Mastering Auto Focus For Cinematic Camera Work

Autofocus has been a staple of documentary filmmaking for years, but it is less common in narrative filmmaking. Cinematographers tend to view autofocus as an amateur tool, but it has improved significantly with technological advancements. While autofocus is not 100% accurate, it can be useful in certain situations, such as when shooting with a gimbal or when the subject is moving towards the camera. Additionally, autofocus can be beneficial when filming uncontrollable subjects, such as animals or children. However, manual focus is still preferred for complex work with multiple moving subjects.

Using autofocus with a gimbal

Benefits of Autofocus on a Gimbal

Autofocus can be incredibly useful when shooting with a gimbal, as it allows you to maintain focus on your subject without needing to constantly adjust the focus manually. This is especially beneficial for self-shooters or small crews who don't have the luxury of a dedicated focus puller. Autofocus is also ideal for filming unpredictable or fast-moving subjects, such as sports, documentaries, or scenes involving children and animals. In these situations, autofocus can keep your subject in focus even when they are moving around or changing distances from the camera.

Choosing the Right Gear

When using autofocus with a gimbal, it's important to have compatible gear. Native lenses that are designed for your camera system will typically provide the best autofocus performance. In some cases, you may need to use an adapter to attach a compatible lens to your camera. Additionally, modern autofocus systems often utilise advanced features such as face detection, subject tracking, and eye detection, so choosing a camera with these capabilities can greatly improve your autofocus performance.

Techniques for Effective Autofocus

To get the most out of autofocus on a gimbal, there are several techniques you can employ:

• Maintain a consistent distance from your subject: By keeping your subject within a certain range, you can minimise the need for constant autofocus adjustments.
• Use a wide-angle lens and a deep depth of field: By increasing your depth of field, you can ensure that more of your frame stays in focus, reducing the need for frequent autofocus adjustments.
• Focus in the middle of your path: This technique helps ensure that your subject remains in focus as you move the gimbal.
• Adjust your aperture: Increasing your aperture (choosing a higher f-stop number) can increase your depth of field, but it will also let less light into the lens, darkening your image. Finding the right balance between depth of field and lighting is crucial.
• Utilise autofocus settings: Modern cameras offer a variety of autofocus settings, such as zone focusing, spot focusing, and face detection. Experiment with these settings to find the ones that work best for your specific shot.
• Consider post-production adjustments: If you find that your footage has some out-of-focus moments, you can always make adjustments in post-production by cropping or using blur effects to hide the areas that are out of focus.

Limitations and Challenges

While autofocus can be a powerful tool, it's important to be aware of its limitations. Autofocus systems can sometimes struggle with complex scenes, multiple moving subjects, or low-light conditions. In these situations, you may need to intervene manually or use other techniques to ensure your subject stays in focus. Additionally, some lenses may not support autofocus, especially high-end cinema prime lenses. Always test your gear and familiarise yourself with the autofocus capabilities of your specific camera and lens combination.

Face detection mode

Sony, for example, offers three options in their Face Detection AF menu: Off, Face Priority AF, and Face Only AF. With Face Priority AF enabled, the autofocus will prioritize human faces as they move within the frame. Face Only AF takes this a step further and only focuses on faces. This is ideal when you want the camera to stay focused on a subject entering and leaving the frame without shifting focus to other objects.

Canon's Face Detection AF, found in cameras like the Canon C300 Mark III, also includes support for touch AF, allowing the camera operator to specify which face to focus on. This can be incredibly useful when shooting with a shallow depth of field or when your subject is moving within the frame.

It's worth noting that face detection technology works better when the image is underexposed. If your exposure level is set too high, you may encounter issues where the face detection box does not appear on the subject. In such cases, lowering the exposure on your lens or reducing the ISO can help improve the performance of face detection.

While face detection autofocus has its benefits, it's important to recognize that it may not always be suitable for complex shots with multiple moving subjects. In such cases, the intuition of a dedicated focus puller or a self-shooter aware of the scene's dynamics can be more effective.

When to use autofocus over manual focus

Autofocus has traditionally been seen as inferior to manual focus, especially in the world of cinema. However, autofocus technology has improved significantly, and there are now several situations in which using autofocus may be preferable.

When shooting a documentary

Documentary filmmaking often involves capturing unpredictable and fast-moving subjects in challenging environments. Autofocus can be beneficial in these situations, as it allows the filmmaker to focus on other aspects of the shoot, such as lighting and audio. The Sony FX6, for example, has 629-point focal plane phase detection AF, which can help keep subjects in focus, even in chaotic situations.

When shooting an interview

Interview footage can be tricky to keep in focus, especially if the subject moves around a lot. Autofocus can help ensure that the subject remains in focus, even if they are animated. The Sony FX9, for instance, has enhanced Fast Hybrid AF, which offers unprecedented speed, smoothness, and precision.

