Preshow field guide
Back Focus · Fujinon B4 zoom lenses
Never adjust back focus during a live shot. Take the camera off program and clear it with the director, TD, or shader first.
Start here
One field guide for both flange back focus systems used on the house Fujinon zoom lenses. Each lens uses one method only: electronic calibration or a mechanical flange focal length ring.
When to run this procedure
- The lens is sharp at telephoto but soft after zooming wide.
- A lens has been mounted on a different camera body.
- The lens or B4 mount has been removed and reinstalled.
- The camera cannot hold focus through the zoom range during setup.
Quick diagnosis. Focus at full telephoto, then zoom fully wide without touching the focus control. If the image becomes soft, flange back focus needs adjustment.
Do not adjust back focus during a live shot. Coordinate with the director, TD, or shader, take the camera off program, and confirm the lens method before touching any calibration control.
Applies to cameras 1 to 3. The pit pair are documented as 16× and the FOH lens as 22×. Barrel model numbers and the method assigned to each position still need field verification, so identify the installed controls before every adjustment.
Prepare the camera
Do all six before you touch a calibration control. Skipping the iris or macro check is the most common reason a good procedure still lands soft.
Setup
- Put a high-detail focus chart at a suitable working distance and light it evenly.
- Set the iris fully open. Use ND or reduce light level to control exposure.
- Disable or disconnect external focus control if it can move the lens during calibration.
- Confirm the lens is not in macro mode.
- Put the camera on a stable shot and make sure you can clearly judge focus with peaking or focus assist.
- Identify which adjustment system is fitted before continuing.
Identify the system
| If the lens has… | Use… |
|---|---|
| A servo-mode switch and electronic calibration button near the servo housing | Method A Electronic calibration |
| A locking knob on the flange focal length or back-focus ring | Method B Mechanical adjustment |
Do not use both methods on one lens. If the controls do not clearly match either description, stop and have the lens identified before adjusting it.
Identify the method
Pick the system fitted to the lens in front of you. The runner then shows only that procedure, one step at a time.
One lens, one method. If the controls do not clearly match either card, stop and have the lens identified before adjusting it.
Run the procedure
Choose a method first.
Method A: electronic calibration
- Set the iris to manual and fully open it.Use ND or reduce the light level rather than closing down.
- Confirm macro mode is disengaged.The green reference line should be visible beneath the macro ring lip.
- Disable or unplug any external focus controller.A demand that creeps mid-routine will bake the error into the stored calibration.
- Set the lens zoom control to servo mode.
- Press and hold the calibration button beside the servo-mode switch for about
5 seconds. - Wait for the lens to zoom in and for the red indicator to blink.
- Use the normal focus control to make the chart as sharp as possible.
- Press the calibration button once. The lens will zoom wide.
- Focus the chart again as directed by the lens calibration sequence.
- Press the calibration button a final time to store the calibration.
- Run the verification check before the camera goes back on program.Section 5 of this guide.
Method B: mechanical flange-back adjustment
- From the FOH camera position, choose one fixed, high-contrast stage reference near center stage.Good targets: a sharp LED-wall seam on a static image, scenic lettering, the edge of a set piece, or a well-lit truss clamp.
- Do not use a performer, moving light beam, animated video, haze edge, curtain movement, or any reference that can change while you adjust.
- Open the iris fully.Use ND or coordinate a temporary light-level change to keep the image properly exposed.
- Loosen the locking knob on the flange focal length ring. Do not remove it.
- Zoom fully telephoto onto the selected stage reference and focus it with the normal focus control.
- Zoom fully wide without touching the normal focus control.Keep evaluating the same stage plane and reference area.
- Turn the flange focal length or back-focus ring until that stage reference is as sharp as possible.
- Zoom fully telephoto again and refocus with the normal focus control.
- Return fully wide and adjust only the flange focal length ring.
- Repeat the telephoto and wide cycle until the same stage plane stays sharp at both ends of the zoom range.
- Tighten the locking knob without shifting the ring, then run the verification check.Section 5 of this guide.
The lens drives the zoom positions during this routine. Do not fight the servo or move the camera while calibration is running.
At telephoto, use the normal focus control. At wide, use the flange focal length ring. Always judge the same fixed stage plane from the FOH position so distance changes do not mislead the adjustment.
Verify and hand back
The adjustment is not finished until focus holds across the whole range twice and the shot is restored to show settings.
Verification pass
- Zoom fully telephoto and focus the selected target: the chart for Method A, or the fixed stage reference for Method B.
- Slowly zoom through the entire range without touching focus.
- Confirm the same target stays sharp at telephoto, midrange, and full wide.
- Repeat the test at least twice.
- Restore the agreed iris, ND, zoom-control mode, focus-control connection, and viewfinder settings.
- Compare the result with the other cameras and tell the director, TD, or shader the camera is ready.
Hand-back checklist 0 / 6
If it still will not hold focus
Work the symptom, not the ring. Repeating a procedure that already failed rarely fixes a seating or mount problem.
| Symptom | Next check |
|---|---|
| Sharp at telephoto, soft at wide | Repeat the correct flange-back procedure and confirm the iris is fully open |
| Soft at every focal length | Check normal focus, chart distance, focus-assist setup, lens seating, and the B4 mount |
| Calibration routine will not start | Confirm servo mode, external focus-control disconnection, and the correct electronic button |
| Mechanical ring reaches its limit | Stop. Check lens seating, mount installation, and whether the correct control is being adjusted |
| Focus shifts after tightening | Hold the flange ring steady while securing the locking knob, then verify again |
Stop and escalate if the lens is loose, the B4 mount is not fully seated, a control binds, the ring reaches its limit, or the exact lens method cannot be identified. Do not force the mechanism.
Watch it done
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