Modern autofocus systems are advanced. Sometimes it seems like you’ll never miss a shot again when it comes to focusing. But autofocus can still fail under certain circumstances. Let’s look at the times when even the most advanced autofocus can give up.
With each new camera, the autofocus system seems more advanced. If you didn’t know better, every frame should be in focus. Sometimes you wonder if it is still possible to get blurry images. The cameras can recognize animals, people and vehicles. It is even possible to focus on the body, the head, the face and even the eyes. Tracking keeps track of your subject and never loses focus.
The truth is that blurry images are still possible, even with these advanced focusing systems and the built-in image recognition software. The camera can be confused, not knowing what the main subject is. In this case, focus can be achieved, but in the wrong place. There are times when it is impossible to concentrate. The system begins to search in an attempt to find a focus point. If it fails, autofocus will eventually stop and taking a photo may become impossible.
To understand why focus can fail, we need to look at the autofocus system in a bit more detail. To keep it simple, the system generally needs contrast to achieve good focus. If there is not enough contrast, the system will fail. A blank sheet of paper does not have enough contrast, for example. But if you’re making a crease, there’s something to focus on.
To see any contrast, there must be enough light available. The darker it gets, the more difficult it is to distinguish this crease in the sheet of paper. Until there is not enough light and autofocus fails.
The exposure value of a scene
If you look at your camera’s specs, you’ll probably find an EV value mentioned for autofocus sensitivity. This indicates the minimum amount of light needed for autofocus to work.
You will find values ranging from 0 EV to -7 EV, depending on the camera you have. The EV value can be traced in the so-called light value tables. These tables describe the lighting situations that correspond to the different EV numbers. For example, a -2 EV situation is similar to the light of a full moon in a snowy landscape.
It’s amazing how an autofocus can work in these lighting conditions, let alone a camera that can focus in even less light. But you have to consider another requirement. Autofocus in these minimum light conditions is only possible with a large maximum aperture, often an f/1.2 lens.
For example, a camera can focus to 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 are using an f/4 lens, which transfers about 3 less light values, the minimum light value for autofocus to work will be +1 EV instead of -2 EV. This is similar to the amount of light from a distant 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 autofocus cannot lock focus?
Now the basics are clear. Let’s look at some situations where autofocus fails. Some of them will be related to the sensitivity of the autofocus system, which differs with each camera. I will also mention a possible solution.
1. If the focus is locked on the wrong subject
No matter how smart modern autofocus systems may seem, it’s just software that offers some sort of image recognition. If you don’t have a bright subject in the frame, autofocus may choose the wrong place to focus.
The solution is simple. Choose a focus point manually. Use the most sensitive center autofocus point, focus and recompose the image. Or, you can choose one of the other available AF points to avoid recomposing the image.
This issue can also occur with head, face, and eye AF. If there are a lot of people in the frame, the system can get confused and focus on the wrong person. Often there is a way to switch to another face. Some cameras have real face recognition, so you can prioritize one person over others.
2. When the subject lacks contrast
If your subject doesn’t have much contrast, it can become difficult to lock focus. The target begins to chase until it gives up. Either an object in the back or in the front is chosen instead of the subject itself, a situation quite similar to the previous point.
The solution to this problem is quite simple. Choose a point with good contrast and focus on it, hold the focus, then do whatever 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 gets too dark, the autofocus will no longer be able to focus. If there is not enough light, it means there is not enough contrast or maybe the subject cannot be recognized from the background. Don’t forget your camera’s autofocus sensitivity specification, as mentioned earlier. If the exposure value of the situation falls below the autofocus threshold, the camera will not be able to focus. Consider the maximum aperture of your lens when checking your camera’s autofocus minimum exposure value.
A solution to this problem may be an autofocus assist light. This can be built into the camera itself, or you can use a flash’s AF assist. A flash projects a pattern onto your subject, giving the camera much-needed contrast to focus. You can also use a flashlight, of course.
4. When using a neutral density filter
The amount of light that passes through the lens is important to your camera’s autofocus system. If you place a dark neutral density filter in front of the lens for long exposures, you will greatly reduce the amount of light passing through the lens. In other words, you make the world look darker or you mimic a smaller maximum aperture.
Let’s go back to the example with an autofocus sensitive to -2 EV. The number is based on the amount of light passing through the lens with an f/1.2 lens aperture. If you place a 10 stop neutral density filter in front of the lens, the amount of light passing through is reduced by 10 stops. In other words, it’s like having a maximum aperture of around f/45. In this case, the light value of the scene must be at least +8 EV for the autofocus to work.
Although most cameras can focus with live view when a ND filter is used, it is recommended to focus before placing the ND filter. Switch to manual focus, then place the neutral density filter. Be sure not to touch the focus ring again.
One last thought
Is this knowledge necessary for your photography? In most cases, no. You’ll know when autofocus fails, and most of the time the correct solution is obvious. But it can be nice to understand why autofocus fails. If this happens too often, knowledge can help you find 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 yourself regularly encounter a faulty autofocus? Please share your experiences in the comments below and let us know how you deal with it.