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I'm trying to accurately measure the size of objects via photogrammetry. The objects are 1 to 2 feet from the front of the camera lens, so where I start measuring from makes a difference.

I think the proper distance measurement is from either the "nodal point" or "entrance pupil" of the lens. Is that correct? I'm not even sure if there's a difference between the two for a non-zoom lens.

Which is the proper measuring point? And how to figure out where that is on the lens?

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    I’m voting to close this question because it is about using a camera as a measuring instrument instead of using a camera for the purpose of producing photographs for artistic, historical, or documentary purposes (as stated in the Photography SE community guidelines and meta discussions).
    – Michael C
    Mar 31, 2021 at 5:37

2 Answers 2

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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.

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The formula for determining the Focal length require to magnify an object so that it fills a sensor axis edge to edge is FL= (subject distance ÷ subject size)Sensor dimension; or FL=(D/S)Se.

For example, say you want to fill the frame width on an APS body (24mm sensor width), taking a picture of a squirrel that is about 1.5ft long including it's tail (.5M long), from about 33 feet (10M), So, keeping everything in metric; FL=(10/.5)24 = a 480mm lens is required.

You can rewrite that formula as S=(FL/Se)D; which determines how big the subject can be and fit on the sensor. And then you can determine the difference between actual object size recorded vs max possible in order to determine the actual size (e.g. 50%).

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