Understanding Autofocus and Manual Focus on Lenses and Cameras

In the world of photography, mastering the principles of autofocus (AF) and manual focus (MF) is crucial for achieving the desired outcome. This article explores the implications of turning on AF on the lens but setting your camera to manual focus (MF), providing insights and practical advice for photographers.

What Happens When AF is Turned on the Lens but Camera is Set to MF?

When you have the autofocus (AF) mechanism turned on the lens but set your camera to manual focus (MF), the following occurs:

Lens AF Mechanism

The autofocus mechanism in the lens is activated, which means the lens is capable of focusing automatically if the camera is set to AF. However, since the camera is in manual focus mode, it does not engage the autofocus system. This means that while the lens can focus automatically, the camera will not trigger this to happen. You will need to manually adjust the focus ring on the lens to achieve the desired focus.

Camera MF Mode

Because your camera is set to manual focus, it will not trigger the autofocus system. This means that while the lens has the capability to focus automatically, the camera will not activate the AF mechanism. You will have to manually adjust the focus ring on the lens to achieve the desired focus.

Potential Confusion

In this setup, you might notice that the focus confirmation (if available in the camera) might not indicate whether the lens is in focus, as the camera is not actively trying to autofocus. This can lead to some confusion when trying to determine if the lens has achieved the correct focus.

Use Cases for Mixed AF and MF Settings

This combination can be useful in several scenarios. For instance, if you want to benefit from the lens's focus ring feel, or if you are in a situation where autofocus might not work well, such as low light or complex scenes. In these cases, you can still have the option to switch to autofocus if needed.

Practical Examples

Photographers often use manual focus (MF) in astrophotography to ensure precise focus. Once they have the lens in focus, they often tape the focus ring and try not to disturb it for the duration of the shoot. In these situations, they will usually turn off auto focus on the camera to avoid accidental adjustment.

For instance, when working with Nikon AF-S lenses, no action typically occurs because the lens controls the focus independently. However, with AF D lenses, the D750 camera model will not drive the auto focus motor if set to manual focus. This behavior was observed recently during a photography session, where no sound and no autofocus were noticeable.

Switching between AF and MF

In most cases, where there is an AF/MF switch on both the lens and the camera body, the switch on the camera body overrides the switch on the lens. This means that if you have the switch on the lens set to AF but the switch on the body set to MF, the camera will be in MF mode.

The one addendum to note is that if the lens is set to MF but the body is set to AF, the camera will not be able to change the lens to AF, so it will operate in MF mode. This is why it is often advisable to leave the switch on the body set to AF and only flip the switch on the lens as needed.

Conclusion

Understanding how the autofocus and manual focus settings work in conjunction with each other is crucial for achieving precise and consistent focus. Whether you are shooting in low light, working on astrophotography, or in any scenario where manual control is preferred, knowing these settings will help you to achieve the best possible results.