Understanding autofocus

Neil Freeman reveals everything you always wanted to know about autofocus on your Nikon DSLR or Z-series mirrorless but were afraid to ask

Sharp, in focus images are key to good photography. Focusing, depth of field, shutter speed and lenses are all crucial factors in achieving this. You can correct a lot during post-production, but poor focusing isn’t one of them; an out-of-focus image will always be out of focus, blurry or soft. Sometimes image blur can be used as an effect in an image, but this is a deliberate creative choice by a photographer. Knowing how to focus accurately and which focus modes to use is an important skill.

Accurate focus is needed as your depth of field stretches around the point of focus. Miss your focus point and the depth of field might be in the wrong place.

Autofocus needs areas of contrast to work. A subject with no texture, like a blue cloudless sky, has nothing to lock onto. This will cause your lens to ‘hunt’ back and forth, as it tries to achieve focus. Areas of contrast that incorporate vertical or horizontal lines or strong edge contrast enable autofocus to work very accurately.

It’s personal preference whether you use the viewfinder or Live View to focus, each have different uses and utilize separate focusing technologies inside the camera.

Using the optical viewfinder on a DSLR activates a separate AF sensor to achieve focus using phase detection. The shape of the AF sensor dictates where the focusing points can be positioned, meaning that you can’t achieve full viewfinder coverage with an optical viewfinder. This sensor uses hardware micro-lenses for focusing.