Automatic Focussing

Allium seed head

A throwaway remark  the other day, "I always use Autofocus", set me thinking.  What is autofocus and what do I need to do on my camera to make it work?

I can remember having to manually focus my first camera in the 1980s, a Canon AE1 film camera, using a split screen prism. 

The prism split your viewfinder in 2 and you had to focus your lens in order to make the 2 halves line up.  It took time, was difficult in poor light, and did not take into account your depth of field setting.  When you were ready you needed to check your composition and stop down your lens to preview the depth of field.  Then you took your picture, if it was still there! It was a good job it was in the days of film because it slowed you down and it was a lot easier with the camera mounted on a tripod.

Canon brought out an autofocus lens in the 80s but it was extremely expensive and out of the reach of amateurs.

We now take autofocus

for granted and, except for very closeup macro work I rarely venture into manual focus.  If I do, the viewfinders have improved so much there is no longer any need for a split prism.

Autofocus has developed and improved a lot over the intervening years, particularly in recent years as the introduction of digital viewfinders has meant a whole raft of new technologies have had to be invented.  This brief article focuses on optical viewfinders and so is predominantly of interest to DSLR/bridge camera users (or aspiring users).

What Does Your Camera Need to Know?

Before your camera can focus it has to decide (or be told) what you want to focus on and what to do if the subject or the camera moves.  Modern cameras have a very large number of focus points and I suspect most of you leave the camera to choose which one(s) to use for any scene. I think we can leave thinking about focus point selection to a future article and, for now, consider the different autofocus modes.  Most cameras come with 3 autofocus modes and because of a lack of standardised terminology all call them different names.

On my camera, Canon, I can select one from 3 autofocus options:

• One Shot
• AI Focus
• AI Servo

Great but no clue as to what each one does and no explanatory screens on the EOS90D as it is aimed at the enthusiast market and we are assumed to know what they mean!

The difference between the 3 modes is the way in which they respond to movement (by the subject or camera).

One Shot AF (Nikon AF-S (single area), Olympus S-AF (Single))

One shot AF is for scenes/objects that are static and will definitely not move while you take the image.  You focus the camera by half pressing the shutter button and the focus point will flash/change colour and the camera will beep.  You cannot progress to taking the photo by completing the button press until it achieves focus lock.  This is all very good provided nothing moves; if the selected subject moves out of the focus point either because it is moving or because you have moved the camera (intentionally or otherwise) the focus will not change and your image will be out of focus.

This mode would therefore suit architecture photography, for example.  It will focus on the building across the street and will not refocus if something moves across the image in the street in front of you.  It will ignore that car going past and will remain focused on the house beyond.  It will, for example allow you to take a photo while an out of focus bus is in front of you.

But, at the same time, you can safely recompose the shot once the focus is locked in without the focus changing, provided you maintain the half pressure on the shutter.

AI Servo AF (Nikon AF-C (continuous), Olympus C-AF (Continuous))

AI Servo AF is the opposite, it is intended for scenes which include a fast moving subject.  The camera will detect movement and assume that is the desired subject and attempt to focus on it.  It will shift the focus to follow the moving object.  It will track the moving object and predict where the object will be when the shutter is open, taking into account any shutter lag.

It will allow you to fully depress the shutter even if focus lock has not been achieved.

It may refocus on and follow any crossing objects that pass between the camera and your subject.

You cannot move the camera to recompose the sene and preserve the focus as it will continue to refocus and track the original subject.

AI Focus AF (Nikon AF-A (automatic), Olympus various options)

AI Focus AF is a combination of the previous two, switching from one to the other depending whether or not the camera detects movement.  The camera switches between One Shot AF and AI Servo AF.  The camera makes the decision based on the speed of movement detected in the scene.

Which One to Choose?

Apart from focusing performance, the only difference you are likely to notice is the ability to fully depress the shutter button.

The only real way to choose is to match the selection to your style of photography at a particular time.

• If you only shoot static objects (e.g. architecture, landscape, fine art, macro) then One Shot AF is the one to choose.
• If you shoot mainly static objects and occasionally a moving object then AI Focus AF might better suit.
• If shooting sports, wildlife, birds in flight etc then AI servo is best.

In practice my selections are:

• In a studio environment (definitely static) - One Shot AF
• For action, sports, wildlife (definitely moving) - AI Servo AF
• For general photo outings, dog walks, holidays (uncertain whether static or moving), then AI Focus AF

Live View

The growth in digital viewfinders (live view on DSLR, mirrorless, or mobile devices) introduces new autofocusing modes including Artificial Intelligence driven options for, face detection and tracking (human or pet faces), vehicle recognition and tracking.  This is a fast moving area of development.