Autofocus System in Cameras Abstract— in this paper,

Autofocus
System in Cameras

Abstract— in this paper, we will discuss the auto-focus system in the commercial
cameras. The autofocus system rely on one or more sensors to determine the
correct focus. In the modern days, the autofocus technology is composed of
active autofocus, passive autofocus or hybrid autofocus.  Each technique is geared with its unique
advantages and disadvantages. Thus, it would resolute to different
applications. This paper compares in detail for each technique and their
applications.

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Index Terms—Optics, Autofocus, Camera

 

I.     INTRODUCTION

W

hile focus can be obtained either manually or
automatically, it is much faster and easier to use autofocus system in modern
day camera. The focus optical system relies on one or multiple sensor with a
dedicated control system as well as a motor to focus automatically on an auto
or manually selected point or area. The focusing system that is currently
available in the market in most commercial cameras are either using the active
systems or passive systems with some that have hybrid systems. The active
systems use a mechanical system that sends some form of energy which actively
finds the distance to the subject of the image, then uses that to dictate the
focus 1. Some of the common method includes using sonar and infra-red light. The
passive systems obtain focus points by performing mathematical analysis on the
raw image data without transmitting anything towards the subject. Some of the
common methods include contrast detection and phase detection.

II.     History of autofocus system

The evolution of
the focus system has been rather short in years when compared to the camera
itself. The initial technology for imaging focusing relied on range finding.
This system relied on a principle called triangulation to determine the
distance between the camera and the subject 2.

The first
mass-produced autofocus camera was the point on shoot Konica C35 AF, released in 1977 5.
The Polaroid SX-70 Sonar OneStep was the first autofocus single-lens
reflex camera, released in 1978.

As the
technology progresses, they started developing lens with motorized lens
adjusting system which lead to the shift into passive detection system. The
Minolta 7000 was the first SLR with an integrated autofocus system, which has both
the AF sensors and the drive motor were housed in the camera body 6.

However the
system has evolved, the main goal has remained around the same idea: how to
effectively render a sharper image accurately and fast.

III.     Autofocus

In general, auto
focus can be divided into two types: active and passive. Active autofocus uses
the reflection of the energy emitted by the device to determine the object’s
distance. On the other side, the passive autofocus system does not actively
search for the object, hence the name passive. Instead, it calculates the focus
point from the image that was captured by the camera using mathematical
computation.

IV.     Methods of Active Autofocus

The active
autofocus technology is a separate unit from the optical system which means
that it is not connected to the photo-taking lens or the viewfinder of the
camera 2. It involves actively sending acoustical or optical signals to
search for the distance to the object. Two of the most popular system involves
sending signals in sonar or infrared.

A.     Sonar System

The sonar system, also known as the ultrasonic echo system was patent
for Palaroid SX-70 Camera. The focusing method is illustrated by Figure 1.

This system
emits ultrasonic chirp of approximately 60 kHz towards the subject 2. The
camera then translates the time-of-flight into distance and the lens position
is adjusted accordingly. The effective range of this method is from 0.2 Meter
to 10 Meters.

Figure 1 Sonar System 2

One of the
biggest flaw of this method is that because it’s emitting a soundwave, it makes
focusing an object that is behind glass impossible. This method also limits the
camera to be on the larger end of the spectrum. Hence, it is very rare that the
modern era commercial camera uses this system.

B.     Infrared Triangulation

The infrared
triangulation, as the name suggests, uses the infrared light to focus on its
subject. This system is based on the triangulation method which determines the
distance between the camera and the subject by looking at two different
reference points. In most cases, the reference points are the angle of emitting
light and the angle of reflection light.

Figure 2 Infrared Light Triangulation 2

As seen from
Figure 2, the infrared is emitted from an infrared emitting diode that is
powered by the discharge of a small capacitor when the user presses the shutter
button halfway 2. Depending on the design, this infrared light can be a burst
of light that last just milliseconds or it could be a series of ultrashort
pules. By adjusting the angles of emitter and receiver for which the maximum
amount of light received is found, the subject is pin pointed 3. The infrared
red that is reflected from the subject is measured in either the intensity (the
amount of) infrared or the flight-of-time in which the data is then translated
into distance. The returning beam is electronically filtered for a control of
accuracy and make sure it matches the frequency of the emitted beam.   In
case that no infrared signal is detected by the sensor, the camera sets the
lens to infinity focus. 

The biggest
drawback of this system is that not all material reflects infrared light. This
would result in inaccurate distance reading. Also, if any infrared source is
present on site, such as flame or bright incandescent light 1, the reading
from the camera can be severely altered, which may cause autofocus inoperable
under those conditions. However, this system is very compact. It is widely used
in the cellphone camera system.

