Two in-focus images can still use different object scales. A dimensional inspection needs a magnification question as well as a sharpness question.
Separate two optical behaviours
Conventional perspective imaging changes apparent size with object distance. Object-space telecentric geometry limits that magnification change. It does not by itself make every depth sharp or eliminate every source of measurement error. Primary reference: Edmund Optics: advantages of telecentricity.
A buyer comparing gauging offers should therefore distinguish the required object-height envelope from the scale behaviour across it. Retain the optical design and its stated limits instead of assuming that a sharp-looking edge has a constant millimetres-per-pixel conversion.
Review a fictional pair of images
Imagine an unchanged reference object produces a 200-pixel span at one height and a 196-pixel span at another, with both images appearing clear. Converting both with a single fixed scale would describe a size change even though the object's identity has not changed. The invented four-pixel difference raises a depth/scale question; it is not evidence from an installed HM system.
Do not resolve the discrepancy by refocusing and declaring the dimension settled. Mark the two object planes and ask which optical and calibration basis applies to each. A telecentric proposal should still provide its usable depth and residual performance specification for the actual task.
Make the scale claim testable
Request a planned assessment using an appropriate known reference at the relevant object positions, with the optical settings retained. Record how apparent size and usable image detail are evaluated separately. The method and reference suitability belong to the technical reviewer.
Keep the resulting evidence with the installed lens, camera and mounting geometry. A later height or support change can reopen the scale question even when the operator can recover focus. This is a configuration review, not an instruction to buy a particular lens family for every inspection.
Customer Questions
Does an in-focus edge prove constant scale?
No. Object distance can change apparent size in perspective imaging.
Does telecentricity guarantee sharpness at every height?
No. Retain the usable depth and task-specific image criteria.
Is this guide a lens-selection guarantee?
No. The actual optical arrangement requires its own evidence.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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