MadSci Network: Computer Science

Re: How does zoom work?

Date: Sat Nov 22 11:23:21 2003
Posted By: Harry Adam, Research Associate
Area of science: Computer Science
ID: 1068444148.Cs

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 -
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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