The following script can be used with the post processing stack. Editing the script at runtime is kind of a pain. Attach the script to an object that has the PostProcessing layer, such as the same object you are using for a post processing volume.

Calculate the distance as needed – I’m just getting the distance between the camera transform and the player transform, but if you have a larger player, you may want to use a raycast and get the distance to the closest point to the camera.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Rendering.PostProcessing;
public class FollowFocus : MonoBehaviour
    Transform player;
    public PostProcessVolume m_Volume;
    public DepthOfField m_DOF;
    public float distance = 10f;
    void Start()
        m_DOF = ScriptableObject.CreateInstance<DepthOfField>();
        m_DOF.enabled.value = true;
        m_Volume = PostProcessManager.instance.QuickVolume(gameObject.layer, 100f, m_DOF);
        player = GameObject.FindGameObjectWithTag("Player").transform;    

    void Update()
        distance = (Camera.main.transform.position-player.position).magnitude;
        m_DOF.focusDistance.value = distance; 

    void OnDestroy()
        RuntimeUtilities.DestroyVolume(m_Volume, true, true);