The modern autofocus systems are advanced. Sometimes, it looks like you will never again miss a shot when it comes down to focusing. But the autofocus can still fail under certain circumstances. Let’s look at the moments when even the most advanced autofocus can give up.
With every new camera, the autofocus system seems more advanced. If you didn’t know any better, the focus of every image should be spot on. Sometimes, you wonder if it’s still possible to get out-of-focus images. Cameras can recognize animals, people, and vehicles. It is even possible to require focus on the body, head, face, and even the eye. Tracking makes it possible to follow your subject and never lose focus.
The truth is, out-of-focus images are still possible, even with these advanced focus systems and the image recognition software built in. The camera can get confused, not knowing what the main subject is. In that case, focus may be achieved, but on the wrong spot. On some occasions, it’s impossible to achieve focus. The system starts hunting in an attempt to find a focus point. If it fails, the autofocus will eventually stop, and taking a photo may become impossible.
To understand why achieving focus can fail, we have to look at the autofocus system in a bit more detail. To keep it simple, the system generally needs contrast to achieve a good focus. If there is not enough contrast, the system will fail. A blank sheet of paper doesn’t have enough contrast, for instance. But if you make a fold, there is something to focus on.
To see any contrast at all, there has to be enough light available. The darker it becomes, the more difficult it is to distinguish that fold in the sheet of paper. Until there isn't enough light anymore, and the autofocus will fail.
The Exposure Value of a Scene
If you look at the specs of your camera, you probably find an EV value mentioned for the autofocus sensitivity. This indicates the minimum amount of light that is necessary for the autofocus to work.
You will find values that range from 0 EV down to -7 EV, depending on the camera you have. The EV value can be traced back in the so-called light value charts. These charts describe light situations that match the different EV numbers. For instance, a -2 EV situation is similar to the light of a full moon in a snow-covered landscape.
It’s amazing how an autofocus can work under these light conditions, not to mention a camera that’s able to focus with even less light. But you have to take another requirement into account. Autofocus under these minimum light conditions is only possible with a large maximum aperture, often an f/1.2 lens.
For example, a camera can focus at a minimum of -2 EV, but only with an f/1.2 lens. If your lens doesn't have a maximum aperture of f/1.2, you won't be able to focus at that light value. If you use an f/4 lens, which transfers roughly 3 stops less light, the minimum light value for the autofocus to work will be +1 EV instead of -2 EV. This is similar to the amount of light of a faraway city skyline at night. If your lens has a maximum aperture of f/5.6, the limit will be +2 EV, and so on.
What If the Autofocus Can’t Lock Focus?
Now, the basics are clear. Let’s look at some situations when the autofocus fails. Some of these will relate to the sensitivity of the autofocus system, which differs with each camera. I will also mention a possible solution.
1. If Focus Is Locked on the Wrong Subject
No matter how clever the modern autofocus systems may seem, it’s just a bit of software that offers some kind of image recognition. If you don’t have a clear subject in the frame, the autofocus may choose the wrong place to focus on.
The solution is simple. Choose a focus point manually. Use the most sensitive center autofocus point, focus and recompose the image. Or, you may choose one of the other available focus points to avoid recomposing the image.
This problem can also occur with head-, face-, and eye-AF. If there are many people in the frame, the system may get confused and focus on the wrong person. Often, there’s a way to switch over to another face. Some cameras have a true face recognition, so you can prioritize some person over others.
2. When the Subject Lacks Contrast
If your subject doesn’t have a lot of contrast, it may become difficult to lock focus. The lens starts hunting until it gives up. Or an object in the back or front is chosen instead of the subject itself, a situation that is quite similar to the previous point.
The solution for this problem is rather simple. Choose a point with good contrast and focus on that one, hold focus, then make for the composition you had in mind. You can also choose something with more contrast at roughly the same distance, but in the other direction. As long as you keep the focus locked, you’ll be fine.
3. If It’s Too Dark
If it becomes too dark, the autofocus won’t be able to focus anymore. If there is not enough light, that means there isn't enough contrast anymore or perhaps the subject can’t be recognized against the background. Remember, the autofocus sensitivity specification of your camera, as mentioned before. If the exposure value of the situation falls below the autofocus threshold, the camera won't be able to focus. Take the maximum aperture of your lens into account when checking the minimum exposure value of the autofocus of your camera.
A solution for this problem can be an autofocus assist light. This can be built into the camera itself, or you can use the AF-assist of a flash. A flash projects a pattern onto your subject, which will give the camera the much-needed contrast to focus on. You can also use a flashlight, of course.
4. When You’re Using a Neutral Density Filter
The amount of light that passes through the lens is important for the autofocus system of your camera. If you place a dark neutral density filter in front of the lens to achieve long exposures, you will reduce the amount of light that passes through the lens significantly. In other words, you make the world a darker place, or you imitate a smaller maximum aperture.
Let's look back at the example with a -2 EV sensitive autofocus. The number is based on the amount of light that passes through the lens with an f/1.2 lens opening. If you place a 10-stop neutral density filter in front of the lens, the amount of light that passes through is reduced by 10 stops. In other words, it’s like having a maximum aperture of about f/45. In that case, the light value of the scenery needs to be at least +8 EV for the autofocus to work.
Although most cameras will be able to focus with live view when a neutral density filter is used, it's good practice to focus before placing the neutral density filter. Switch over to manual focus, and then, place the neutral density filter. Make sure you don’t touch the focus ring anymore.
One Last Thought
Is this knowledge necessary for your photography? In most cases, no. You will know when the autofocus fails, and most of the time, the proper solution is obvious. But it can be nice to have an understanding of why the autofocus fails. If it occurs too often, the knowledge may help you in the search for a more permanent solution. The solution may be a lens with a wider maximum aperture, a camera with a more sensitive autofocus system, or perhaps a focus assist light.
Do you encounter a failing autofocus on a regular basis yourself? Please share your experiences in the comments below, and tell us how you deal with it.