How to use Auto Focus to make sharper images…
Are you struggling to get sharp images with auto focus?
Do your images sometimes look soft or not quite in focus?
Does your camera sometimes focus on the wrong thing?
Auto focus is great but the way you use it makes a big, Big, BIG difference in the pictures you make.
In this article we’re going to look at how you can use your camera’s auto focus (AF) to make sharper images. We’re going to cover simple techniques to give you an immediate improvement in any situation; we’ll cover more advanced techniques for specific scenarios and we’ll look at how to configure the AF in your camera for best results.
AF points: one is enough!
As you look through the viewfinder of your camera, how many auto focus points does it have? All mine have 9 arranged as a diamond but yours may well have more. (Leave a comment and let me know together with which camera you have ;-))
When it comes to auto focus points, more is bad.
To make a step change in your photography, change the settings on your DSLR to only use a single AF point. Don’t worry, you can always change it back later if you wish or select a different point.
What’s the advantage of only using a single auto focus point?
If you have multiple AF points enabled (most likely the default setting for your camera), every time you focus your camera will use all the points to decide what to focus on. The problem with this is your camera may focus on the wrong thing, just like the image below.
This can occur if your camera’s AF circuitry finds a stronger focus point in the background than the foreground. In this example, the lamp in the background has well-defined lines, better illumination and higher contrast than the people in the foreground. The camera’s AF system gets a stronger signal from the lamp and so decides that’s where to focus.
Using just one AF point will cure this problem.
Secondly, using a single AF point can help improve your composition skills too. If you move the active point from the centre to one of those surrounding it, you’ll naturally place your subject slightly off centre. This isn’t quite ‘rule of thirds’ but it’s a bit better than a ‘bullseye’ photo with the subject dead centre.
WARNING: Not all AF points are equal!!!
As I mentioned above, all my cameras have 9 AF points. But they’re not all equal. Far from it…
The centre AF point can detect both horizontal and vertical lines unlike the others which are only sensitive to either horizontal or vertical lines. Thus the centre point is a lot more versatile and potentially more accurate than the others.
For lenses with a maximum aperture of f/2.8 or greater, the centre AF point works as a High Precision AF point. It’s still sensitive to horizontal and vertical lines but is about twice as sensitive as the other AF points.
For some DSLRs, Auto Focus will not work with lenses where the maximum aperture is f/8 or smaller.
Using Auto Focus – the basics…
Step 1: Make sure Auto Focus is ON
First things first: if you want to use it, you have to switch it on. Chances are that unless you’ve specifically turned it off your camera will already be set to use auto focus. If not, now’s the time to switch it on. How you do this will vary from camera to camera. Mine has a switch on the barrel of the lens marked “AF MF” to change between auto and manual focussing respectively.
Step 2: Choosing a good spot to focus on
Auto focus will work better on some parts of the image than others. It’s far easier for your camera to focus on high contrast areas, edges and in good light than flat colours or low light.
Why is this? Well, it’s down to the way your camera works internally — I’m not going to go into great detail as that’s beyond the scope of this article but there’s a great description here if you’re interested.
When you’re composing your image, look at the main subject (the part of the image you want in focus). Make sure your AF point is over that part of the image before you hit the button to auto focus. If it’s a low contrast area or a flat colour look for something nearby with a higher contrast or an edge. That’s the spot where you need to place the AF point. For a face the general rule is to use the eye nearest the camera.
Be wary of subjects close to the camera…
There are two issues to consider with subjects that are close to the camera:
- Minimum focus distance
- Reduced Depth of Field
Minimum Focus Distance of the Lens
If your subject is closer than the minimum focus distance of your lens, you’re not going to get it in focus. What’s worse, your camera won’t give you a warning and you’re unlikely to notice this in the viewfinder either.
e.g. my favourite portrait lens for beauty images is the Canon EF 70-200 f/2.8 LIS. It has a minimum focus distance of 1.4m. When I started using it I often had soft images because the subject was too close. Taking a pace backwards cured the problem resulting in pin sharp images making it my ‘go to’ lens for this kind of work.
Effect of Depth of Field
Not having enough Depth of Field, whilst not strictly an auto focus issue, can also give an image that’s too soft.
e.g. at f/2.8 if you focus on the subject’s eye, it will be pin sharp but her nose and ear will be out of focus.
The depth of field reduces as…
- the focal length increases
- the focus distance decreases
- the aperture increases.
Nature and wildlife photographers often use this to great effect. A shallow depth of field makes a pin sharp subject pop out of an extremely blurred background.
In the above example to make more of the subject in focus you’ll need to increase the depth of field. Do this with a smaller aperture, increasing the camera-to-subject distance, a shorter focal length, or a combination of all three.
Step 3: Wait for the lens to achieve focus
Auto focus in not instant. Some lenses are faster than others but it still takes time for the motor to move the focus ring.
Your camera should give you an indication it’s achieved focus. Depending on your particular model the AF point could flash, you might have an indicator in the viewfinder or maybe even a beep.
To get a pin sharp image you need to wait for the lens to focus. Release the shutter too early and you’ll get a soft image.
Use the correct AF mode
Everything has modes these days, even Auto Focus (and I don’t mean ON and OFF).
Typically your DSLR’s AF will come in two different flavours: One Shot (aka Single) and Servo. The one you use will depend on what you’re photographing. Most likely it’ll be One Shot.
One Shot AF (AF-S on Nikon)
The One Shot AF mode (or if you’re a Nikon owner, AF-S) will auto focus on the subject when you 1/2 press the shutter release. Once the lens has focused you’ll see an indication in the viewfinder and auto focus will stop. The camera will not re-focus on the subject, no matter how long you leave your finger there. To re-focus you need to remove your finger from the button and start again.
