I’m not going to spend a lot of time in this chapter on the mechanical aspects of digital SLRs. If you’ll examine Figure 1.3, which shows light entering a lens, evading capture by the diaphragm, bouncing off the reflex mirror up to your eye, and/or wending
its way to the sensor, you’ll realize that you probably already know that stuff. Here’s a
quick review with brief descriptions of the main components (by no means comprehensive; I’ll have more detail on some of them later in the book).
Light path. The yellow arrows represent an overly simplified path for the light
entering the lens and making its way to the viewfinder and sensor. In reality, the
light is refracted at angles as it passes through the lens elements (2), and is redirected down to the autofocus sensor by the mirror (3) or up to the focus screen
(4) or through the shutter to the sensor (9).
Lens elements. Lenses contain a varying number of elements made of glass, plastic, or another material. These elements are fixed in place, or can move in relation
to other elements to focus or zoom the image, or, in an image-stabilized lens, be
shifted to compensate for camera movement.
Mirror. The mirror is a flip-up partially silvered component that directs most of
the light upwards towards the viewing system and exposure meters, and some of
the light downwards toward the autofocus components. (In some SLR-like cameras, a non-moving mirror may bounce light upwards to an autofocus system, while
allowing the rest of the illumination to pass through to the sensor for live viewing
in an electronic viewfinder to back-panel LCD.)
Focus screen. When the mirror is flipped down, light that has passed through the
lens is focused on this screen.
Pentaprism/pentamirror. This optical component can be a solid glass block, or
pentaprism, silvered on two surfaces to reflect the light, or a less-efficient (in terms
of light transmission) hollow structure, a pentamirror, that uses only mirrors. The
reflective surfaces reverse the focus screen image received from the lens/lower mirror both laterally and vertically, producing a right-reading view.
Exposure meter. In the typical dSLR, the exposure meter detects light in the viewing path, using an array of points in the frame that range in number from a dozen
to more than 2,000, depending on the camera. Readings may involve brightness
only, or capture full red/green/blue information to allow the camera to make exposure decisions based on a matrix of points, a small “spot meter” set of points, or a
center-weighted average of the points interpreted.
Viewfinder. This optical window shows the image as displayed on the focus screen.
It includes a diopter adjustment that allows fully or partially compensating for the
user’s eyeglass prescription if the user chooses to shoot without using them, or needs
additional correction. Some viewfinders have a shutter that can be closed to keep
light from entering the viewing system and affecting the exposure meter (6).
Vendors also may provide a rubber or plastic cover to slide over the viewfinder window to block light. Additional accessories, such as a magnifier lens or right-angle
viewer, may be offered for the viewfinder.
Autofocus sensor. Some of the light from the lens is reflected downwards to this
sensor. It uses pairs of lenses (typically 3 to 51 pairs, depending on the camera) to
split off portions of the beam to form autofocus points or zones that are lined up, in
rangefinder fashion, to allow phase detection autofocus functions.
Shutter/sensor. The shutter is a pair of vertically traveling curtains (9, left) that
open consecutively to create a gap for light from the lens to pass through when the
main mirror flips up. The sensor (9, right) contains light-sensitive photosites that
record the image and pass it off to the camera’s analog-to-digital signal processing
chip, and then to a memory card for storage.
