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2.3 The digital camera 67

Usually, a higher shutter speed (less motion blur) makes subsequent analysis easier (see Sec-
tion 10.3 for techniques to remove such blur). However, when video is being captured for
display, some motion blur may be desirable to avoid stroboscopic effects.

Sampling pitch. The sampling pitch is the physical spacing between adjacent sensor cells
on the imaging chip. A sensor with a smaller sampling pitch has a higher sampling density and
hence provides a higher resolution (in terms of pixels) for a given active chip area. However,
a smaller pitch also means that each sensor has a smaller area and cannot accumulate as many
photons; this makes it not as light sensitive and more prone to noise.

Fill factor. The fill factor is the active sensing area size as a fraction of the theoretically
available sensing area (the product of the horizontal and vertical sampling pitches). Higher
fill factors are usually preferable, as they result in more light capture and less aliasing (see
Section 2.3.1). However, this must be balanced with the need to place additional electronics
between the active sense areas. The fill factor of a camera can be determined empirically
using a photometric camera calibration process (see Section 10.1.4).

Chip size. Video and point-and-shoot cameras have traditionally used small chip areas
1
12
1
( -inch to -inch sensors ), while digital SLR cameras try to come closer to the traditional
4 2
size of a 35mm film frame. 13 When overall device size is not important, having a larger chip
size is preferable, since each sensor cell can be more photo-sensitive. (For compact cameras,
a smaller chip means that all of the optics can be shrunk down proportionately.) However,
larger chips are more expensive to produce, not only because fewer chips can be packed into
each wafer, but also because the probability of a chip defect goes up linearly with the chip
area.

Analog gain. Before analog-to-digital conversion, the sensed signal is usually boosted by
a sense amplifier. In video cameras, the gain on these amplifiers was traditionally controlled
by automatic gain control (AGC) logic, which would adjust these values to obtain a good
overall exposure. In newer digital still cameras, the user now has some additional control
over this gain through the ISO setting, which is typically expressed in ISO standard units
such as 100, 200, or 400. Since the automated exposure control in most cameras also adjusts
the aperture and shutter speed, setting the ISO manually removes one degree of freedom from
the camera’s control, just as manually specifying aperture and shutter speed does. In theory, a
higher gain allows the camera to perform better under low light conditions (less motion blur
due to long exposure times when the aperture is already maxed out). In practice, however,
higher ISO settings usually amplify the sensor noise.

12 These numbers refer to the “tube diameter” of the old vidicon tubes used in video cameras (http://www.
dpreview.com/learn/?/Glossary/Camera System/sensor sizes 01.htm). The 1/2.5” sensor on the Canon SD800 cam-
era actually measures 5.76mm × 4.29mm, i.e., a sixth of the size (on side) of a 35mm full-frame (36mm × 24mm)
DSLR sensor.
13
When a DSLR chip does not fill the 35mm full frame, it results in a multiplier effect on the lens focal length.
For example, a chip that is only 0.6 the dimension of a 35mm frame will make a 50mm lens image the same angular
extent as a 50/0.6=50 × 1.6=80mm lens, as demonstrated in (2.60).