It is a focus movement method of interchangeable lenses for mirrorless cameras (DN lens) in relation to the rotational angle of the focus ring during manual focusing.
With “non-linear focus,” the amount of focal point movement varies depending on the focus ring rotational speed.
With linear focus, if the focus ring rotational angle is the same, the amount of focal point movement remains the same regardless of the focus ring rotational speed.
With non-linear focus, turning the focus ring quickly will change the focusing amount by more than if it is moved slowly, even if the ring is turned by the same amount.
By changing the turning speed, it is possible to achieve both large focus movement and fine focus adjustment, but since the amount of movement of the focus position changes depending on both the turning angle and speed, it can be difficult to predict how much the focus moves when operating the focus ring.
With linear focus, the amount of focus movement is constant regardless of the speed at which the focus ring is turned. If you have a camera that supports linear focus and you set it so that a small movement of the focus ring results in a large movement of the focusing element(s), it will be difficult to ensure focusing accuracy. Conversely, if the focus ring is set so that a large movement is needed to move the focusing element(s) by only a small amount, greater focusing accuracy can be achieved, but operation will be slower as the ring has to be turned further.
With compatible lenses, you can switch between linear and non-linear focusing.
For compatible lenses, please check the correspondence table. Compatibility of Linear focus