Back Focus Lens Repair & Calibration USA
What Is Back Focus — and Why Does It Drift?
Back focus — also called flange focal distance — is the critical measurement between the rear flange of the lens mount and the camera’s image sensor or film plane. For a broadcast lens to produce sharp images, this distance must be set precisely to the manufacturer’s specification.
When back focus drifts, the lens appears to focus incorrectly — images look soft, particularly at longer focal lengths, and zoom lenses fail to hold focus when zooming in to confirm the shot. Many operators mistake a back focus problem for a lens fault or camera fault when the actual issue is a small mechanical shift in the flange focal distance.
Back focus drift is the single most common broadcast lens issue we repair — caused by impact, mount wear, extensive field use, temperature cycling, or a lens mount replacement.
Signs Your Lens Needs Back Focus Repair
Sharp at Wide, Soft at Tele
The clearest sign of back focus drift — the lens appears sharp when zoomed wide but soft at telephoto, even when focus is set correctly.
Focus Shifts During Zoom
Focus that appears correct when zoomed in to set, but drifts when zooming back out — classic zoom tracking and back focus drift.
Correct Ring, Soft Image
The focus ring reads the correct distance, but the image is soft on the monitor. The actual focal plane is in the wrong position.
After a Drop or Impact
Any impact to a lens or camera can shift the flange focal distance — always recalibrate back focus after a drop or significant impact.
After Mount Replacement
Changing or replacing a lens mount changes the mechanical flange distance. Back focus recalibration is essential after any mount work.
Gradual Soft Focus Onset
Back focus can drift gradually over years of use — if a lens that was once sharp has slowly become inconsistent, back focus is the likely culprit.
How We Calibrate Back Focus
Back focus calibration is a precision process — not a quick external adjustment. Here’s what’s involved:
Measurement
We measure the current flange focal distance against the manufacturer’s specification using precision gauges and collimation equipment.
Mechanical Adjustment
The mount position is adjusted — using shims, adjustment screws, or mount repositioning — to set the correct flange focal distance.
Optical Verification
We verify the correction optically using calibrated test charts and collimation — not just mechanically — to ensure true image sharpness at the sensor plane.
Zoom Tracking Check
For zoom lenses, we verify focus holds through the full zoom range after back focus adjustment — a critical step often omitted by less experienced technicians.
Lock & Document
The adjustment is locked and sealed. A written calibration report is produced documenting the pre- and post-calibration state and test results.
Back Focus Repair — Frequently Asked Questions
Everything you need to know about our back focus repair service.
Back focus (flange focal distance) is the precise distance between the lens mount flange and the camera's image sensor. When correctly set, the lens achieves accurate, sharp focus at the correct plane — critical for zoom lenses to hold focus through the zoom range.
Common causes include impact or drop damage, lens mount wear, mount replacement, extended field use over time, and temperature cycling in outdoor environments. Even small shifts produce visible softness.
Classic signs: sharp at wide angle but soft at telephoto; focus that shifts when zooming; images that appear correctly focused on the ring but soft on the monitor. A back focus test chart will confirm it definitively.
Back focus calibration alone typically takes 1–3 business days. If additional repairs are required alongside, we'll provide a full quote at the estimate stage before starting work.
Yes. Every back focus calibration comes with a written report documenting the pre- and post-calibration measurements and optical test results.
Most broadcast cameras have a basic back focus ring on the lens mount for minor field adjustments. However, if the lens requires internal flange adjustment, shim changes, or re-locking — that requires workshop tooling and should be performed by a qualified lens technician.
GET IN TOUCH
2260 Hendersonville Hwy, Pisgah Forest, NC. 28768
828-884-5822
Info@alpine-optics.com
Mon - Fri : 10.00am - 05.00pm