Do pixels correspond to a fixed real-world distance across an entire photo?
Asked 8/5/2020
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If I photograph something from 3 feet away, can I assume the same number of pixels represents the same real-world length everywhere in the frame? For example, if a 6-foot pole has marks every inch, would each 1-inch segment take up the same number of pixels from top to bottom of the image?
I tested this by photographing graph paper with a cellphone. Near the bottom of the image, 30 pixels measured about 1/10 inch, but near the top it was closer to 37 pixels for the same spacing. Was my experiment flawed, or is this expected? Would a camera like a Canon G11 show similar behavior?
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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In a camera with the usual lenses, the size in pixels in the image corresponds to an angle. Things that are far away elicit a smaller angle of view and so correspond to less pixels in the image. Take a picture of a building from the street, the windows in the upper floors will appear smaller than the ones at street level. This is also happens what happens with your pole, assuming the pole is parallel to the sensor (or perpendicular to the lens) the part of the pole at the center of the photo is closer to your camera than the extremities, so there are more pixels between marks at the center.
However, if you step back with the lens and zoom in (or take a lens with a longer focal lenth) to compensate, then the distance difference between the camera on one side and the center and the ends of the pole on the other will be less and you will see closer sizes in pixels (but still no totally equal)
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
5y ago
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Not necessarily. Pixels in an image mainly correspond to angle of view, not a fixed real-world size everywhere in the frame.
If all measured points lie on a flat plane parallel to the sensor, and the lens is an ideal rectilinear lens, equal-sized objects should project to equal-sized image sizes. But in real photos, two things can change that:
- Perspective / distance differences: Parts of the subject that are farther from the camera appear smaller. If the pole or graph paper was not perfectly parallel to the sensor, spacing will vary across the image.
- Lens distortion: Real lenses are not perfectly rectilinear, especially wider lenses and many phone cameras. This can make spacing near the edges differ from spacing near the center.
So your result is plausible, not necessarily a bad experiment.
To get more uniform pixel-to-length measurements, move farther away, use a longer focal length (zoom in optically), keep the subject plane parallel to the sensor, and avoid the frame edges. A Canon G11 can show similar effects, though using its longer zoom setting and better alignment should improve consistency.
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