V.     Methods of Passive Autofocus

Passive
autofocus system determine the correct focus by performing passive (mathematical)
analysis of the image. In this school of method, it generally does not direct
any energy toward the subject. However, under low light condition, a beam of
infrared light can be used to assist the measurement. Two of the most common
passive detections are Phase detection and Contrast detection.

A.     Phase detection system

Phase detection
is a technique where the primary image (the one that is recorded by the image
sensor) is projected onto a specialized autofocus sensor unit located behind
the primary image 4. This system was first introduced by Minalta Maxxum 7000
in 1985.  Due to the layout, this type of
system is called Through The Lens (TTL). In simplified term, there are two optical
systems which would produce two individual images of the same object but from
different perspectives.

Of course, in
real life, there would only have one photo taking lens in a camera. To have the
image split into two perspectives, a beam splitter is used to divide the incoming
light into the photo-taking lens to two halves.

Modern day Phase detection is built on top of
this principle and uses the light coming through opposite halves of the
photo-taking lens. Each optical system receives light only from its half of the
camera lens and forms a secondary image on individual, one-dimensional CCD (Charge-Coupled
device) sensors.

Figure 3 Simplified Phase Detection Principle 4

As seen in Figure 3, when the subject is in
focus, two secondary images are formed on the “in-focus-spots” of the linear
detectors. These in-focus-spots are a reference for the focus condition. The
system detects a focusing error when the secondary images leave the
in-focus-spots of their respective CCD sensors 4, shifting towards or away
from each other. The distance between the secondary images indicate the
direction in which the camera lens must be moved to achieve proper focus.

The above explanation is only assuming that the camera is
looking at a single bright spot on a black background, producing two sharp
peaks on each of the CCD sensors. In reality, the photographic scene has a
widespread object which include a large number of points. Thus, instead of
comparing single peaks, the CCD would be comparing waveforms.

Also, the phase detection system must be able
to analyze widespread object that includes both on axis and off axis points. A
condenser lens (also called field lens) is required for any off-axis image. The
condenser lens bends the cones of light from off-axis objects so that they
always illuminate both separator lenses. Contrary to on-axis focusing, the shift
in secondary imaging for is in opposite direction. The autofocus system can
recognize that the signal is off-axis as long as the they also shift the same
distance toward or from the reference point.

Depending on the scene to be photographed, TTL
phase detector can only be applied when the scene’s brightness is above a
certain minimum level. Furthermore, if a subject contains a contrast edge that
is oriented in the same direction as the pair of CCD sensors, as shown in
Figure 4 situation 1, the signal is unable be brought into focus. It is also
not possible for the system to focus if the subject is high repetitive surface/patter
like in Figure 4 situation 2. Tt is also impossible to determine focus if no
contrast edge is available at all, like in a clear blue sky.

Figure 4 Problems of Linear PD Sensors

To avoid focus inabilities, most focus points have
cross type focus points which is basically two pairs of the linear CCD strips around
in a right angle. The installation of cross-type AF points would require four
separator lenses with individual masks compare to the two separator lenses for
the Linear PD sensors as shown in Figure 5. Therefore, is not an option of all
AF points due to space limitations.

Figure 5 Type of PD Sensors

Usually the AF points in the central area have more
cross-types design whereas the outer AF points have a linear design. The number
of AF points and the type of AF points are usually dependent on the camera model.
The higher end the DSL usually result in higher AF points.

With these cross-type autofocus points, a
vertical contrast edge can be easily registered by the horizontal pair of CCD
strips as a cross-type autofocus point compares four secondary images being
formed by four bundles of light within the photo-taking lens.

Phase detection is the most common type of
autofocus system applied in today’s DSLR cameras due to its various advantages.
The autofocus unit itself does not require any moving parts and is therefore
not vulnerable to vibrations. Also, the position of the AF unit behind the
photo-taking lens offers very high accuracy as the primary image is analyzed
directly. Possibly the greatest advantage of phase detection autofocus is its
high speed. Once the comparator has determined the phase-difference, the system
already knows to which position the camera lens has to be moved. Unfortunately,
the disadvantages are its low usability in dark environments and for scenes
with low contrast or highly repetitive fine patterns. Also, phase detection can
only be applied as long as the camera’s mirror is in its idle position. In
live-view mode where the mirror is flipped up, another technique must be
applied to determine proper focus. Finally, higher production cost is a slight
drawback for phase detection systems.