Camera Focussing Technologies – An Introduction

Camera Focussing Technologies – An Introduction

A camera's autofocus system intelligently adjusts the camera lens to obtain focus on the subject, and can mean the difference between a sharp photo and a missed opportunity.
AF system and its Objectives known in Digital Camera:

1. Contrast Detection AF - Focus Precision
2. Phase Detection AF - Speed Priority
3. AI Servo – Predictive AF - Moving Subject precision
4. Face Detection AF - Face & Tracking Priority
5. Face ID AF - Pre registered Face Priority
6. Hybrid AF (Sensor AF + Contrast AF) - Best of both Contrast Detection as well as Phase Detection AF

Note: Autofocus (AF) works either by using contrasts sensors within the camera (passive AF) or by emitting a signal to illuminate or estimate distance to the subject (active AF). Passive AF can be performed using either the contrast detection or phase detection methods, but both rely on contrast for achieving accurate autofocus.

Fig: Illustrative Diagram on working of AF System

Contrast Detection AF: Establishes focus by moving lens forward and backward until maximum subject contrast is achieved at the image sensor. AF accuracy is high, but speed is slow.

Phase Detection AF: Subject image is split in two and transmitted to the AF sensor, which detects necessary focus direction and focus difference by calculating the distance (phase difference) between the two images. Because it instantly calculates focus direction and out-of-focus amount, and moves the lens to focus the subject, AF speed is fast.

AI Servo: AI Servo AF is designed to predict subject movement. It anticipates where a subject is headed and maintains focus as long as the subject is kept within the AF frame. Hence enables effective shooting of moving subjects.

Face Detection AF: Face Detection uses facial features determined by the algorithm. It uses Contrast AF system. It is effective while shooting portrait, group etc.

Face ID AF: Face ID uses pre registered facial features; it uses the same techniques as Face Detection with preference over location and placement of the subject. This is effective while shooting group and prioritizing the registered faces.

Hybrid AF: This system predicts the subject location and approximates the focus using phase-detection AF on the sensor then fine tuning the final focus using contrast-detection AF. In actual term it uses benefits of both the system.

Points to Remember:

AF modes are not well-suited to shooting low-contrast scenery, fences, recurring patterns, and other such elements that can be difficult to bring into focus. When attempting to shoot these kinds of elements, you can do so using manual focus, or you can use One-Shot AF by first focusing on another object the same distance away as the subject that you wish to shoot, locking the focus, and then re-adjusting the composition with the intended subject.

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