Lens optical axis decentration detection and star test: Using an artificial star device to identify lens assembly tolerances and edge corner defocus

1. The lens disease that photographers are most afraid of: Decentering

When buying an expensive, top-of-the-line Red Circle or GM lens, or purchasing a second-hand lens, many photographers discover that:

1. The left side is as sharp as a knife, and the right side is mushy:

• When photographing a flat brick wall or a scenic skyline, the leaves on the left side of the picture are fully visible, while the leaves at the same distance on the right side are blurred and blurry no matter how you focus;

2. It’s not that the focus is inaccurate, but that the optical lens is physically tilted and shifted:

• The interior of the lens is assembled by gluing 10 to 20 precision lenses;
• The assembly tolerance requirements of modern lenses are at the micron level;
• Once the lens has suffered an accidental drop impact, or the injection molding tolerance of the plastic inner barrel during original factory assembly exceeds the standard, resulting in a slight tilt (Tilt) or lateral displacement (Shift) of less than 0.1 mm between the optical central axis of one or two key lenses and the mechanical optical axis;
• The image field plane of the entire lens is completely distorted into an irregular tilted surface, causing one side to be in focus and the other side to be severely out of focus. This devastating hidden disease cannot be detected by ordinary inspection cameras!

2. The ultimate telescope inspection skill in the world of astronomical optics: The Star Test

In optical physics, the most sensitive and stringent method to test the coaxiality of a lens is the "Star Testing" derived from astronomy:

1. Essence of point light source: Airy disk diffraction pattern:

• When an ideal aberration-free light source is converged by a perfect lens, strictly concentric Airy Disk & Diffraction Rings will appear before and after the focal plane;
• As long as there is micron-level off-axis, coma or astigmatism in the lens group, the symmetry of the diffraction ring will be completely broken in an instant!

3. Homemade low-cost artificial star generator (Artificial Star Generator)

Photographers do not need to spend thousands of dollars on professional optical instruments and can build a 5-micron artificial star point at home:

1. Tool Materials:

• A piece of thick aluminum foil (kitchen baking foil);
• A very fine insect needle (No. 00 insect needle) or a medical ultra-fine insulin needle;
• A small high-brightness single high-power LED flashlight.

2. Micropore production and darkroom setup:

• Place aluminum foil flat on hard flat glass;
• Use a very fine needle tip to very slightly prick a microscopic hole (about 5 microns to 10 microns in diameter) that is almost invisible to the naked eye in the center of the aluminum foil;
• Wrap the perforated aluminum foil tightly in front of the light outlet of the strong flashlight, and wrap it tightly with black tape to prevent light leakage;
• Fix this miniature "artificial star" at the end of a long dark corridor or dark room 10 to 15 meters away from the camera.

4. Practical detection steps and off-axis graphic interpretation

1. Camera Installation and Full Manual Setup:

• The camera is set up on a heavy-duty tripod, and the optical axis is strictly aligned with the artificial star point;
• Lens aperture Wide Open, ISO set to 100, use the body screen to magnify 10 times for manual focus;

2. Observe the diffraction ring with slight defocusing:

• First, focus accurately on the star point;
• Then rotate the focus ring very slightly counterclockwise (Extra-focal) to blur the star point into a tiny circular spot;
• Then turn the focus ring a little bit clockwise (intra-focal);
• Magnify the screen 20 times to observe the microscopic concentric ring structure inside the blurred spot!

3. Diagnosis comparison table for life and death decisions:

• Perfect Centering:
• The diffraction rings of the inner focus and the outer focus present absolutely perfect concentric circles, with the bright spot in the center of the circle strictly centered, and the thickness of the light and dark rings completely uniform and symmetrical;
• Severely Decentered Lens:
• The concentric rings inside the light spot appear as severely eccentric ellipses, even dragging a long coma tail like a comet;
• The bright spots are obviously squeezed in a specific direction;
• Even if you move towards the center of the screen, the diffraction ring is still asymmetrical;
• Conclusion: If an obvious asymmetrical comet spot is found, it indicates that the internal lens group of the lens has suffered serious internal damage and is off-axis. You must immediately apply for a return or exchange or send it to the factory for re-alignment of the optical axis!