Tips & Tutorials

Camera Basics #9: AF Modes

When establishing focus on a subject, it is vitally important to anticipate the subject’s movement and capture it at the right moment. This means it is essential to know the appropriate autofocus (AF) mode to use for a stationary subject, and which to use when the subject is moving. Let us take a closer look at the 3 types of AF modes. (Reported by Tomoko Suzuki)

 

Select an AF mode that best suits the subject’s movement characteristics

Points-to-note

- When the subject is stationary, select One Shot AF.
- When the subject is in motion, select AI Servo AF.
- When a subject’s motion is unpredictable, select AI Focus AF.
 

There are 3 types of AF modes that you can choose from to best fit your shooting situation: One Shot AF, AI Servo AF, and AI Focus AF which automatically switches between One Shot AF and AI Servo AF where necessary.

In One Shot AF mode, half-press the shutter button. Once focus is established, the camera will then fix the focus. This is the AF mode that is most suitable for shooting stationary subjects. In AI Servo AF mode, AF is activated as long as you keep the shutter button half-pressed, and continuously keeps focus on the subject. Meanwhile, AI Focus AF is ideal for shooting subjects with unpredictable movements. In this mode, the camera switches between One Shot AF and AI Servo AF according to the subject’s movement. which AF mode to use according to the situation.

 

One Shot AF

EOS 5D Mark III/ EF50mm f/1.4 USM/ FL: 50mm/ Aperture-priority AE (f/1.4, 1/6400 sec, EV+1)/ ISO 100/ WB: Auto

Benefits: Easy to establish and fix focus
Drawbacks: Focus is easily lost if the subject moves

Establish focus with the shutter button half-pressed. Keep the shutter button half –pressed until focus is established and the camera will fix the focus. This is most ideal for shooting still subjects such as on the streets, tabletop and landscapes. However, if the subject moves after the focus is fixed, the photo will go out of focus.

 

AI Servo AF

EOS 7D Mark II/ EF70-200mm f/4L IS USM/ FL: 135mm (216mm equivalent)/ Aperture-priority AE (f/4, 1/1250 sec, EV±0)/ ISO 160/ WB: Auto

Benefits: Is able to sustain focus on a subject in motion
Drawbacks: Difficult to establish focus onto the precise spot or location you want to

The camera keeps on establishing focus as long as the shutter button half-pressed, which makes it best suited for shooting subjects that are constantly moving away from or towards the camera. However, it may be hard to obtain that perfect picture composition you want since fine focus is not so easily achieved.

 

AI Focus AF

EOS 5D Mark III/ EF70-200mm f/2.8L IS II USM/ FL: 200mm/ Aperture-priority AE (f/5, 1/160 sec, EV+0.3)/ ISO 160/ WB: Daylight

Benefits: Best for subjects with unpredictable, erratic movements
Drawbacks: Hard to determine which mode is currently in use

In AI Focus AF mode, the camera automatically switches between One Shot AF and AI Servo AF mode depending on whether the subject is moving or staying still. This is very useful when shooting pictures of subjects with unpredictable movement characteristics such as animals and children, who can be dashing around one moment and completely still the next. It is harder to fix the composition with this mode compared to One Shot AF mode, as this mode puts heavier effort on maintaining focus.

 

Related concept: AF area selection mode

Single-point AF

Zone AF

Automatic Selection AF

The AF area selection mode is a function that determines the AF frame area when establishing focus. You can choose to carry out autofocusing by using either one point or an entire zone. Although the types of functions featured vary among cameras, most of the time, entry-level models will contain at least the three types of AF modes mentioned above: Single-point AF, Zone AF and Automatic Selection AF. If the emphasis is on establishing focus on a point, Single-point AF is good. If you are using a zone to establish focus, select Automatic Selection AF. For everything else in between, use Zone AF.

It is very important to know which AF area selection mode to use according to the size and the movement characteristics of the subject. When shooting a small-sized subject or scenes where very precise focus is required, use Single-point AF. For subjects with unpredictable movements, Automatic Selection AF is best. For scenes that require consideration for both moving subjects and good photo composition, Zone AF is most suitable.

 

Good combination 1: Single-point AF and One Shot AF

Single-point AF takes photos by letting the camera decide which AF point will achieve the best focus. If you want to achieve precise focusing on one part of a subject, this is a good mode to used, especially when combined with One Shot AF.

Using this combination not only allows you to establish focus on your intended areas in scenes with depth, but also achieve tack-sharp pinpoint focusing on scenes with a shallow depth of field, such as when photography flowers or taking close-up shots.

EOS 6D/ EF50mm f/1.8 STM/ FL: 50mm/ Aperture-priority AE (f/1.8, 1/100 sec, EV+0.3)/ ISO 100/ WB: Auto

In this image, I established focus on the subject in the center and defocused the areas in the front and back. If you are trying to get a foreground bokeh effect, the combined use of One Shot AF + Single-point AF is most useful.

 

EOS M3/ EF-M28mm f/3.5 Macro IS STM/ FL: 28mm (45mm equivalent)/ Aperture-priority AE (f/5.6, 1/60 sec, EV+0.3)/ ISO 250/ WB: Auto

When shooting photos of flowers, extreme precision focus is essential. In order to establish focus on the petals, I used One Shot AF + Single-point AF.

 

Good combination 2: AI Servo AF and Automatic Selection AF

When Automatic Selection AF is enabled, the camera automatically selects the best AF point(s) to use from among all of its AF points. In principle, focus is established on the subject that is nearest to the camera.

If you select just a single point to carry out focusing, it is sometimes difficult to maintain focus continuously. Automatic Selection AF comes in useful when photographing animals or taking sports photos, where it would be hard to maintain focus on a moving subject by using just a single AF point. When shooting scenes of moving subjects in AI Servo AF mode, switching on the Automatic Selection AF too will help you achieve the best focus since it uses an entire zone for autofocusing.

EOS 5D Mark III/ EF40mm f/2.8 STM/ FL: 40mm/ Aperture-priority AE (f/14, 1/1600 sec, EV-0.3)/ ISO 1600/ WB: Auto

I took this picture of a seagull that popped up suddenly. The camera is set in Automatic Selection AF and AI Servo AF mode. With Automatic Selection AF, focus-hunting is carried out by multiple AF points, increasing the chances of accurate focus. This makes it great for capturing sudden shutter opportunities like this one.

 

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Digital Camera Magazine

A monthly magazine that believes that enjoyment of photography will increase the more one learns about camera functions. It delivers news on the latest cameras and features and regularly introduces various photography techniques.
Published by Impress Corporation

Tomoko Suzuki

After graduating from the Tokyo Polytechnic University Junior College, Suzuki joined an advertisement production firm. She has also worked as an assistant to photographers including Kirito Yanase, and specializes in commercial shoots for apparels and cosmetic products. She now works as a studio photographer for an apparel manufacturer.

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