Why does a known-size object measure different pixel widths at different positions, and how can I estimate image magnification more accurately?
Asked 10/20/2016
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2 answers
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I’m trying to estimate a camera’s image magnification for research. My basic method is to photograph an object of known size, measure how many pixels it spans in the image, and use that to estimate scale (for example, a 5 cm object appearing 10 pixels wide gives 0.5 cm per pixel).
However, when I place the object at different positions in the scene, it sometimes measures 9 pixels, sometimes 10, sometimes 11. That changes the estimated scale.
Is this variation mainly caused by perspective, or is it more likely due to measurement/pixel-sampling error? Ideally I would expect a constant scale if the setup is fixed.
Is there a better way to estimate magnification or image scale than counting pixels from a photographed object?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
1
You're dealing with aliasing. Your method is correct (yes, you can judge magnification by the number of pixels the subject takes on the sensor if of course the manufacturer specs are all correct), but in the specific case the measures will be off because you're lacking precision. Imagine that the subject is projected onto 10.5 pixels, but it's seen as taking up 10 pixels - here's an error of 5 %. Whether or not this is critical is up to you.
Originally by user31714. Source · Licensed CC BY-SA 4.0
user31714
9y ago
0
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Your pixel-counting method is valid in principle, but the 9–11 pixel variation is likely dominated by sampling/aliasing error rather than true magnification changes. If the object projects to about 10.5 pixels, one measurement may round to 10 and another to 11, which is already about a 5–10% difference.
Perspective can also change apparent size if the object’s distance or orientation relative to the camera changes. So the scale is only constant when the object stays on the same plane, at the same distance, and with the same orientation.
A more accurate approach is to use geometry:
- measure the real object size
- measure the object distance from the lens
- use similar triangles with the lens focal length
The ratio of object size to object distance equals the ratio of image size on the sensor to focal length. From that you can estimate the image size formed on the sensor, and then convert to pixels if you know the sensor/image dimensions.
So: yes, perspective can matter, but with such small pixel counts, limited pixel precision/aliasing is probably the main reason for your inconsistent results.
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