Why does a sphere measure larger in pixels than a ruler in the same photo?
Asked 8/29/2012
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2 answers
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I’m photographing a ball with a ruler in the same shot and trying to estimate the ball’s diameter from the image. I calculate a pixels-per-mm scale from the ruler, then measure the ball’s diameter in pixels and convert it back to millimeters. But the ball appears larger than its real physical diameter.
The ruler is aligned with the ball’s center, and I expected them to be in the same plane. I’m using a 5.5mm lens with flash, and the effect is less noticeable with a 4.5mm lens.
Why would a sphere appear oversized compared to a ruler, even when distortion seems low?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
8
For an image to be a valid measurement tool two criteria must be met:
- The objects must be exactly in the same plane.
- The plane must be parallel to the plane of the image sensor.
Criteria one is easy to verify, are both objects sitting on a flat surface? Do any of the objects have a significant depth when compared to the distance between the camera and the object plane? Eg: A 1 inch ball from 10 inches would likely have noticeable effects, a 1 inch ball imaged from several hundred inches would have no noticeable effect. This is using foreshortening to your advantage.
Criteria two is slightly more complex. Placing two rulers perpendicular to each other on the image plane will allow verification of this criteria. Count the pixels per unit of ruler on both ends of the ruler. If the number is equal then that ruler is parallel to the image plane. Compare the measurement between rulers to verify if your pixels are square.
I have created the following image to explain what happens when the object significantly protrudes from the measurement plane. In my example the ball fills the height of the image. The height of the image on the ruler is 1.35 units. The viewer then assumes that the ball is 1.35 units wide as well. This is not true. Any object can be made to fill the frame if placed close enough to the lens.
Originally by user3335. Source · Licensed CC BY-SA 4.0
user3335
13y ago
0
Generated from our catalog & community — verify before relying on it.
A photo only works as a measuring tool if the subject plane is truly flat and parallel to the sensor. A sphere breaks that assumption.
The ruler is a flat reference, but the visible edge of a ball is not in the same plane as the ruler marks. The ball’s apparent outline is formed by the sphere’s curved surface and viewing angle, so its image doesn’t map to a flat scale the same way a ruler does. When the camera is relatively close, perspective makes this worse.
So even if the ruler passes through the ball’s center, the ball’s “diameter” in the image is not directly comparable to the ruler’s pixel scale unless the setup is carefully controlled.
Also check camera alignment: if the ruler/object plane is not parallel to the sensor, pixels-per-mm will vary across the frame.
Why lens choice matters: the effect changes with focal length mainly because a shorter focal length often means shooting from closer distance for the same framing, increasing perspective error. It’s not necessarily lens distortion.
For better measurement, use a longer camera distance, keep the ruler and subject plane parallel to the sensor, and avoid using a spherical object as if it were a flat target.
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