Advanced modern autofocus system. Sometimes it seems like you’ll never miss a shot again once focus starts. But autofocus can still fail under certain circumstances. Let’s take a look at those moments that even the most advanced autofocus can give up.
With each new camera, the autofocus system seems to get more advanced. If you don’t know any better, the focus of every image should be on the spot. Sometimes you wonder if it’s still possible to get an out-of-focus image. The camera can recognize animals, people and vehicles. It may even require focusing on the body, head, face, and even eyes. Tracking makes it possible to keep track of your audience and never lose focus.
The truth is that out-of-focus images can still occur, even with this advanced focusing system and built-in image recognition software. The camera may be confused, not knowing what the main subject is. In that case, the focus might be on, but in the wrong place. In some cases, you cannot achieve concentration. The system starts searching in an attempt to find the focus point. If it fails, autofocus will eventually stop and taking pictures may become impossible.
To understand why focusing might fail, we have to look at the autofocus system in a bit more detail. For simplicity, systems often need contrast to achieve good focus. If there is not enough contrast, the system will fail. For example, a blank sheet of paper does not have enough contrast. But if you do a fold, there’s something to focus on.
To see any contrast, there must be enough light. The darker it is, the harder it is to distinguish that crease on the paper. Until there is not enough light left and the autofocus will fail.
Exposure value of a scene
If you look at your camera’s specs, you might find an EV value mentioned for autofocus sensitivity. This indicates the minimum amount of light required for the autofocus to work.
You’ll find values that range from 0 EV to -7 EV, depending on the camera you have. The EV value can be traced back in the so-called light value graph. These charts depict lighting situations suitable for different EV numbers. For example, a -2 EV situation is similar to the light of a full moon in a snow-covered landscape.
It’s amazing how the autofocus can work in these lighting conditions, not to mention a camera that can focus even in low light. But you have to take into account another requirement. Autofocus is only possible in this minimal light with a large maximum aperture, usually an f/1.2 lens.
For example, a camera can focus as low as -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 transmits about 3 stops less light, then the minimum light value for autofocus to work will be +1 EV instead of -2 EV. This is similar to the amount of light of a distant city skyline at night. If your lens has a maximum aperture of f/5.6, the limit will be +2 EV, etc.
What if Autofocus can’t Lock Focus?
Now, the basics are clear. Let’s look at some situations when autofocus fails. Some of these will be related to the sensitivity of the autofocus system, which varies from camera to camera. I will also mention a possible solution.
1. If the focus is locked on the wrong subject
No matter how clever modern autofocus systems may seem, it’s just a bit of software that provides some sort of image recognition. If you don’t have a clear subject in the frame, autofocus may choose the wrong place to focus.
The solution is very simple. Select the focus point manually. Using the most sensitive center AF point, focus and recompose the image. Or, you can choose one of the other available focus points to avoid recomposing the image.
This issue 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. Usually, there is a way to switch to a different face. Some cameras have real face recognition, so you can prioritize some people over others.
2. When the subject lacks contrast
If your subject doesn’t have a lot of contrast, it can become difficult to lock focus. The lens starts hunting until it gives up. Either an object in the background or front is selected instead of the subject itself, a situation quite similar to the previous point.
The solution to this problem is quite simple. Pick a point with good contrast and focus on that point, keep the focus, then create the composition you have in mind. You can also choose something with more contrast at the same distance, but in a different direction. As long as you always lock the focus, you should be fine.
3. If it’s too dark
If it becomes too dark, autofocus will no longer be able to focus. If there’s not enough light, it means there’s not enough contrast left, or perhaps the subject can’t be recognized in the background. Remember, your camera’s autofocus sensitivity specs, as mentioned before. If the exposure value of the situation falls below the autofocus threshold, the camera will not be able to focus. Take into account the maximum aperture of your lens when checking the minimum exposure value of your camera’s autofocus.
The solution to this problem may be the AF-assist illuminator. This can be built into the camera itself, or you can use the flash’s AF assist. The flash projects a pattern onto your subject, which will give the camera much-needed contrast for focusing. Of course, you can also use a flashlight.
4. When you are using a neutral density filter
The amount of light that passes through the lens is important to a camera’s autofocus system. If you place a dark neutral density filter in front of the lens to achieve a long exposure, you will significantly reduce the amount of light passing through the lens. In other words, you make the world a darker place or you mimic a smaller maximum aperture.
Let’s look again at the example with sensitive autofocus of -2 EV. The number is based on the amount of light passing through a lens with an aperture of f/1.2. If you place a neutral density filter 10 stops in front of the lens, the amount of light that passes through will be 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 landscape needs to be at least +8 EV for the autofocus to work.
While most cameras will be able to focus with live view when using a neutral density filter, it is recommended that you focus before setting a neutral density filter. Switch to manual focus, then set 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’ll know when the autofocus has failed, and most of the time, the proper solution is obvious. But it can be great if you understand why the autofocus is failing. If it happens too often, knowledge can help you in your search for a more permanent solution. The solution could be a lens with a wider maximum aperture, a camera with a more sensitive autofocus system, or perhaps an AF-assist lamp.
How often do you experience autofocus errors? Please share your experience in the comments section below and let us know how you solved it.
