Backlight is the right first choice for holes, tips, and counting because the feature becomes a shape against a bright field. It is also the easiest way to teach perspective. A circular hole at the center of the field stays circular. The same hole moved off axis becomes a football or canoe under a conventional lens. That matters for counting openings, where an off-center presentation can change the apparent shape.
Telecentric optics are the hardware answer when the measurement cannot tolerate that cone. Collimated light and a controlled working distance keep the edge in focus through the depth of the part. If the job only needs presence, the distorted outline may not matter. If it needs a diameter or a count of mouths, it does. Center the hole, then walk it off center, so the distortion is seen before anyone argues about thresholds.