Pre-focusing is a technique that allows us to take focused and sharp photographs in the situations where focus in real time is usually not possible or difficult. In this article on Pre-focusing, we will see a simple technique that is very useful when photographing moving subjects or scenes are action-packed.
Introduction to Pre-focusing
As our camera always will be in situations where performance of focus engine is too slow, so there will be issues with auto-focus. This is true for both DSLR and compact digital cameras. As we saw in a previous article on Autofocus to Prevent Blurry Photos, to take a picture every time we have to follow a precise procedure:
- Pressing the shutter button halfway
- Wait until the camera has focused
- Press the button all the way down to finally take pictures
The amount of time to wait at point 2 varies depends on the camera model and lens attached (the latter only in the case of DSLR). As this time may be reduced, there will be scenes for which it will be too slow. We can think of two large families of such situations:
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- Situations in which subjects are moving very quickly, such as automobile or motorcycle races
- Situations in which the action is unpredictable, take for instance a photo of children playing or nature photos
Situations such as these can lead to shots out of focus because the movements of the subject movement are too fast or too unpredictable to allow the camera to focus at the point and at the correct time.
Pre-focusing to Prevent Blurry Photos for Moving Subjects
The solution to the problem described is relatively simple. It comes within a few words for pre-focus :
- If, for example we were photographing a motorcycle race, we could focus on a point on the asphalt that, we are sure will pass through the bikes and take photos only when the bikes come into the frame
- If we want to take pictures of pets, after studying their movements, we could pre-focus a point where we expect will pass and shoot only when the birds actually reach that point.
The answer to the question, how to pre-focus; derives from the procedure described above. When you press the shutter button halfway, the camera puts the focus at the point of focus selected. So, what you’ll do is, point on which we want to make the prefocusing on and press the shutter button halfway. Until pressing to full or not releasing the button, the focus will remain locked. Another way would be to use the AF-L button, to hold instead of the shutter button, but personally we do not see it to be a comfortable option. Two things, however, care must be taken when using the prefocusing, after having focused:
- If we change our position with respect to the point just focused, this will no longer be in focus, then we will have to go for re-prefocusing
- We could go wrong to predict in which, pointing to the subject that we want to photograph; therefore it is better to choose an opening that is not too wide, so that the portion of the image focus is remains greater (to get the depth of field that we want to achieve).
How to Choose the Correct Focus Point
The choice of the point on which to prefocusing must be done very carefully. Above all, keep in mind that the camera will focus on a plane that is parallel to the sensor that will pass through the point identified. Therefore, it is necessary to focus on a point that is the same, where you will find the object to be photographed when it enters the frame. For this reason, in the previous example of a motorcycle race, a point on the asphalt is a good choice as a point of focus, because the bikes we will pass right infront your nose. Plus there will be shutter lag.
In general, choosing a point on the ground is often a good idea. When the scene may not include the ground, you need to find an object that has the same distance from the camera. Aiming any object in the background could lead to a blurred subject. This is because the movement of the subject to be photographed could intersect the object in the background, but this object could be several meters distant from the camera and then on another plane of focus.
Given the simplicity of implementation, the technique of pre-focusing may seem silly. In addition, the real difficulty lies in predicting the movements of subjects to photograph and identify the most appropriate point on which to focus. Like many other photographic techniques, proper use of pre-focusing demands exercise.
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