Focus First (How to highlight your subject in the best way possible)
Sharp, clear pictures are usually one of the first few requirements of a good photography. Read on to learn about Auto Focus – a handy tool in your cameras that helps you highlight your subject in the best way possible.
Autofocus – you might have heard this term frequently. As the name suggests, it is the capability of a digital camera to lock its focus on the subject automatically without you having to manually rotate the focusing ring of the lens. Yes, it’s a fantastic artificial intelligence within the camera which works almost like a human eye. But have you ever wondered how it works? How does it lock the focus on the subject of interest out of so many things in a frame? Let’s try to understand it in a simple way.
Autofocus happens with the help of certain sensors called Autofocus (AF) sensors. Take the DSLR in your hand and look through the viewfinder. Your camera manual will tell you which button will display the AF sensors. You will be able to see that there are multiple AF sensors distributed in arrays across the field of view (Cover Photo). Each of these sensors measures the relative focus based on detection of contrast in the frame. Digital compact cameras use the image sensor itself as an AF sensor. Therefore when there is no contrast, the camera lens may keep moving back and forth in search of the ideal focus point. I am sure you have come across this situation. Try shooting a light-brown bird perched on a dry brown branch against a dry brown grassland background. Your lens will keep moving back and forth because the AF sensors are unable to find a contrast area. Look for an area of contrast, say the eye of the bird and now try to focus there. AF sensors will find a contrast area now and you will be able to focus.
AF depends a lot on the number and type of AF points in your camera. High-end DSLRs may have 45 or more autofocus points. Others may have as little as one. But remember, not necessarily all 45 can be used at all apertures of the lenses attached. As and when f-stop decreases (i.e. aperture widens) the number of autofocus points that can be used also decreases.
Additionally, the number and accuracy of the available AF points also depend on the maximum achievable aperture of the lens that you use. We have discussed before the widest apertures of various lenses. Some lenses can open wide up to f/2.8 and some up to f/5.6. The wider the maximum achievable aperture of a lens, the more is the number of available AF points. Now pay attention here – this means that a f/2.8 lens will give you more AF points at f/5.6 than a f/5.6 lens will give. That means a DSLR using a lens (who widest aperture is f/2.8) will always have a number and better accuracy of AF points at f/5.6 than what can be achieved by a lens whose widest aperture itself is f/5.6. Hence even if you don’t use the lens at its widest aperture, this is a very important thing to keep in mind while buying a lens, because it affects the focus immensely.
I know it’s complicated – I think the figure will simplify this. (Above Fig).
Many cameras have the facility of an AF assist beam. This proves to be very useful in situations where your subject is not well lit or has insufficient contrast. But remember the AF beam will result in slower autofocus.
Now let’s come to different AF modes that you have in your DSLR. A commonly used AF mode is one-shot focusing. While best for still subjects, this one-shot mode is vulnerable when focussing on fast moving subjects since it cannot anticipate the subject motion. One shot focusing requires a focus lock before the photograph can be taken. But most DSLRs also support another autofocus mode which continually adjusts the focus distance for moving subjects. Canon cameras call this “AI Servo” focusing, on fast moving subjects since it cannot anticipate the subject motion. One-shot focusing requires a focus lock before the photograph can be taken. But most DSLRs also support another autofocus mode which continually adjusts the focus distance for moving subjects. Canon cameras call this “AI Servo” focusing, whereas Nikon cameras refer to this as “continuous” focusing. It works by predicting where the subject will be slightly in the future, based on estimates of the subject speed from previous focus distances. The camera then focuses at this predicted distance in advance to account for the shutter lag (the delay between pressing the shutter and the start of the exposure). This greatly increases the probability of correct focus for moving subjects (Fig Below). So go out and start experimenting. It is fun and an excellent way to learn and better your focusing instincts.
Cover Photo by Pratik Chorge
Read also: Playing Tricks with Light
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