Casio QV-5000SX User Manual download pdf (Page 2)

ADC

CCD

Figure 2: Expanded view of the image pre-processing

stage

The Autofocus Sub-System – Background

Information

Autofocus has been used in mid-range film cameras for

quite some time. The use of AF is governed by two kinds

of issues. On the consumer side, AF may not be used

primarily because of cost. AF lenses are much more

expensive than

fixed focus

lenses. On the professional side,

AF may not be used more because of mistrust of the AF

algorithm. Increasingly, however, most high-end cameras

only come with AF lenses and their acceptance is improving

– especially amongst photojournalists who are shooting

sports events where manual focus may not be possible.

The low-end market, when trying to choose a lower cost

alternative to AF will choose a fixed focus approach. A

fixed focus lens with a wide depth-of-field (large range to

object distance, that remains in acceptable focus) is used

instead of an AF lens. This is typically a 35-mm wide-angle

focal length lens for a 35-mm format camera. The tradeoff

of this approach is that it does not allow selective focus

control, and a wide-angle lens is not the optimum choice for

applications like portrait photography.

Increasingly, both at the low end and high ends of the

market the user/professional is demanding AF cameras and

lenses. In camcorders, the existence of AF is assumed.

The basic idea behind AF is to determine the distance of

the subject from the lens and move the motorized lens to

correctly focus the subject onto the image plane of the

optical system. The “high cost” generally associated with

AF systems come from the cost of these motorized lenses.

There are numerous approaches that manufacturers have

taken to implement AF. Amongst the largely popular ones,

we can identify two approaches that stand out, namely:

1. Contrast-based algorithms:

These algorithms typically analyze the image and determine

the highlight and shadow values. The difference between

these values – the contrast – is at a maximum for optimum

focus. This approach is analogous to

manual focusing

by the

human eye. When a photographer focuses the image on

the ground glass of the cameras, he is essentially

maximizing contrast. An example of determining AF using

this technique is to place 3 linear CCD's in the focal plane,

in front of and behind the focal plane. The output of the

CCD is analyzed for maximum signal variations (contrast).

The disadvantage of this approach is that it requires high

level of light and the operation is also slowed by the signal

processing algorithms used. An alternative approach, which

images the “focused” image through a lenslet array and

determines the separation of the signals on a CCD array,

overcomes the limitations of this approach (Minolta

developed it in 1985). In low light or low contrast

situations, these algorithms employ the use of projected

beams (from an LED in the camera body) or the use of a

pre-flash (“Flash-assist”) to illuminate the scene.

In conventional cameras, this approach required finding

an appropriate location for the sensors. In digital cameras,

the advantage lies in the fact that the main image sensor

itself can be used for focus sensing. One approach involves

taking the image, computing its Fourier Transform and

analyzing the frequency content for maximum frequency

gain between frames. The image in focus will have the

maximum frequency gain. This technique will also perform

poorly in low-light conditions. However, image-processing

techniques can be used to maximize the signal under low

light conditions.

2. Ranging algorithms:

These algorithms involve scanning a scene with infrared

light, which is emitted from a photodiode. The Infrared

light that is reflected off the subject is used by another

photo-diode to determine the distance from the cameras

using triangulation. The use of Infrared allows for focusing

in low and no light conditions. Some subjects, such as

window glass or black objects may have very high or very

low IR reflections, which generate false calculations of

distance. Visible light is finding increasing use as it allows

the use of visible laser that can be pointed to the subject of

interest (much like handguns that have a laser pointing) and

the photographer can see the subject that the beam is

focusing on. This beam must be disabled when the

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Casio QV-5000SX User Manual Page 2

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