Digital cameras use different methods of autofocus to obtain sharp focus on the subject.

DSLR cameras use a phase detect focus module below the sensor to compare light from a sub-mirror split into two separate images by prisms and micro-lenses in the AF sensor unit, each aimed at two lines on the AF sensor corresponding to the active AF point. If the images hit the two lines precisely, the subject is in focus, any distance between the two images tells the camera how much the lens is out of focus, the lens is driven until the images match, and accurate focus is thus achieved. In “Live View” many DSLR cameras use the contrast detect focusing pixels directly on the sensor.

Mirrorless cameras can use a contrast detection focus method, a phase detect autofocus, or a combination of both called a hybrid autofocus system, but without mirrors, or prisms, all the focus points are located directly on the sensor.

Contrast detect autofocus is the simplest system, it compares contrast areas within the subject, and moves the lens focusing elements backwards, and forwards until the highest contrast is achieved. Contrast autofocus is extremely accurate, but can be slow.

Phase detection autofocus (PDAF) pixel pairs are incorporated into the digital sensor and consist of partially covered sensors receiving light from either side of the lens, left, and right pairs. The camera compares both, and when they match, knows that the subject is in focus. If the subject is out of focus, it calculates how much it needs to adjust the lens to bring the subject into focus and drives the lens there.

Hybrid autofocus uses the phase detection to achieve close focus and finalises accurate focus with contrast detection.

Now, knowing all the technical details of camera autofocus is good, but how best to employ that knowledge? The best advice I can give is to get to know the various settings of autofocus that your camera has, and I shall use some images to illustrate those settings that I use.

Portraits – a lot of digital cameras have “face and eye” detect modes, and I’ve used this mode to good effect for photographing this model. My camera model has four autofocus selections, single point, zone, wide & tracking, and finally “all”, for this shot I used the wide & tracking selection, with the “face and eye” detection turned on to focus accurately on the eyes. Marika was photographed with the Fuji X-T20, and the XF50mm f/2 (75mm ff) lens f/4, 1/80, ISO 400

Derelict building – for this shot I used single point autofocus, I was taking practice images with my new Fuji X-T3 camera, and the XF35mm f/2 (50mm ff) lens, f/11, 1/52, ISO 1000. This photo is a technical failure, not being as sharp as it could have been, I needed a faster shutter speed, and should have either widened the aperture to f/8 or increased the ISO to 1600 to eliminate the shutter shake.

Robin – I was very pleased with this image, taken with the Fuji X-T3, and the XF70 – 300mm (105 – 450mm ff) using single point autofocus on the eye of the bird. Photo details: 434mm ff equivalent, f/8, 1/280, ISO 800.

Cadwell – my first outing with the Fuji X-T3, and the XF70 – 300mm lens, using the wide & tracking selection, and continuous autofocus, following the bikes with the camera as they come past, a technique known as “panning”. Photo details: 126mm ff equivalent, f/5.6, 1/300, ISO 200.

All images were captured in Aperture Priority mode, with the aperture and ISO adjusted to give the correct shutter speed (hopefully) for the scene. I very rarely use the “All” selection of autofocus as the camera focus can sometimes lock onto unwanted areas in the frame, a blade of grass in the foreground of a landscape perhaps, or the nearest tree. For a landscape scene, I usually use a single point autofocus point and focus a third of the way into the scene.

For macro images autofocus can be of use, but many photographers prefer to use manual focus, be aware that many lenses are “focus by wire”, which means that the lens focus ring does not control the focusing element within the lens directly, so there may well be a lag in obtaining focus. I find that manual focusing aids such as magnification, and / or focus peaking work well, and instead of turning the focus ring “rock” the camera physically slowly back and forwards until sharp focus is obtained.