Camera Lens Mount Shims (Shims / Mount Flange Fine Adjustment Washers) for Calibrating Lens Infinity Focus and Back Focus

1. The physical definition of back focus and the prevention of parfocal lenses

Back focal length refers to the precise physical distance between the metal reference flange surface of the rear bayonet of the lens and the focal plane sensor:

• Back focus misalignment performance of fixed focus lenses:
• The back focus is too long (the flange is too thick): When the focus ring is turned to the physical infinity ((\infty)) dead stop, the focus still falls 50 meters away, and the distant mountains will always be virtual!
• The back focus is too short (the flange is too thin): It can focus to infinity, or even achieve focus at infinity early, but the closest focusing distance is severely stretched and deprived, and the physical feet/meter distance scale originally precisely engraved on the lens barrel of the lens is completely out of touch with the real distance!
• The disaster of cinema zoom lenses: Loss of Parfocal Ability:
• A true movie zoom head must be "parfocal": after locking the actor's eyeball focus at 200mm, no matter whether the zoom is pushed or pulled to 70mm, 35mm or 24mm, the focus must be locked on the eye like a nail and never shift;
• Cruel Physical Reality: If there is a mechanical manufacturing tolerance deviation of only 0.02 mm (20 microns, thinner than a hair!) in the camera mount or lens back seat, when the zoom head is pushed to the wide-angle end, the focus will drift by tens of centimeters catastrophically, and the entire zoom lens will be completely reduced to scrap metal!

2. Adjustment artifact: High-precision bayonet fine-tuning shims (Shims Kit)

In order to tame this 20 micron micro-tolerance, the mount back seats of all professional cine lenses and high-end adapter rings (such as PL to E / PL to L mount) have a spacer fine-tuning chamber under the fixing screw:

• Gasket material and thickness specifications:
• Made of special beryllium copper (Beryllium Copper) or tear-proof polyester film (Color-Coded Polyester) that has been micro-cut by high-precision laser;
• Each set contains color-coded ultra-thin precision washers in thicknesses from 0.005mm (5 microns), 0.010mm, 0.020mm, 0.050mm to 0.100mm;
• Absolute optical laws for adding and subtracting spacers:
• The focus at the wide-angle end falls behind the subject (Back-focused): It means that the rear group of the lens is too close to the sensor $\rightarrow$ You must add spacers (Add Shims) to move the lens outward to a micron distance;
• The focus at the wide-angle end falls in front of the subject (front-focused): It means that the rear group is too far $\rightarrow$ The removal shims (Remove Shims) must be removed.

3. Siemens Star calibration standard SOP

1. Target paper layout and environment locking:

• Stick a high-contrast printed Siemens Star Chart vertically and flatly on a flat wall;
• The camera is set up on a heavy-duty tripod, the optical axis is absolutely 90° vertically facing the center of the star pattern, and the distance is strictly measured as the standard scale object distance (for example, 3.00 meters);

2. Telephoto Baseline:

• Push the zoom lens to the longest focal length (such as 70mm or 135mm);
• Fully open the aperture, observe the starburst chart with 100% magnification on the monitor, and rotate the lens focus ring until the smallest black-and-white ray convergence point in the center of the starburst chart is extremely sharp;
• At this time, the physical scale mark on the lens barrel should be accurately aligned with the 3.0 meter mark;

3. Push to the wide-angle end to check the drift (Wide-End Verification):

• Never touch the focus ring!
• Only turn the zoom ring and push the lens to the widest angle end (e.g. 24mm);
• Observe the starburst center of the monitor:
• If the center is still sharp $\rightarrow$ and the back focus is absolutely perfect, congratulations, you're done!
• If the center of the starburst turns into a blurry halo $\rightarrow$, move the focus ring slightly to record the focus direction;

4. Gasket replacement fine-tuning calculation:

• Remove the 6 torx screws behind the bayonet and remove the original bayonet ring;
• Add or remove a 0.01mm spacer according to the amount of drift, and re-tighten the screws evenly with a torque wrench using the cross diagonal method;
• Recheck again until 100% zero-drift parfocality is achieved at both wide-angle and telephoto ends before allowing the crew to start shooting.

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