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Digital photography All-in-one desk reference 3rd edition - Busch D.

Busch D. Digital photography All-in-one desk reference 3rd edition - Wiley publishing, 2006. - 755 p.
ISBN: 0-470-03743-1
Download (direct link): digitalphotographyallinone2006.pdf
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Magnifications and focal lengths
Comparing zoom ranges of digital cameras can be confusing because the exact same lens can produce different magnifications on different cameras. Thatís why you usually see the zoom range of a digital camera presented either as absolute magnifications or in the equivalents of the 35mm camera lenses that the zoom settings correspond to.
Zoom range doesnít relate to lens quality. Youíll find excellent 4:1 zooms and average 4:1 zooms. However, the longer the zoom range, the more difficult it is to produce a lens that makes good pictures at all zoom settings. You should be especially careful in choosing a lens with a longer zoom range (8:1 or above); test the camera and its lens before you buy. However, lenses from the major manufacturers (Canon, Sony, Nikon, Fuji, and so forth) are all generally quite good.
Until the advent of digital cameras, figuring the magnification of consumer camera lenses was relatively easy because, in recent years, most consumer (and the workhorse professional) film cameras used a standard film size ó the 35mm film frame ó which measures a nominal 24mm x 36mm. Some cameras also used Kodakís now-discontinued Advanced Photo System (APS) film format, which produced images in three different configurations.
Book II Chapter 1
Choosing the Right Camera
Understanding How Lenses Work
Optical zoom versus digital zoom
Non-SLR digital cameras have two kinds of zooming capabilities: optical and digital. Optical zoom uses the arrangement of the lens elements to control the amount of magnification. Usually, optical zoom is the specification mentioned first in the camera's list of features. Digital zoom is a supplementary magnification system in which the center pixels of an image are enlarged using a mathematical algorithm to fill the entire image area with the information contained in those center pixels.
Digital zoom doesn't really provide much in the way of extra information; you could zoom in on
an image in your image editor if you like. However, digital zoom is a way of turning a 4:1 zoom into an 8:1 (or better) zoom lens even if the results aren't as good as those you'd obtain with a true optical 8:1 zoom. I tend to discount digital zoom capabilities when buying a camera because I want the sharpest picture possible, and many of the digital zoom pictures I've taken have looked fuzzy and pixelated. The feature doesn't cost you anything, and you can usually switch it off so you won't accidentally grab a digital zoom picture by mistake.
The magnification of any particular lens with a standard film size is easily calculated by measuring the distance from the film the lens must be positioned to focus a sharp image on the film. (This is the focal length of the lens.)
By convention, in 35mm photography, a lens with a 45-50mm focal length is considered a normal lens. (The figure was arrived at by measuring the diagonal of the film frame; you can calculate the focal length of a normal lens for any size film or digital camera sensor by measuring that diagonal.)
Lenses with a shorter focal length, such as 35mm, 28mm, 20mm, or less, are described as wide-angle lenses. Those with longer focal lengths (such as 85mm, 105mm, or 200mm) are described as telephoto lenses. We've lived happily with that nomenclature for more than 75 years, since the first 35mm camera was introduced. Wide-angle and telephoto images are shown in Figure 1-3.
Then came the digital camera, and all the simple conventions about focal lengths and magnifications went out the window. For good and valid technical reasons, most digital camera sensors do not measure 24mm x 36mm. You wouldn't want a sensor that large (roughly 1" x 1.5") anyway in a compact digital camera because the camera would have to be large enough to accommodate it. In addition, as with all solid-state devices, the larger a device such as a sensor becomes, the more expensive it is to manufacture. Sensors that are as large as the full 35mm film frame are available for an increasing number of digital SLR cameras, which means the magnification effect I'm about to describe doesn't apply for those models.
Most sensors for non-SLR cameras are more likely to measure, say, 16mm x 24mm or less. Even the larger digital camera sensors might be no bigger
Understanding How Lenses Work 89
than about 38mm x 38mm. So, a normal lens on one digital camera might be 8mm, whereas another normal lens on another camera might be 6mm. A 4:1 zoom lens can range from 8mm-32mm or 5.5mm-22mm. What a mess! How can you compare lenses and zoom ranges under those conditions?
Figure 1-3: Telephoto (top) and wide angle shots (bottom) provide different perspectives on a subject.
Book II Chapter 1
Choosing the Right Camera
Understanding How Lenses Work
Camera vendors have solved the problem by quoting digital camera lens focal lengths according to their 35mm equivalents. If youíre already familiar with 35mm camera lenses, thatís great. If youíre not, at least you have a standard measurement to compare with. Thatís why you often see digital camera zoom ranges expressed as ď35mm-135mm equivalent (roughly 4:1)Ē or some similar expression. You can safely use these figures to compare lenses in your quest for the perfect digital camera.
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