Apple Aperture Digital Photography Fundamentals - Manual (Page 14)

14 Chapter 1 How Digital Cameras Capture Images
Zoom
A zoom lens, also known as an optical zoom lens, has the mechanical capacity to change
its focal length. A zoom lens can be extremely convenient, because many zoom lenses
can change their focal lengths from wide-angle to standard and from standard to
zoom. This eliminates the need to carry and change multiple lenses while shooting a
subject or project. However, because of the movement between focal lengths, the
f-stops aren’t always entirely accurate. To achieve a greater level of accuracy with
apertures, many manufacturers have multiple minimum aperture values as the lens
moves from a shorter focal length to a longer one. This makes the lens slower at longer
focal lengths. (See “
Understanding Lens Speed ” on page 15 for an explanation of lens
speed.) Plus, a zoom lens requires additional glass elements to correctly focus the light
at different focal lengths. It is desirable to have the light pass through the least amount
of glass in order to obtain the highest-quality image possible.
Prime
A prime lens , also known as a fixed lens, has a fixed focal length that is not modifiable.
Prime lenses often have wider maximum apertures, making them faster. For more
information about lens speed, see “
Understanding Lens Speed ” on page 15. Wider
apertures allow for brighter images in low-light situations, as well as greater control
over depth of field. Prime lenses are primarily used by portrait photographers. For more
information on depth of field, see “
Depth of Field ” on page 15.
Aperture
The aperture is the opening in the lens (created by an adjustable iris or diaphragm) that
allows light to pass through. The exposure of the image is determined by the
combination of shutter speed and the opening of the aperture. The larger the aperture,
the more light is allowed to pass through the lens. The aperture is measured in f-stops,
and each stop represents a factor of two in the amount of light admitted. The aperture
setting (f-stop), combined with the focal length of the lens, determines the depth of field
of an image. For more information on depth of field, see “
Depth of Field ” on page 15.
Understanding Digital Zoom
The digital zoom feature offered by some camera models does not really zoom in
closer to the subject. Digital zoom crops into the center area of the captured frame,
effectively enlarging the pixels. This results in a picture with a lower overall image
quality. If you don’t have a telephoto or optical zoom lens and you want a close-up,
physically move closer to the subject, if you can.