Page 81 - (DK) Advanced Photography Guide

FO CUSING 79

CONTRAST DETECTION SYSTEM
ON-SENSOR
Contrast-detection AF is far 8 9 1 PHASE DETECTION
simpler and more robust than
phase-detection AF: there are no On-sensor phase detection helps to
components other than the lens improve CSC AF speeds, using paired
optics and digital sensor. It’s the photodetectors on the camera’s
digital sensor that calculates image sensor. One of each pair is
focus distance, often along with masked so that it only receives light
the camera’s image processor. from the right half of the lens; the
Despite its simplicity, contrast other, so it receives light from the left
detection is very accurate, but it’s half. This mimics conventional phase
also relatively slow, although the detection and enables the camera
technology is improving with each to focus with a similar speed. Phase
new generation of camera. Both detection is usually only applied
accuracy and speed can also be across a small area of the image.
compromised in low light. 7 3

LENS FOCUSING

There are two basic types of AF motors used in camera lenses. switch to MF. Ultrasonic motors, found in newer lenses, are
Most older or less expensive lenses use an arc-form drive much quieter and faster, which makes them more suitable for
motor, which can be slow and noisy and doesn’t permit full- video work, too. You can also manually focus an ultrasonic
time manual focus; to focus the lens manually, you have to motor lens at any time without first switching it to MF.

PHASE DETECTION FOCUSING
Inside the phase-detection module are several paired The lens optics
AF sensors, each of which is linked to a specific AF point in rotate left or right
the viewfinder. The images received by the paired AF sensors to adjust the
are compared to see if they match each other. If they don’t focus point
match, it means the image is out of focus, so the AF module
adjusts the lens optics until the images
match or are in phase.

FRONT FOCUS BACK FOCUS
SUBJECT CORRECT FOCUS

CONTRAST DETECTION FOCUSING
The contrast-detection system moves the optics in the
camera through a range of different focus distances,
analyzing the degree of image contrast at the selected
autofocus point. The image is sharpest when the
greatest amount of contrast is detected.

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