Camera Autofocus: The Complete Guide

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INTRODUCTION
AUTO IS NOT AUTO

In this part of the photography beginner series, we shall understand more on camera 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

Section E
Auto Focus Tips

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

Infographic – Autofocus Contrast Detection

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

Infographic – Autofocus Phase Detection

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.

Infographic – Why Cross-Type sensors are better

 

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.

 

Infographic – Autofocus 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.

 

Infographic – Autofocus Area Modes
  • 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.

 

SECTION E
AUTO FOCUS TIPS

In this section, we will run through a few autofocus tips and techniques. This is where you will learn some kick-ass kung fu to taking some of those very sharp photos. Don’t worry, techniques do not necessarily mean technical, so here are just some “best practices” to remember while you shoot.

 

GENERAL TIPS FOR STABILITY

Infographic – Autofocus Tips

How do we get a good shot without blur? By simply holding the camera right… which most beginners get it wrong. Here are a few general tips you want to remember.

  • If your camera is heavy, try to keep it close to your body.
  • Rest your arm against your body instead of holding it mid-air. This will add to the stability.
  • Lean against something stable if you can – a wall, chair or railing.
  • You can open your feet a bit wider to create a stable base.
  • Holding your breath while shooting handheld will add to the stability too.
  • If you cannot deploy a tripod at certain locations, use the railing, chair or even rubbish bin as a temporary support.

FOCUS AND RECOMPOSE

Infographic – Focus & Recompose Technique

Ever find yourself in the situation where the autofocus does not work? Here’s what to do when AF refuses to work properly – Use the center AF point. This is because the center point is usually the “best” and most sensitive. Simply frame your subject in the middle of the frame, autofocus on, readjust the frame, then take the shot.

 

 

USE THE BACK BUTTON FOCUS

Normally, cameras have “by default”, set the auto-focus to trigger by half-pressing the shutter button. While that is not wrong, the better way will be to use a dedicated back button to activate AF. Why so? Because it prevents you from accidentally taking a shot while half-pressing, and it also makes focus-and-recompose easier.

The true advantage comes when you are shooting fast moving subjects in the continuous focus mode. You can easily track and lock the focus with the back button, while taking photos with the shutter button. In the half-press scenario, it will be clumsy to do so with only one button.

 

SHARP. TACK SHARP.

Infographic – How to take TACK sharp photos

Landscape photography lovers, this is for you to get very sharp photos, or what we call “tack sharp”. You are going to need a tripod and remote shutter release. [Selens] offer decent and affordable tripods, or go for the good old reliable [Manfrotto]. As for the shutter release, it does not matter if it is wired or wireless, as long as you get hands off the camera. [Nikon] [Canon] [Sony]

  • Start with a sufficiently small aperture like f/8.
  • If you are using a DSLR, set your camera to the “mirror up” mode.
  • Mount the camera to the tripod, use a wireless release.
  • Frame and auto-focus 1/3 into the scene.
  • Switch to the live view, zoom into the foreground and background to check the sharpness. Manually adjust the focus if required.
  • If you cannot get everything sharp with f/8, use a smaller aperture.
  • When you are comfortable with the sharpness, take the photo.

 

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: Metering


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