How to use autofocus
Autofocus or autofocus for most photographic scenes is the preferred solution over manual focus. In skilled hands, autofocus focuses more precisely, and, most importantly, faster than the average photographer. However, autofocus is far from being as simple as it might seem to a novice amateur photographer, and its proper use is very far from the point-and-shoot principle. There are a number of subtleties that you should learn if you want autofocus to stop living its own life and begin to do what you want from it.
I highly recommend that you re-read that section of the manual for your camera that focuses on autofocus – these are some of the most useful pages in the entire manual, and the information contained therein should not be neglected. At a minimum, you should be aware of which controls are responsible for switching between different autofocus modes and choosing the focus point you need.
Most cameras have two main autofocus modes: single and tracking.
Single or single-frame autofocus (in Nikon cameras it is called Single Servo AF (S), and in Canon devices it is called One-shot AF) is intended for shooting still scenes, such as, for example, most landscapes. When you press the shutter button halfway, the camera focuses on the subject located within the pre-selected focusing point, after which the focus is locked, allowing you to change the composition of the frame (without changing, of course, the distance to the subject) and only then release the shutter.
It should be understood that in reality the lens does not focus on the object as such, but on a certain distance. Thus, if I allow the camera to aim at a certain object located at a distance of 5 meters from me, then all other objects that are 5 meters away from me, i.e. lying in the focal plane will come out sharp, and while the focus is locked, and the distance to the object does not change, I am free to rotate the camera to please the composition, without fear of losing focus.
This method is good when the distance to the subject is relatively large and measured at least meters. At close distances, which are unavoidable during macro photography, rearrangement of the frame, entailing a change in distance of only a couple of centimeters, can result in a noticeable shift in focus relative to the subject, which will be especially critical at a shallow depth of field.
Tracking or continuous autofocus (for Nikon – Continuous Servo AF (C), for Canon – AI Servo AF) is indispensable when shooting moving objects, such as athletes or animals. As long as the shutter button remains half-pressed, autofocus continues to work continuously, keeping the subject in focus, even when the distance between it and you changes. Focus lock does not naturally occur in this case, since the lens of the lens is in constant motion, tracking the movement of the object.
Obviously, when using tracking autofocus, you cannot arbitrarily change the composition of the frame, because if the active focusing point leaves the subject, then the focus will shift from the subject to the background after the point. In order to lock focus in AF tracking mode, use the back button focus.
The intermediate or automatic mode (AF-A or AI Focus AF), which itself decides whether to use single or tracking autofocus, does not inspire me much confidence, since it is not always able to distinguish between camera movement and object movement.
Focus point selection
The number of focusing points in modern cameras can reach fifty or even more. The abundance of focus points is, of course, nice, and sometimes useful, but even if your camera has a small number of points (nine or eleven) by modern standards, you still have enough of them with your head.
When shooting stationary objects, I use only one single point, most often – the central one. One point allows me to focus in the most precise way on the object I need, or even on its individual details, and then, having locked the focus, recompose the frame as I want.
Automatic selection of focus points is very convenient when you are in a hurry, but you should remember that the camera usually tries to focus on the object closest to it or on the area with the highest contrast, and this is far from always what you want. Autofocus cannot know which of the objects is the most important and requires unconditional sharpness, and which is secondary, and therefore may not be in focus, and therefore do not be lazy to choose the focus point yourself, if the camera’s automation does not cope with this.
I use auto focus point only in the following situations:
The object moves very fast, and I simply don’t have time to select points – the camera will do it much quicker.