Autofocus is something we take granted. It is normal to think that, by pressing the shutter button halfway, the camera immediately catches the focus properly. And we support the view ! That is why we wrote about AutoFocus in Digital Photography and How to Use Properly Autofocus to Prevent Blurry Photos. There are more advanced guides like What Camera Mode to Choose Depending on the Situation, AF Area Selection Modes etceteras. This is another type of article on Auto Focus.
How Auto Focus is Betraying You?
But sometimes it happens that the lens runs back and forth in search of focus and can not stop to find the right point. Indeed, there are some situations in which the focus has obvious difficulties. In these cases, we have to intervene in order to be able to focus on what we want. In this article, We’ll explain why and when the autofocus will face difficulty and workaround these problems. The information except when specified, is applicable to any type of digital camera.
Autofocus is Betraying You? Here’s How to Resolve
Because the camera can not focus
There are two main technologies to focus on modern digital cameras : a phase detection and measurement of contrast. We are not going to explain them here in detail. We will suggest you to read the above linked articles. The importance is to understand what the lens both analyze sees and then decide how to focus, both need to identify the margins, the contours of framed objects. Often, if the autofocus is hard, it means that it is not easy to identify these margins. We see the various situations where autofocus may not work and how to address them.
Light too tenuous
In this case, the camera does not have enough “information” to find a point of focus. Although your naked eyes ‘think’ that the light is sufficient, it may not be for the camera. In fact, cameras, even the best do not “see” all the light that we see. The first help can come from ‘AF illuminator that many cameras have. It provides a soft light, but sometimes enough to focus. Unfortunately, it is totally useless if the point to focus on a point far more than a few meters. In addition, in certain situations the illuminator bothers you, you just can not activate it. Applying the same concept, you can use a flashlight, more powerful illuminator. Here, the problem is not to show the ‘torch light’ in the shot. You can proceed as follows : light the area you want to focus, Compose the picture, press the shutter button halfway, targeting the torch (or whatever) from outside of the scene, snap. Of course, so you need have one free hand. To overcome this difficulty, or use a tripod, or find a way to support the torch.
Too little contrast
Sometimes the point of focus is placed on an area that has no contrast, e.g. a surface with uniform color, without sharp shadows. The contrast is crucial for auto-focus, so these subjects are decidedly problematic. Even in this case, the illuminator or a torch are useful.
Through a glass
When we look through a glass, our brain is very good at identifying what part of the scene is not belonging to the glass and which part of the subject of our interest. The camera is not so smart, what it sees is only a set of lines with various values ??of contrast. If the point of focus is partially located on a glass, you will have problem to focus on the subject. The same applies to the glass, if the reflections on it become preponderant with respect to the scene that is located behind. The only solution in this case may be the manual focus. You can also help with the distance scale, if this is your goal.
Shallow depth of field
When using very low values ??for the opening (from f2.8 down), especially in combination with high focal lengths lens, reduce the very shallow depth of field. This means that the area to focus is very small, so the autofocus must be extremely precise. If the situation is not optimal (for example, due to lack of contrast or light), the autofocus can fail. Sometimes the problem is accentuated if you use good quality objectives. Even in this case, the only solution may be to resort to manual focus. In addition, in combination with this, you can use Live View to get a preview of depth of field. Camera screen is much larger than the viewfinder, then it will be easier to focus precisely, even with a reduced depth of field. This problem also occurs with higher values ??of aperture, but only when photographers are at a close range.
Action too fast
When you pursue a fast-moving subject, the focus must continually changing. In fact, the distance between you and the subject is constantly changing. It is a frequent situation in sports photography and nature photography. In these types of photography, in fact, often can not be ignored by owning the lens of fastest speed. If you do not you can afford it, you can approximate with the technique of prefocusing. It basically consists of predict where the subject will move and focus on that point before the subject gets there.
Solution Universalis !
Two solutions are always available in all occasions when the autofocus is in crisis. The first is the manual focus, which we have already mentioned, in some cases above. It is not suitable for highly dynamic situations, but remember that it is always available. The second option is to focus on a point at the same distance compared to what you want to focus. When you focus on a point, which are in focus that are at the same distance from the sensor and makes is easier to focus. Change selection of the point of focus in the camera (not applicable for all cameras) and snap.