Casio QV-5000SX User Manual download pdf (Page 4)

subject

In 'Single-Spot Auto Focus', the distance to the

center of the image field is measured. These measurements

and adjustments, along with ambient light metering and

exposure calculation, are performed when the shutter button

is depressed halfway.

Since it supplies its own light source, and is independent

of the imaging system, the AF system can operate in

complete darkness and still obtain accurate focus. This

system is similar to, but not exactly like the IR autofocus

system on many point and shoot film cameras, and Kodak's

earlier DC120 digital camera.

The camera also has a manual focus setting that permits

the camera to be set to fixed focus at any one of nine

distances (20m, 10m, 5m, 3m, 2m, 1m, 70cm and 50cm).

When in fixed focus mode, the delay between shutter press

and shutter opening is slightly less than when either of the

autofocus modes are enabled. By holding the shutter

release in the half-depressed position, a shot can be taken

almost instantly.

Independent methods to measure distance to the target,

like in the DC260, entail the use of additional hardware

over contrast detection focus systems that use the cameras

lens and CCD to measure image contrast. The advantage of

distance measurement schemes is their ability to operate in

no light conditions, on subjects with inherently low contrast.

Since the hardware for this system is independent from the

image chain of the camera, it is also possible to make the

system much faster. On the down side, the additional

hardware adds cost and complexity, and can also be tricked

by subjects that are highly reflective like glass or water

surfaces.

The Casio QV-5000SX employs passive phase-difference

detection AF, using a separate sensor pair. Similar AF

sensor systems are in use on a number of middle to high-

end autofocus point and shoot film cameras. Active AF has

the advantage of being able to focus in low contrast or low

light situations, since the light source for range

determination is carried in the camera. Multi-beam active

systems with multiple sensors also have fewer electronic

and optical components than passive systems.

The QV-5000SX includes an easy to operate manual

focus mode, which allows in-focus pictures to be taken

when the AF system fails. The manual focus mode is

accessed from the setup menu in 'Record' mode, and

thereafter is actuated by the +/- button on the top of the

camera. The rear panel LCD becomes active automatically

in this mode so that a correct focus determination can be

made. There are eight separate focus zones from infinity to

10 cm, so that the lens automatically switches from 'normal'

to 'macro' mode.

The Casio uses standard phase detection AF ('passive

AF') similar to that used in many of today's high-end point

and shoot cameras. The phase detection sensor is located

directly above the camera lens and consists of two plastic

injection molded lenses in front of a two segment linear

CCD array. Nearly identical sensors have been found on

the Kodak DC260 as well as many film AF cameras. The

linear CCD lies in the horizontal plane of the camera, so

that the system is only sensitive to vertical lines. Since

there is no auxiliary light or projected target (as there was

on the Kodak DC260) in low light or low contrast scenes

the camera will fail to AF.

The current trend in AF (either film or digital) is to use

active infrared or NIR (near infrared) (range-finding). AF

on low-cost cameras and passive (contrast-detection)

systems on higher cost cameras, where a microprocessor

and other necessary electronics are already available.

Active systems have the advantage of rapid AF and accurate

determination of focus even in zero contrast and no-light

scenes, but are limited in range and suffer from interference

from very bright lights. Passive AF can work at any range

but requires some light and scene contrast to achieve focus

lock. Some very recent APS film camera designs (notably

the Canon ELPH series) use a combination of active (at

short distances) and passive (at long distances), the passive

system assisted at middle distances by a projected near

infrared target.

Camera Product Feature Focus

Kodak DC260 Visible light ranging AF - center

weighted, AF 3 zone weighted, 9-zone

manual focus. Can focus in low

contrast and low light

Fuji MX-700 Contrast detect autofocus

Nikon Coolpix

900

Contrast detect AF, fixed focus at 30"

Table 2: Feature Comparison

Common Approaches for Automatic Focus &

Their Implementation Issues

There are two approaches commonly used in implementing

AF. For the sake of our discussion, we will refer to them as

ranging and contrast sensing. The differences in the two

approaches are characterized by the following design

decisions:

Power Consumption

Shutter Latency (time from shutter press to image

capture)

1. Power consumption issues:

1 2 3 4 5 6 7

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