The biggest reason to use One Shot AF is it allows you to focus and recompose the image. This is great for making more interesting photographs with a stronger composition. It’s very simple too:
- Aim the AF point over the subject (and adjust zoom if necessary)
- 1/2 press the shutter to focus and set the exposure (without removing your finger!)
- Recompose the frame to taste
- Fully depress the shutter button to make the image
Servo AF (AI Servo on Canon, AF-C on Nikon)
The big difference with Servo AF is it’s continuous. As long as you keep the shutter button 1/2 pressed, the focus will track whatever the AF point is on.
The big advantage of Servo mode AF is maintaining focus with (fast) moving subjects. It’s great for panning shots or when the subject is moving towards you provided you can keep the AF point over the subject. If you miss and the AF point moves off the subject, the camera will refocus on the background. In many cases that will be infinity. When the AF point comes back over the target the lens will refocus but this takes valuable time; time in which you will miss the shot because you haven’t got a focus.
To successfully use Servo AF you must have an unobstructed view of the subject. That means no fences, no windows, no trees. If you have anything between you and the subject, your camera will focus on it if the AF point crosses it. The result? A beautifully sharp panning shot of a fence with a very out of focus F1 car taking the checkered flag behind it. Doh!
NOTE: This means you can’t use the focus and recompose technique with Servo AF.
A second problem with Servo mode auto focus is battery drain. Your camera will constantly be focusing on the subject and that consumes charge from the battery. If you’re covering a big sporting event and are likely to use Servo AF you’d be wise to take a spare battery or two with you.
Which is the best AF mode to use???
It ultimately depends on your choice of subject at the time. In most scenarios where the subject is fairly static One Shot AF (AF-S) is the one to use. The only time I ever use Servo mode (AF-C) is when I have a moving subject with an unobstructed view. Panning shots are a good example as are sports events, fashion shows and some wildlife photography.
Back Button Focus — Advanced Auto Focus Technique
There is a major problem with the shutter button on a DSLR: it has too much to do. A half press will focus on the subject AND take a meter reading to set the exposure. A full press then releases the shutter and makes the image.
The problem is with the half press.
Why? Because focusing and metering are now coupled together. You can’t separate them. Unless you switch to manual focus you cannot focus on one part of the image and take a meter reading from another. You have to focus and meter from the same point.
That’s not always desirable. It creates images that are too dark or too light. It stops you from choosing where to focus, where to meter and thinking about how to frame the image.
But there is a solution: it’s called Back Button Focus
On your DSLR you can configure the camera to use one of the buttons on the back for auto focus instead of the shutter button. This decouples focusing from metering enabling you to focus on one part of the image, meter from another, recompose and shoot.
It’s the way I’ve configured all my DSLRs and was one of the tips that helped me make a massive step change I’m my images several years ago.
Configuring Back Button Focus
To configure your camera for back button focusing you’ll need to refer to the manual. I shoot with Canon bodies and it’s available through one of the custom functions (you know, that really scary part of the manual that nobody dares to read for fear of changing something that you shouldn’t touch and aren’t able to fix it).
On the Canon cameras that I use, there is a button on the back labeled “AF-ON” that you can reconfigure through a custom function to start auto focus. Pressing AF-ON will cause the camera to focus on the AF point, releasing it stops the AF function even if it’s in Servo mode. Then a half press on the shutter locks the exposure and you’re good to go.
The sequence we mentioned in the One Shot AF section above is then modified to include the extra steps:
- Aim the AF point where we want to focus (and adjust zoom if necessary)
- Press the AF-ON button to focus
- Aim the camera where you want to make your exposure reading
- 1/2 press the shutter button to lock the exposure (aka AE Lock)
- Recompose the image to taste
- Fully depress the shutter button to take the shot
It might seem complicated but believe me, it isn’t. It also makes a huge difference in your images once you get use to this way of working.
Using AF in low light
Auto focus doesn’t really work well in low light. Some cameras are better than others but, it needs a reasonable level of light to get a good focus for you. Increasing the ISO won’t make a difference either as it just amplifies the signal from the sensor and is separate from the auto focus system.
So what can you do if, like me, you love working in low light?
Option 1: AF Assist Beam
An AF Assist beam is a light on the camera body that illuminates the subject to help the auto focus work correctly. Not all cameras have them but if yours does this will significantly improve your camera’s ability to focus in low light conditions.
The only drawback is if you want to grab a quick candid shot, shining a light at the subject really can give the game away.
Option 2: Speedlite AF Assist
If your camera body doesn’t have an AF Assist beam, your speedlite might. You’ll need to check the manual to find out how to enable it. (For all my Canon speedlites it’s controlled by a custom function)
Option 3: Use a torch
Yes, I am serious, I do use a torch! In situations where light levels are low and the camera won’t focus on the subject a torch will illuminate the subject and help the AF. You can think of it as a ‘poor man’s AF assist beam’
Option 4: Manual focus
If none of the above are possible, and there’s no other way to put more light on your subject, the only other option is to switch to manual focus.
Other things you might not know about auto focus
- For lenses with a maximum aperture of f/8 or smaller, Auto focus may not work .
- When using AI Servo mode on some Canon cameras, the central AF point have up to 6 other ‘hidden’ AF assist points when using f/2.8 or f/5.6 lenses.
- Auto focus may not work with all types of polarising filter so if you want to use one, make sure it’s a circular polarising filter.
Auto Focus is a big subject for such a simple concept. There are a lot of ‘gotchas’ to trip you up along the way (I usually get four or five people a week telling me they’re struggling with AF). If you’re aware of its idiosyncrasies you can really tame it, learn how to push it to its limits and make some amazing images.
If you’ve found this post helpful, please do leave a quick comment below and let me know :-). Or if you prefer, share it with your friends and followers on your favourite social media platform.