The only reason is to do the cropping in the camera instead of afterwards in the computer – which is impossible to do if you don’t have a computer or are going to get your digital images printed straight from the camera before you get to a computer. If you are interested in seeing how complicated the elements and groups in zoom lenses are - try this link - http://www.star.ucl.ac.uk/~rwesson/esif/om- sif/lensgroup.htm - and click on almost any of the zoom lens examples which provide a diagram of all the glass elements in their layout in the lens. I hope this all helps answer your question, and also stimulate your interest in photography. Have fun!
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