Understanding Camera Focus: What's In Sharp Relief?

Focusing is a fundamental aspect of photography, and while modern cameras have made it easier, understanding how it works is key to getting the best image. In simple terms, focusing involves adjusting the lens to ensure that the desired object is as sharp as possible in the photo. This can be done manually by adjusting the focus ring on the lens, or automatically with autofocus. Autofocus systems use a motor in the camera or lens to focus on a subject, and this can be selected manually or automatically. The two main types of autofocus systems are phase detection and contrast detection. Phase detection is very fast and good at tracking moving objects, while contrast detection is more precise as it measures data directly from the camera sensor but is slower.

Manual focus vs autofocus

Focus is the result of a combination of your camera's lens aperture and light. The aperture of your lens will determine how much light is let in and, as a consequence, how well you'll be able to focus.

Autofocus

Autofocus (AF) is the function of a camera to automatically focus on a subject. Autofocus allows the camera to set the focal point, handling most of the work in ensuring a clear shot. Most general digital cameras have this function. There are various AF methods, and the available methods differ depending on the model of your camera.

AF-S (Single-shot AF)

Autofocus operates when the shutter button is pressed halfway down. Once the subject is in focus, the focus is locked. It is suitable for shooting motionless subjects such as landscapes or snapshots.

AF-C (Continuous AF)

Autofocus continues to operate and keeps adjusting the focus while the shutter button is pressed and held halfway down. It is suitable for shooting moving subjects such as sports or railway photography.

Manual Focus

Manual focus (MF) is the function that lets the photographer adjust the focus manually instead of the camera. Manual focus enables the user to set the focal point, providing more creative control over the image. Although autofocus (AF) shooting is more typical in digital cameras, MF is effective when focusing is difficult with autofocus, such as in low light conditions or when there are many objects in the scene.

When to Use Autofocus

Autofocus is ideal for situations where the subject is in motion, such as sports, wildlife, or street photography. It is also useful when the photographer does not have much control over the scene, as in street photography.

When to Use Manual Focus

Manual focus is ideal for static scenes, such as still life, landscape, studio, and macro photography, where the photographer has time to set up the shot and adjust the focus. It is also useful when autofocus struggles, such as in low light conditions, when there are many objects in the scene, or when you need to nail a precise point of focus.

The choice between manual focus and autofocus depends on the photographer's needs and shooting conditions. Neither is universally superior, and both have their advantages and disadvantages.

Phase detection vs contrast detection

Phase detection autofocus and contrast detection autofocus are the two autofocus systems used in modern digital cameras. Each has its own advantages and disadvantages, and they are suited to different types of photography.

Phase detection autofocus is the system most commonly found in DSLR cameras. It is very fast and, therefore, great for tracking moving objects. However, the system is not always accurate, especially in low-light conditions or when using wide apertures. It also doesn't work in Live View or movie mode, as the camera's mirror blocks the light to the autofocus unit.

Contrast detection autofocus, on the other hand, is commonly used in mirrorless cameras, point-and-shoot cameras, DSLRs in live view mode, and smartphone cameras. It is slower than phase detection but much more accurate, making it better suited to genres like product or landscape photography. This system works by analysing the contrast on pixels on the camera's sensor and pushing the lens back and forth until it finds the right focusing point. When the contrast is at its highest, the subject is in focus.

Some cameras use a hybrid of the two systems. For example, a digital SLR camera may use contrast detection autofocus in Live View or movie mode and phase detection autofocus when you look through the viewfinder.

Single shot/Single-servo focus mode

Single-shot or Single-servo autofocus mode is one of the main autofocus modes available on most camera brands. It is called One-Shot AF on Canon cameras, and AF-S on many non-Canon cameras.

Single-shot AF is used when neither the photographer nor the subject is moving. To use this mode, the shutter button is half-pressed, which locks the focus on the subject. The focus will remain locked on the subject even if the shutter button is released, until the photo is taken. This is particularly useful for focusing on a subject, and then recomposing the shot.

