Can you estimate an object's real size from a photo using EXIF data?
Asked 5/15/2012
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Is it possible to estimate the real size of a subject in a photo from EXIF and camera information alone? For example, if I photograph a bird in flight against an empty sky, there is nothing else in the frame for scale. If I know things like sensor size, focal length, image dimensions, and possibly focus distance, can I work out the bird's approximate size?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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It would never be more than an estimate, without some external reference or accurate distance to the subject. If you have estimated distance to the subject, FOV (/length of the lens) and the sensor size it's not too hard to calculate. The biggest uncertainty would be the distance to the subject.
To find the spatial resolution, i.e., how large (wide or high) a pixel is in the subject, use:
$$ \frac{\text{distance}\times\text{sensor size}}{\text{focal length}\times\text{number of pixels}} $$
Make sure the sensor size and focal length are using the same scale (both are in millimetres or inches).
Then you can count the number of pixels for the subject, and multiply that with the spatial resolution for one pixel.
I'm assuming a linear change in FOV over the whole area of the lens, which isn't 100% accurate, but quite close for normal and longer lenses.
Originally by user7271. Source · Licensed CC BY-SA 4.0
user7271
14y ago
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Yes, but only as an estimate unless you also know the subject distance very accurately or have some external reference.
EXIF can provide helpful inputs such as focal length, image size, and sometimes focus distance. If you know the distance to the subject, sensor size, focal length, and the number of pixels covering the subject, you can estimate its size from the image scale.
A useful approximation is:
spatial size per pixel ≈ (distance × sensor size) / (focal length × number of pixels)
Use the sensor dimension, image pixel count, and subject pixel count in the same direction (width or height), with distance and focal length in the same units.
Then:
subject size ≈ subject pixel count × spatial size per pixel
The biggest source of error is distance to the subject. Focus distance in EXIF, if available, may help, but it may not be precise enough for exact measurement. Perspective, lens distortion, and the subject not being perfectly perpendicular to the camera also reduce accuracy.
So: yes, you can estimate size from a photo, but EXIF alone is usually not enough for a reliable exact measurement.
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