Camera under control: how to set the autofocus
With autofocus, the camera focuses automatically. But when and, above all, on what? That depends on how you set the autofocus.
The term "autofocus" is a little misleading - it's not as if everything happens automatically. The camera measures the distance and adjusts the lens correctly. But you usually have to give it a few instructions on exactly how to do this. Take a look at this image:
The focus here is on the air bubbles in the drink. Not only the plate is blurred, but also the closer area of the glass. This is intentional. But how would the camera know that I wanted it that way?
Of course, there are pictures where the automatic system can guess what should be focussed on. In the picture below, for example, the camera recognises that there is something large in the centre that is closer than the rest. It therefore correctly assumes that this is the main subject and should be focussed on. If face recognition is switched on in the camera, it also recognises that this is a portrait.
Fully automatic
In other words, you can switch the autofocus to fully automatic in certain cases. This would work for the portrait image above. However, this does not always produce the desired result. If you deliberately want to use sharpness (or blurring) as a stylistic device, the fully automatic mode is no good. It is primarily intended for snapshots.
In this article, I will therefore explain what options you have for configuring your camera's autofocus.
The basics: Switching on, fully automatic
When the camera is in full automatic mode, the autofocus is always fully automatic. Only when you switch to another mode, such as P, A, S or M, can you make the AF settings that I describe in this article.
In order to set the autofocus correctly, you should also check whether it is switched on at all. That's clear. I mention this because it is done very differently depending on the camera brand and model, so you may get stuck here if you are not familiar with a camera. If in doubt, the user manual is your friend. You can simply download it from the manufacturer and save it to your mobile or Dropbox.
Case 1: Switch on the lens
Canon or Nikon SLR cameras have a switch on the lens called AF/MF or similar. Of course, this must be set to AF. Some SLR cameras (Nikon) also have such a switch on the camera. This is only intended for the case that a lens without an AF switch is used.
Case 2: Focus mode button or menu
On many cameras, you switch on the autofocus where you also change the focus mode. I will come to the focus mode in the next section. Some cameras have a separate button for the focus mode, which is typically labelled "AF". In some cameras, however, you have to call up a menu item for this (usually a quick access, i.e. not in the depths of the menu settings).
Case 3: On-off button
On Canon mirrorless cameras, there is usually a button on the control wheel labelled "MF" that doesn't actually do anything other than switch between autofocus and manual focus.
Case 77: Sony's AF/MF-AEL combo switch
To illustrate that every manufacturer comes up with its own ideas, here is the example of the Sony Alpha 7S II: There is the on/off switch on the lens (case 1), but there is also a switch on the camera that switches between AEL and AF/MF (see previous or next image). If this switch is set to AF/MF, you focus manually while holding down the button.
The mode of the autofocus
As mentioned, you can access the camera's AF mode via the AF button or via a quick access menu. On the Sony A7S II, for example, this quick menu looks like this:
In practically every camera there is single and continuous autofocus.
One-time AF
The one-shot autofocus is called "One Shot AF" (Canon), AF-S (Nikon, Sony), S-AF (Olympus) or similar, depending on the brand. The S stands for single. If you press the shutter button halfway to focus, the camera only focuses once and then remains in this position. There is visual and/or acoustic feedback, for example when the focus field changes from red to green. This lets you know that focussing has worked and the image is in focus.
Continuous AF
Continuous autofocus (AF-C or similar) keeps refocussing as long as the shutter release button is pressed halfway. Therefore, there is no feedback when the camera is in focus. This is useful for moving subjects. Continuous AF is often used in combination with continuous shooting mode.
Canon users take note: Canon calls continuous AF "AI Servo". However, some Canon cameras (mirrorless, e.g. M5) have an option called "Continuous AF" in the settings menu. However, this means something different here. If this option is activated, the camera will always continuously refocus, even if you don't touch the shutter button.
Automatic mode change
Many cameras have an automatic switch between single AF and continuous AF. If the camera detects that the conditions in the image are changing, it switches to continuous focus, otherwise single AF is active. At Nikon this automatic is called AF-A, at Canon "AI Focus AF".
