How to Adjust Sirui IronStar Lens Back Focus: Step-by-Step Guide

What Is Lens Back Focus Adjustment?

• The image appears sharp but the focus scale does not match the actual focusing distance.
• The subject cannot be brought into sharp focus even when the focus ring is set correctly.
• Focus accuracy changes after replacing the lens mount.

What You Need Before Adjusting Lens Back Focus

• Mount the camera and lens securely on a tripod.
• Prepare a clear, high-contrast subject or focus chart.
• Ensure the camera sensor plane is exactly 1 meter away from the subject.
• Perform the test in a stable environment with sufficient lighting.

Step 1: Set the Subject 1 Meter from the Camera Sensor

1. Place the subject exactly 1 meter away from the camera sensor plane.
2. Rotate the lens focus ring to the 1-meter focus position.
3. Check whether the subject appears sharp.

• If the subject is clear: The back focus is properly adjusted, and no further adjustment is required.
• If the subject is blurry: Back focus adjustment or shim adjustment is required.

Step 2: Adjust the Lens Back Focus Ring

1. Remove the back focus cover and loosen the adjustment screw carefully.
2. While observing the subject, slowly rotate the back focus ring.
3. Continue adjusting until the subject appears sharp.

1. Tighten the adjustment screw.
2. Recheck the image sharpness.

Step 3: Check Whether the Focus Scale Matches the Actual Distance

1. Keep the subject at exactly 1 meter from the camera sensor plane.
2. Rotate the focus ring until the subject is perfectly focused.
3. Check the focus distance marking on the lens.

Case 1: Focus Scale Reads Less Than 1 Meter

• Actual subject distance: 1 meter
• Lens focus scale reads: 0.9 meter

Case 2: Focus Scale Reads More Than 1 Meter

• Actual subject distance: 1 meter
• Lens focus scale reads: more than 1 meter

Step 4: Fine-Tune Back Focus with Lens Shims

To Add Shims:

1. Remove the lens mount.
2. Insert shims with appropriate thickness.
3. Start with thicker shims first.
4. Reinstall the lens mount securely.
5. Attach the lens to the camera again.
6. Repeat the 1-meter focus test.

To Remove Shims:

1. Remove the lens mount.
2. Take out existing shims.
3. Reinstall the mount.
4. Repeat the focus test.

Step 5: Verify the Lens Back Focus Adjustment

1. Place the subject at exactly 1 meter.
2. Set the focus ring to the 1-meter position.
3. Confirm that the subject is sharp.
4. Verify that the focus scale matches the actual distance.

Important Tips for Lens Back Focus Calibration

• Always measure the 1-meter distance from the camera sensor plane, not from the front of the lens.
• Make adjustments gradually and test after each change.
• When using shims, change only one variable at a time to avoid over-adjustment.
• After changing lens mounts, back focus calibration may need to be repeated.
• Avoid loosening or tightening screws excessively to prevent damage.

We recently expanded the IronStar Series with three new focal lengths: 75mm, 100mm, and 135mm. Together with the existing 35mm, 45mm, and 60mm options, these additions complete the focal length lineup and create a more comprehensive lens system.

If you have recently added the IronStar Series to your kit, learning how to perform back focus adjustment will greatly help you get the best performance from your lenses and ensure accurate focusing in different shooting situations.

Frequently Asked Questions

1. Why Does My Lens Need Back Focus Adjustment?

Back focus adjustment may be required when the distance between the lens optical system and the camera sensor is not perfectly matched. This can happen after changing the lens mount, using a different camera system, or when the focus scale no longer matches the actual focusing distance. A properly adjusted back focus ensures that the lens reaches accurate focus and that the focus markings correspond correctly to real-world distances.

2. Can I Adjust Back Focus Without Using Shims?

Yes. Many lenses with an adjustable back focus mechanism can be fine-tuned by rotating the back focus ring first. If the adjustment range of the back focus ring is not enough to achieve accurate focus, shim adjustment may be required. Shims provide a more precise way to adjust the flange focal distance by adding or removing small spacers between the lens mount and lens body.

3. Does Changing a Lens Mount Affect Back Focus Accuracy?

Yes. Changing a lens mount can affect the flange focal distance between the lens and the camera sensor. Even a small difference in mounting distance may cause the focus scale to become inaccurate or prevent the lens from achieving precise focus. For interchangeable-mount cine lenses, checking and adjusting back focus after changing mounts helps maintain consistent focus performance across different camera systems.