Apple Aperture Digital Photography Fundamentals - Manual (Page 15)

Chapter 1 How Digital Cameras Capture Images 15
f-stop
The photographer adjusts the opening of the aperture by setting the f-stop. An f-stop
is a ratio of the focal length of the lens to the diameter of the opening of the aperture.
For example, a 50 mm lens with an aperture opened up to a diameter of 12.5 mm
results in an f-stop of f4 (50 ÷ 12.5 = 4). Therefore, the larger the numerical value of the
f-stop, the smaller the opening of the aperture. The speed of a lens is determined by its
largest f-stop value (smallest number). Thus, the larger the aperture, the faster the lens.
Depth of Field
Depth of field is the area of the image that appears in focus from foreground to
background and is determined by a combination of the opening of the aperture and
the focal length of the lens. A small aperture setting results in greater depth of field.
Controlling depth of field is one of the easiest ways for a photographer to compose the
image. By limiting the depth of field of an image, the photographer can turn the
attention of the viewer on the subject in focus. Often, limiting the depth of field of an
image helps eliminate clutter in the background. On the other hand, when shooting a
landscape, you want the image to have great depth of field. Limiting the depth of field
to the foreground would not make sense.
Understanding Lens Speed
A lens’s speed is determined by the maximum amount of light the lens is capable of
transmitting—the largest f-stop value. When a lens is capable of transmitting more light
than other lenses of the same focal length, that lens is referred to as
fast . Fast lenses
allow photographers to shoot at higher shutter speeds in low-light conditions. For
example, lenses with maximum f-stop values between 1.0 and 2.8 are considered fast.
f2 f2.8 f4 f5.6
f8 f11 f16 f22