When working solo

If you are operating the camera yourself, it can be challenging to adjust the focus manually while also handling other aspects of the shoot. In these situations, autofocus can be a valuable tool, allowing you to ensure that your footage is in focus without needing to constantly monitor it. The Sony FX3, for example, has high-precision touchscreen AF, which can be set with a simple touch of the screen.

When shooting fast-moving subjects

Autofocus is generally better suited for capturing objects in motion, as the focus point changes rapidly and requires constant adjustments. This is true for scenes involving sports players or birds in flight, for instance. Sony cameras, in particular, have robust Hybrid AF, combining phase detection and contrast-detection AF for excellent speed and precision.

When shooting in low light

Autofocus systems rely on light to function properly, and they may struggle in low-light conditions. However, manual focus can be more precise in low-light settings, as it allows the user to tell the camera precisely where to focus.

When shooting with a shallow depth of field

When shooting with a shallow depth of field, the focus point is very narrow, and it is easy for the camera to lose focus. Manual focus gives the user the precision needed to maintain focus where they want it.

When shooting magnified scenes

Autofocus systems often struggle with macro photography, as they find it challenging to determine the object of focus at high magnifications. On the other hand, manual focus is well-suited for this purpose and is easy to use for picking out objects of interest from a chaotic scene.

When shooting uncontrollable subjects

When filming unpredictable subjects, such as dogs or young children, autofocus can be incredibly useful. Cameras with subject tracking and facial recognition features can help keep the desired subject in focus, even when they are moving around.

When using a gimbal

When using a gimbal, it is challenging to manually adjust the focus without throwing the camera off balance. Autofocus can be beneficial in these situations, ensuring that the subject remains in focus while the camera is in motion.

Limitations of autofocus

Autofocus has its limitations, and while it has improved over the years, it is not always reliable.

Firstly, autofocus is not supported by all lenses, especially high-end cinema prime lenses. This is a significant limitation, as it means that filmmakers who wish to use autofocus may be restricted in their choice of lenses.

Secondly, autofocus is not always accurate. While it has improved with advancements in technology, it is not perfect and can sometimes take too long to focus, or fail to focus altogether. This is particularly true in low-light conditions or when filming subjects with low contrast or repeating patterns. Autofocus also struggles when there are multiple moving subjects within the frame, as it can be difficult for the camera to track the selected subject.

Thirdly, autofocus can be slower than manual focus, especially when used in conjunction with a gimbal. This is because it can be difficult to manually focus the lens while moving the gimbal, and autofocus may not be able to keep up with the movement of the camera.

Another limitation of autofocus is that it is not always possible to achieve the desired focus point. For example, when filming a subject with a shallow depth of field, it can be challenging to get the camera to focus on the desired area.

Finally, autofocus can be unpredictable. While it is designed to focus on the subject, it may sometimes latch onto other objects in the frame, resulting in an unintended focus point.

Autofocus lenses for cinema cameras

Benefits of autofocus lenses

The main advantage of autofocus lenses is that they can save time and effort by automatically adjusting focus, allowing the camera operator to focus on other aspects of the shot. This can be particularly useful in situations with a single or small number of roving subjects, documentary-style projects, walk-and-talk interviews, or sports coverage. Autofocus can also be beneficial when dealing with constant light changes or when the camera is mounted in a vehicle without remote-control options.

Drawbacks of autofocus lenses

However, there are also situations where autofocus lenses may not be the best choice. If you already have timed or slow focus adjustments or are working with a focus puller on a complex sequence, manual focus lenses may be more suitable. Additionally, if you are using specific prime lenses with manual focus and have your setup tuned for that lens line, switching to an autofocus lens for a brief shot may slow down the production.

Available autofocus lenses for cinema cameras

• Canon CN-E cine servo lenses
• Sony's E-mount cine lenses
• Rokinon AF line of cine primes for E-mount cameras
• GLOBAL DYNAMICS UNITED EF-mount electronic-only lenses
• Sigma EF-mount AF ART lenses
• Canon EF Mount EOS lenses
• Sony GMaster E-Mount AF lenses
• Samyang V-AF Autofocus Cinema Lenses

Factors to consider when choosing autofocus lenses

When choosing autofocus lenses for cinema cameras, it is important to consider the compatibility with your camera model, the type of projects you will be working on, and your personal preferences for focus control. It is also worth noting that not all lenses support autofocus, especially high-end cinema prime lenses, so you may need to use photography-style lenses that offer autofocus on cinema cameras. Additionally, the technology is still evolving, and there may be challenges in low-light conditions, changing environments, and tracking very fast subjects.

Frequently asked questions