Will a flat subject photographed straight-on have the same scale at the center and edges?
Asked 2/2/2017
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2 answers
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If a camera is pointed straight down at a perfectly flat surface, with no tilt, will the same real-world distance (for example 1 cm) be recorded as the same number of pixels in the center of the image and near the edges? I expected 1 cm near the center to cover more pixels, but when I photographed a measuring tape I couldn’t see a difference. Is the scale actually uniform, or is my phone correcting lens distortion automatically?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Yes, if you ignore barrel/pincushion distortion.
Distortion aside, a camera projection obeys the pin hole camera model. Under this model straight lines in the scene are always rendered as straight lines in the image. Now imagine photographing some squared paper head on. There's no way the squares in the centre of the image can be larger or smaller than squares at the edges whilst also obeying this straight line rule, thus all squares must be the same size in the image.
Real lenses always have a degree of barrel/pincushion distortion so in practice elements near the edges can appear larger or smaller. However you can usually correct this distortion in camera (enable lens profiles) or during RAW conversion.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
9y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—if the camera is truly parallel to the flat subject, the image scale is the same across the frame in the ideal pinhole-camera model. That means 1 cm on the surface should correspond to the same number of pixels whether it’s near the center or near the edge.
The main exception is lens distortion. Real lenses can introduce barrel or pincushion distortion, which makes areas near the edges appear slightly stretched or compressed. Phones often correct this automatically, so your test may already have been compensated in-camera. Even without correction, the difference may be small enough to be hard to notice.
So in principle: same scale across the image. In practice: small edge differences can appear because of lens distortion, not because flat subjects naturally shrink or grow toward the edges when photographed straight-on.
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