Zoom Lens Parfocality and Repeatability

A zoom lens keeps magnification adjustable, and in exchange it introduces three tolerances that a fixed-focal lens does not have: the image plane moves as the groups travel (parfocality), the image can shift laterally, and the magnification at a given setting depends on how precisely the mechanism returns to that setting. None of these is a defect — they are the physical cost of a moving optical system, and each one can be measured, budgeted and, where necessary, compensated per zoom position.

Why the image softens or shifts after zooming

Parfocality is a tolerance, not a promise. Clearances, cam or screw geometry and the repeatability of the drive let the image plane wander by a fraction of the depth of field, which softens the image first at the edges of the field and at higher spatial frequencies. Lateral shift comes from the same causes: the moving groups are never perfectly centred and their travel is never perfectly straight. Judged by eye, both effects look like "the lens is bad". Measured on a target, they become two numbers — a focus metric spread and a position spread in pixels — that can be compared with the tolerance of the measurement and with the size of the region of interest.

How to quantify it with the machine's own camera

1. Fix a resolution or scale target at the working distance so that it fills most of the field.
2. For each zoom position used in production, drive the lens, wait the settling time you will use, capture, and record a focus metric from a fixed region plus the centroid of one distinctive feature.
3. Repeat each position at least ten times in both directions of travel, and repeat the whole set at the start and end of a shift.
4. Report three numbers per position: focus-metric spread, position spread in pixels, and hysteresis between the two travel directions.

Rising values across the shift indicate thermal or mechanical settling; a wide vertical spread indicates poor repeatability; a systematic gap between up and down travel indicates backlash. The test takes minutes and replaces speculation.

Recipes, calibration and when to stop zooming

Because magnification changes with the zoom setting, the millimetres-per-pixel scale factor belongs to the position, not to the lens: calibrate at the positions actually used and store the scale with the recipe together with zoom, aperture, focus and illumination. Verify with a reference feature at the start of each production run and after any long stop. Where the tolerance is tight and the height varies, treat the zoom range as qualified only at the specific positions you validated rather than assuming it is uniform — and where a line runs one product for months, a fixed-focal lens will generally show better MTF, better stability and a smaller error budget.

Related reading: motorized zoom lens selection, zoom lens FOV calculation, parts with different heights, and the machine vision lens range. The individual questions on parfocality, hysteresis and calibration are answered below — tell us the zoom positions and the tolerance and we will suggest what to qualify.