Photography Basics #4 : Autofocus

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INTRODUCTION
AUTO IS NOT AUTO

Welcome to part 4 of the photography beginner series – understanding autofocus. I am sure that some beginners might be thinking “what’s there to learn about autofocus, isn’t it automatic”? Well no. When it comes to autofocus in photography, it is not really automatic. Confused? Read on, and you will find out why.

 

 

NAVIGATION
TABLE OF CONTENTS

Section A
Auto Focus Types

Section B
Auto Focus Points

Section C
Auto Focus Modes

Section D
Auto Focus Area

Closing
What’s next

 

 

SECTION A
TYPES OF AUTOFOCUS TECHNOLOGIES

Let us start with the different kinds of auto-focus (AF) technologies out there. But is it really necessary to know them? I will say yes, if you do not want to walk around like a clueless newbie… Plus, it will be cool to shoot down salespeople who give the wrong information, just trying to sell things quickly.

 

CONTRAST VS PHASE DETECTION

When it comes to auto-focus, there are only 2 types of systems that are commonly used in cameras these days – That is contrast detection and phase detection.

 

HOW CONTRAST DETECTION WORK

Well, I don’t think anyone is going to like the technical bits. So here goes the human version – In contrast detection, the system continuously adjusts the focus point and compares which is better. This is why you see that “hunting back-and-forth” when focusing on cameras that use contrast detection.

 

HOW PHASE DETECTION WORK

Phase detection is a slightly more complicated system, but in simple terms :

  • Phase detection uses something called a beam splitter.
  • When the light beams are not converged, a “mirror image” is created and it is not-in-focus.
  • The camera simply has to adjust the focus accordingly by “matching the mirror images”.

 

CONTRAST VS PHASE DETECTION

So which is the better auto-focus system? Personally, I do think that phase detection has a slight edge over detection speed, and it is better for videos. But performance wise, phase detection is poor in low light, while contrast detection is poor when it comes to a flat picture (e.g. An empty sky). Each has it’s own strength and weakness, so it’s all up to you to decide.

 

 

SECTION B
AUTO FOCUS POINTS

Autofocus points are another basic of auto-focus systems, which are the “squares” that you select on the screen, and your camera will try to focus on that spot. Probably not very exciting, but here are some trivial for you.

 

NOT ALL CAMERAS ARE THE SAME

AF Points are those squares on the screen

Before some of you guys start to think that all cameras have the same auto-focus points, they don’t. If you pick up different cameras, you will notice a somewhat different layout to the AF points. Especially when it comes to older cameras, they might only have 5 selection points, while the newer systems have 51 points.

Do more AF points mean a better system though? Technically it does, since it will mean that the camera is able to track moving subjects better. But at the same time, also know that the underlying technology matters too.

 

NORMAL VS. CROSS TYPE

You will probably hear this a lot, cross type and “normal” focal points. I am not going to the technical parts, read it on Wikipedia if you want.  But remember the phase detection earlier? Now think about it this way – the beam splitter creates a mirror image, and you just have to match them horizontally.

This makes sense when you are taking portraits, but what happens when you are taking a photo of the horizon on the beach? Horizontal matching probably does not work as well now, since it is difficult to match a horizon horizontally. This is where cross-type sensors shine, as they match both horizontally and vertically.

 

SECTION C
AUTO FOCUS MODES

The most common form of focusing that people probably know of, is to select a spot, press the focus button and wait for things to come into focus. But good cameras are more than that. They are capable of tracking subjects, and here are the few common AF modes.

 

 

  • Single Mode: Attempts to focus once, and stops once in focus. Good for static or slow moving subjects.
  • Continuous Mode: Continuously hunt for a good focus point, good for moving subjects.
  • Auto Mode: Let the camera decide if it is single or continuous.
  • Manual: Human eye power.

 

SECTION D
AUTO FOCUS AREA

On a smartphone, you tap on a spot to focus on it. But when you press the (hardware) focus button on some phones, it attempts to automatically find a subject to focus on. Ever notice that? You can actually specify the focus area, and you can also do it on a “full sized” camera. Of course.

 

  • Single Point: You specify on a single spot in the frame to be in focus. Good for a choosing single subject, isolated from the background.
  • Dynamic: Similar to the single point, but takes the immediate surround of the chosen point into consideration. Good for a small group of objects.
  • Auto: Fully lets the camera choose which is to be in focus.

 

CLOSING
WELL DONE, WHAT’S NEXT?

Here comes the end of the tutorial. Hope this has opened your eyes to “auto-focus” a little more, and whoever said automatic is easy? Well, it sure is not straightforward. But all you need to do is to try all the different modes on your camera, and figure out which works best for you.

If you have questions, feel free to leave them in the comments section below. Happy shooting!

 

Previous Lesson: Lens Filters Next Lesson: Autofocus Tips


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