For close-range photogrammetry, where should camera-to-subject distance be measured from?
Asked 3/30/2021
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I'm using photogrammetry to measure objects accurately, with subjects about 1–2 feet from the front of the lens. At that distance, the reference point for measuring camera-to-subject distance matters.
Should distance be measured from the lens's nodal point or from the entrance pupil? If those are different, which one is correct for this purpose, and how can I determine its location on a lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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The object distance is measured from the front nodal. The image distance is measured from the rear nodal. Depending on the design of the lens, the locations of these points can be flip-flopped. You can google “nodal slide” and make one however ---- position the lens so that it projects an image of a ruler on a screen (white paper). Adjusts lens / screen / object position and obtain, as accurate as possible, a lifesize image (1:1 “unity”). Used another ruler to measure the image of the ruler on the screen and adjusts until unity is achieved. At unity, object distance will be two focal lengths forward of the front nodal. Additionally, the image distance will be two focal lengths downstream from the rear nodal. The object to screen distance will be four focal lengths plus the spacing of the nodal points.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
5y ago
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For this kind of optical distance measurement, the relevant reference is the lens’s nodal points: object distance is measured from the front nodal point, and image distance from the rear nodal point. On some lens designs these points are not physically centered in the lens and can even be positioned in unintuitive places.
A practical way to locate them is to set up the lens to project an image of a ruler onto a screen and adjust until you get 1:1 magnification (life size). At 1:1, the object is two focal lengths in front of the front nodal point, and the image plane is two focal lengths behind the rear nodal point. From that setup, you can infer the nodal point locations.
If your goal is instead framing or estimating subject size on the sensor, you can also use the magnification relationship:
FL = (subject distance / subject size) × sensor dimension
and rearrangements of it to estimate field coverage. But for the physical object-distance reference itself, use the front nodal point.
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