Single-shot AF is the most commonly used autofocus method and is likely the default setting on most cameras. However, it is not suitable for tracking moving subjects. For example, if you are photographing a biker riding towards you, the focus will remain locked on the original spot, and the biker will soon be out of focus.

Single-shot AF is the best option for focusing on still subjects, such as landscapes, still life, portraits, flowers, and architecture.

Continuous focusing/Continuous-servo

Continuous focusing, also known as Continuous-servo AF, is an autofocus mode that is designed to track and focus on moving subjects. This mode is particularly useful when shooting sports, animals, urban life, or any other non-stationary subjects.

In this mode, the camera will continuously adjust the focus to keep the subject sharp, even if the subject or the photographer are moving. This is achieved by the camera system capturing, retaining, and analysing information regarding the subject's positioning and movement trajectory.

Continuous-servo AF is especially helpful when shooting action photos of moving subjects, and when shooting at super-shallow apertures, such as f/1.4, and at close distances. In these conditions, even the slightest movement can cause the desired subject to fall out of focus.

It is important to note that continuous focusing has some limitations. While it can predict the movement of a car or a pedestrian, it may struggle with more erratic subjects, such as a bird or a cheetah chasing prey. To address this, camera manufacturers have developed variations of continuous focus modes, such as Animal Eye AF, Vehicle AF, and Deep Learning AF.

Additionally, continuous focusing can result in increased battery consumption and potential focus inaccuracies due to unexpected changes in the subject's movement.

Some cameras offer different names for this mode, such as AI Servo for Canon cameras, AF-C for Nikon cameras, and Continuous AF for Sony cameras.

When using continuous-servo AF, it is recommended to shoot multiple images to ensure you capture the desired moment with the subject in focus.

Autofocus area modes

Single-Point AF Area Mode

Single-Point AF Area Mode enables photographers to select a single focus point for static elements within a scene. This mode is ideal for stationary subjects such as landscapes, architecture, and still life. By keeping the subject framed over the selected focus point and using Continuous Autofocus (AF-C) mode, the camera will automatically adjust its focus to maintain a sharp image. Advanced camera models offer a higher number of focus points, allowing for more precise selection, while basic models typically provide a smaller number of focus points. Regardless of the camera model, the central focus points are generally the fastest and most reliable, making them the preferred choice for critical focus areas.

Dynamic AF Area Mode

Dynamic AF Area Mode is a versatile tool for capturing moving subjects. Once you manually select the initial focus point, the camera will utilise this point and its surrounding points to maintain focus on the subject as it moves. This mode is particularly useful for wildlife and sports photography, where subjects are often in motion. Most cameras offer different dynamic AF area modes, typically grouped into sets of focus points such as 9, 21, or 51. The choice of mode depends on the predictability and speed of the subject's movement. For slower, more predictable motion, a smaller area mode like 9 points may suffice. In contrast, faster or more erratic movement may require a larger area mode, such as 51 points, to effectively track the subject.

Group AF Area Mode

Group AF Area Mode is a valuable tool when you want to focus on a specific area or zone within the frame. This mode is ideal for wildlife, sports, and group portrait photography, where multiple subjects are present in a defined area. It allows you to select a small number of autofocus points to ensure accurate focus on the desired area. Canon and Sony offer similar group area modes, dividing the focus points into 9 areas to capture subjects within the assigned zone. Group AF area mode is a powerful tool when you need more precision than a single AF point but still want to maintain control over the focused area.

Auto AF Area Mode

Auto AF Area Mode is a fully automatic mode where the camera takes charge of selecting the focus points based on various factors. It considers subject distances, motion relative to the camera, and even the presence of detectable eyes to determine the most important elements in the scene. This mode is excellent for novice photographers who are still learning the intricacies of autofocus modes. It is also useful when you need to quickly focus on something close to the camera. However, it is not recommended when precise control over the focus point is required, as the camera has the most control in this mode.

Frequently asked questions