Focus geometry can imitate a defect
A camera that is not square to a flat target can place one side farther from the focus plane. Field curvature can also make the corners focus differently from the center without implying a decentered lens. Center sharpness alone does not describe the whole image, and a single soft corner in an uncontrolled scene is insufficient evidence.
Repeat the setup carefully and inspect whether the pattern survives. Keep each comparison internally consistent. Separate experiments at another aperture or focus distance into a new report; changing those variables inside one set prevents a fair comparison.
The image pipeline changes the evidence
Noise reduction can smooth fine detail. Sharpening can add halos and increase edge contrast. JPEG artifacts, local contrast edits and AI enhancement can alter the very features you are trying to compare. Identical processing is a necessary capture control even when EXIF settings match.
A mostly blank crop has too little texture for a useful edge measurement. Clipped black or white regions remove tonal information. The tool blocks interpretation for obvious low-texture or strongly clipped crops, but these thresholds are conservative product safeguards, not an imaging standard or a comprehensive quality test.
Understand what the numbers actually measure
The tool computes browser-rendered brightness statistics and a simple image gradient on matching 256 × 256 crops. These are repeatable pixel calculations for a particular set of images. They are not optical modulation transfer function, line pairs per millimeter, sensor resolution or a score that can rank different lens models.
A higher gradient can come from a genuinely crisper edge, a noisier file, stronger sharpening, different target texture or changed sampling. The workbench deliberately withholds a lens verdict. Use the numbers to accompany visual evidence and repeatability, not to replace them.
Escalate with evidence when a difference persists
If an asymmetry survives multiple careful captures, prepare the originals, matching crops, capture notes and exported measurement report. A professional assessment can examine alignment and optical behavior with equipment and procedures this browser does not provide. That evidence is more useful than a single unexplained sharpness percentage.