3D tracking
Also quite often available is a mode called "3D Tracking" or something similar. This is also suitable for moving subjects. Here, the camera attempts to recognise an object and track it when focussing. In contrast to continuous autofocus, there is no fixed image area in which the focus is set, but the area is always where the tracked object is. Or rather: it should always be there, because the detection sometimes fails.
Define measuring range and size
To determine which region of the image should be in focus, you not only specify the position to the camera, but also the size of the focus area. Within this size, the camera itself decides what to focus on. If the entire image is selected as the focus area, this means fully automatic autofocus.
Large or small?
Accordingly, the larger the metering field, the more "automatic" it is. A large field is more convenient, but less precisely controllable. The smaller the area, the more precisely you can target what you want to focus on. The automatic system tends to focus on what is closest in the measuring field. If you want to focus on something behind it, you have to reduce the size of the field. That is the advantage of a small field. The disadvantage: you also have to set it more precisely. You also have to move the field more often, both of which can be a bit fiddly.
Small focus fields have another disadvantage. If there is nothing in the focus area that has a contour (for example, if you are aiming at a flat, uniformly coloured surface), the autofocus will have great difficulty focusing. Of course, the smaller the focus area, the greater the likelihood of this happening.
Move the field of view - or rather the camera?
Let's assume you don't want the main subject in the centre of the image, but to the right of the centre. Then you focus on it by moving the focus point, which is usually in the centre, to the right. Of course, there is also the old trick of pre-focussing: first place the subject in the centre of the viewfinder, press the shutter release button halfway and then move the camera to the left so that the subject moves to the right in the picture. This keeps the focus at the original distance. Of course, this does not work with continuous AF.
Prefocussing is often faster, as it is simply more intuitive to move the camera than the metering fields. But this method can lead to inaccuracies. Panning the camera changes the distance very slightly. If you take photos at close range with an open aperture, this can already produce a visible blur. The focus will also be inaccurate if the subject has moved slightly in the time between focussing and moving. A typical example is a flower in the wind. In this case, it is better to set the field correctly so that you can release the shutter immediately after focussing.
Okay, and how does it work with the field shifting?
In the touchscreen age, the field can be defined by tapping, just like on a smartphone. But if you're using the viewfinder, that's nothing, of course. You also have to use the buttons.
Most cameras have four direction buttons, sometimes also a wheel. This is usually used to move the field. However, to prevent this from happening accidentally, you usually have to enter a kind of "shift mode" first. The corresponding button looks like this or similar:
Nikon has a lock switch for this, which is only labelled with an "L" (for "lock").
More settings
Separate focus button
Some photographers want two different buttons for focussing and pressing the shutter release button. This is possible in many cameras. On my camera, the Nikon D7500, I can use the AEL/AFL button for this and I can also specify whether the shutter release should still focus or not. The best way to find out how to do this with your camera is to read the manual.
Focus priority vs. shutter priority
Somewhere deep in the camera's settings menu, you will find a setting for the priority of the respective AF mode on advanced cameras. Focus priority means that the camera only triggers when the focus has been achieved. This is the sensible setting for single AF and is the default setting. With release priority, the camera will always take a photo, whether focussing was successful or not. This is the usual default setting for continuous AF and is particularly useful in combination with continuous shooting. After all, if you are following an action scene at 10 frames per second, you don't necessarily need to fully adjust the focus for every single frame. It is more important to capture every moment of the motion sequence in order to select the best one later.
You can therefore use single AF with shutter priority or, conversely, continuous AF with focus priority. However, I can't think of any scenario in which this would be better in any way.
By the way, these are the cameras I tried out for this article: [[productlist:6281347,6136596,5999970,5910947]]
My interest in IT and writing landed me in tech journalism early on (2000). I want to know how we can use technology without being used. Outside of the office, I’m a keen musician who makes up for lacking talent with excessive enthusiasm.