Datasheet

5
■ BRIDGING THE FILM/DIGITAL GAP
Lenses: Getting the Numbers Right
Digital cameras, just like film cameras, rely on lenses to collect and focus light onto the
sensors. If you’ve used a 35mm film camera, you are probably familiar with how lenses
are numerically differentiated in terms of focal length. (The stated focal length is the
distance between the optical center of a lens and the medium that captures the image—
either the film or sensor surface—when the lens is focused at infinity.) For example, a
lens with a focal length between 40mm and 55mm is considered “normal” and covers a
field of view similar to human vision, about 50 degrees. Focal lengths less than 40mm
are considered wide angle and focal lengths over 55mm are considered telephoto, with
a narrower field of view. A lens designated as wide angle will expand the field of view,
and long telephoto lenses narrow the field of view and appear to bring far objects
close. Zoom lenses provide a range of fields of view, often going from wide angle to
telephoto. A versatile zoom lens might provide a range of focal lengths, say from 28mm
to 200mm. And so on.
Note: Because the sensor of a digital camera is physically different and responds to
light differently than film, digital camera lenses are often designed from the ground up to
take into account these differences. Some camera manufacturers are more successful than
others in producing lens/sensor combinations that contribute to overall image quality. If
you use a digital camera that accepts interchangeable lenses, it is important that you choose
lenses carefully. Not all lenses will give you optimal quality. A lens that works fine with a
film camera can produce color aberrations, flare, and lack of sharpness when used with
a digital camera.
When shopping for a digital camera it’s natural to want to compare focal lengths
of digital camera lenses with those of familiar 35mm film cameras. But although a digi-
tal camera’s “normal” lens will capture approximately the same field of view as a film
camera’s “normal” lens, the physical focal length considered normal is usually much
shorter. (Because of this difference, it’s typical to refer to “35mm equivalent” focal
length when discussing lenses for digital cameras, and I’ll use that term throughout this
book.) A 50mm lens that is considered “normal” for a 35mm film camera would be
considered a telephoto lens for many digital cameras.
Why? Because most digital cameras use sensors that are smaller than 35mm
film—sometimes only a quarter of the size—and the smaller the sensor, the shorter the
normal focal length. Figure 1.3 illustrates this point. With the same physical focal
length, a 35mm film camera (which has a 24mm
× 36mm frame size) would show a
somewhat larger field of view, with less magnification, than a digital camera with a rel-
atively large sensor (such as the Nikon D200, with a 23.7
× 15.6mm sensor). With the
8.8
× 6.6mm sensor on the Sony DSC F828, we have a much smaller field of view and
more magnification.
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