Push-to-Focus Video With a Sony A7xxx

Discussion in 'Video' started by Ed_Ingold, Oct 17, 2019.

  1. As many of you have found out, auto focus is unreliable when shooting video. If the focus slips in still photography, you take the shot over again. If it slips in video, you lose the entire clip, with no guarantee it will go right on subsequent tries. Manual focus is reliable but slow. It's the best way for selective focusing, if you have time to set up the shot (or hire a focus puller). Since most AF lenses for a Sony are focus-by-wire, you can't mark focus points on a dial. Follow-focus devices have a limited range of motion, less than 360 degrees, which is often not enough.

    Sony includes a valuable feature in their professional video cameras (e.g., FS5) - push-to-focus. It works with any Sony lens, with or without an AF/MF switch on the lens. The lens is set to AF and the camera to MF. There is a button to engage AF as long as it is pushed, then revert to MF and stay in focus when released.

    When in video mode, Sony A7/A9 cameras are either AF-C or MF. DMF has no effect, nor does the AF button on the camera, and AF is active even when not shooting. The hack is both simple and effective.
    • If the lens has a switch, set it to AF
    • In the camera menu, set focus to MF
    • Assign the command "AF/MF Control Hold" to a programmable button (e.g., AEL)
    • Now the button acts as a Push-to-Focus control. The lens focuses automatically as long as the button is held, then returns to manual focus when released, with the focus fixed.
    The downside is that if the lens has an AF/MF switch, the focus ring is inactive, so you can't touch up manually. If the lens has no switch (e.g., Batis), the focus ring is active, and any focusing aids (e.g., magnification) you select in the menu. Older Sony cameras my disallow the menu MF option if the lens has a physical switch.
     
  2. Face recognition can be a boon or a bust, depending on the circumstances. It will lock on to someone facing the camera over another turned sideways. This happens whether you are in full AF mode, or using the push-to-focus I describe above. One solution is to use a center focusing spot, which can be centered on the subject of interest, then re-composed for the shot. This adds a little time for acquisition, which can be covered with B-roll footage. When face recognition works, it's best to use a wide-area focusing pattern.

    Manual focusing is probably the best, but takes 2-3x as long for acquisition, and hard to do when the camera is on a tripod. Focus is critical with large sensors, and lenses are generally used wide open. Peaking helps, but is not accurate enough in many cases. Digital magnification is the best solution, at about 6x. That's usually available in the EVF, but my external monitor only magnifies by 2x. Using AF in a push-to-focus manner is fast and accurate - ideal for live shows where you can't back up and retake the shot.
     

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