Camera focus: Types and Functions

For a photograph be good and presentable, arguably the most important thing that must be maintained is a sharp focus on whatever subject is within the frame.

This blog post will be a guide to learn about the focus modes and functions that a camera has, and how to properly utilize them to take a sharp photo.

Focusing in a camera can be done by either manual focusing, which involves focusing on the subject by adjusting the focus ring on the lens manually, or by autofocus, where the camera detects the subject in the frame and adjusts the lens to get the sharpest image.

Manual focus

Manual focus in a camera, as explained before, is done by adjusting the focus ring on a camera lens while looking at the camera from either the viewfinder or display to determine the sharpness of the image before actually taking the shot.

Manual mode can be used by changing the focus switch on the lens from Autofocus to Manual focus or from the camera settings for some cameras

Manual focusing is commonly used in landscape photography where the ISO is kept as low as possible and shutter speed is slowed down and the camera is on a tripod to prevent camera shake and stopping the autofocus to change the point of focus. It is also used in dark scenes where the autofocus fails to detect the subject. Astrophotography also primarily utilizes manual focus set to infinity.

Focusing to infinity is when the lens is set to focus on an object in an infinite distance away.

In mirrorless cameras, there is a useful function called focus peaking that helps in manually focusing the lens. It highlights the area under sharp focus with a distinct color (commonly red, yellow or blue). It helps in making the manual focusing more easier and faster.

Focus peaking with a red highlight. Image source: Phototrend

Autofocus (AF)

In autofocus mode, the camera focuses on an object or subject by automatically, based on contrast levels of the subject and scene, and placement within the frame by using the focusing motor in the lens. In this mode, the camera will determine the subject and will focus on it via. the focus point(s) nearest to it.

Focus or autofocus points are the points within the focus area shown in the camera either on the viewfinder or the LCD screen in live view mode. These are the specific areas of an image or scene the camera can lock focus on. The more the focus points a camera has, the more accurately the focus points be chosen, and there is lesser chance of missing focus in focus tracking mode when shooting moving objects.

Focus area is the space within the frame in a camera where the focus points can be chosen.

In DSLRs with a mirror in front of the sensor, the focus points will be limited in the middle part of the frame when looking through the viewfinder, so the focus are will be limited. But in the case of modern mirrorless cameras, the autofocus point will cover almost all the screen even in viewfinder mode, thus giving the user freedom when choosing the focus point.

Quora: Why are the autofocus points clumped in the middle in DSLRs and not spread throughout the frame like in mirrorless cameras?

There are mainly three types of focus area mode:

(i) Automatic selection mode

Depending on how many focus points a camera offer, the camera decides automatically the focus points within the bounds of the frame where the AF points exist.

(ii) Zone autofocus mode

In zone autofocus mode, the camera will limit the active autofocus points within a defined square or zone and will only focus on the objects within it.

This AF zone can be moved within the areas where the rest of the focus points exists by using the navigation buttons, joystick or a touchscreen.

It is useful for focusing on subjects at a smaller area within the frame.

In DSLRs the AF Zone can be moved within the area covered by the AF points in the middle of the frame

In mirrorless cameras, the AF zone can be moved even to the edges to the screen.

(iii) Single point autofocus

In this mode, the user have access to choose a single focus point from all the points the camera offer by using the navigation keys or a joystick at the back of a camera.

This is used when the user wants a single point of a subject to be in focus.

Types of autofocus mode:

1. Single-shot autofocus

In single- or one-shot autofocus mode, the camera focuses on to a subject located in any area within the frame depending on the selected focus point and keeps the focus locked on to it after half-pressing the shutter button, until it is released. This mode is used for shooting subjects that don't move around like in landscape photography, art, inanimate objects etc.

But it must be kept in mind that the subject should be in a stationary position, otherwise it will be out of focus if it moves after the focus is locked.

2. Continuous autofocus

Continuous autofocus or focus tracking mode is the mode where the camera focuses on a subject after half-pressing the shutter button and keeps tracking the subject by continuously focusing on it within the boundary of focus points as long as the shutter button is kept half-pressed.

This mode is known as Servo AF in Canon cameras, and AF-C (Continuous) in Nikon and Sony Cameras

This mode is used in photography that involves moving subjects like kids, cars, animals, sports etc.

Recomposing shots

If you own a DSLR, there is an extra step to take for focusing on a subject near an edge or corner within the frame. This step is called focusing and recomposing. This process is necessary because the focus points in DSLRs are clumped in the middle.

To focus on an subject at the edge of the frame, you must first aim the camera to put the subject in the middle of the frame, then half-press the shutter button (and keep holding it),  re-compose the shot i.e. go back to placing the subject at the edge of the frame, and then take the shot. If you want to take continuous shots with the focus locked on the subject, then after the initial focus lock with the half-pressed shutter button, just simply change the focus mode from autofocus to manual, while still holding the shutter button.

In the case of a mirrorless camera, the camera is able to choose any single AF point to focus on any subject regardless of its location within the frame without having the trouble of recomposing. And also, the single autofocus point can be chosen at any point within the frame while looking through the viewfinder by using a finger on the touchscreen like a trackpad to move the focus point across the screen.

Conclusion

You must decide from these settings, which focus area and mode to choose according to the type of photograph that's going to be taken for the best, sharp focused result.

For example, if I wanted to take photo of a flower, I would choose Zone AF and Single-shot AF mode. And if I'm taking photo of someone running, I would choose either Auto selection AF or Zone AF and continuous AF.

Thank you for reading, enjoy your day (or night).