How can I find a phone camera’s focal length if the sensor size and aperture aren’t published?
Asked 3/9/2015
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Some phone cameras don’t list full specs such as actual focal length, aperture, or sensor size. Is there a reliable way to work these out myself, or a source that usually provides them? Also, can the thin-lens equation 1/u + 1/v = 1/f be used to calculate a phone camera’s focal length from subject distance and image distance?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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No the lens equation \${1\over u} + {1\over v} = {1\over f}\$ is not useful in this circumstance, as the image distance \$v\$ (labelled \$S_2\$ in the diagram above) is unknown, and the only way to find that distance would be to cut the camera in half and measure it. Even if you did that (bearing in mind it changes as the lens focuses, so make sure you focus before cutting it in half) it's worth noting that the formula applies to single element lenses, in a multi-element lens \$v\$ (\$S_2\$) refers to the distance from the second principal plane to the sensor. There are various ways to determine the location of the second principal plane but you don't want to go there.
You can determine the 35 mm equivalent focal length very easily by measuring the angle of view, but unless you know the sensor size you can't relate that back to the actual focal length.
If you knew the aperture you could obtain a rough estimate for the focal length by trying to measure the depth of field, but it you don't know the sensor size or f-number then you're out of luck.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
0
Generated from our catalog & community — verify before relying on it.
The thin-lens equation is not a practical way to determine a phone camera’s focal length. You usually don’t know the image distance inside the module, and phone lenses are multi-element designs, so the simple single-lens formula is only an approximation anyway. The effective image distance also changes with focusing.
What you can estimate more easily is the 35mm-equivalent focal length by measuring the camera’s angle of view. But to find the actual physical focal length, you also need the sensor size (or crop factor). Without sensor dimensions, you cannot accurately derive the true focal length from field of view alone.
So in practice:
- You cannot reliably calculate all missing specs from the outside.
- The lens equation will not give an accurate answer for a phone camera.
- If you know the angle of view, you may estimate the full-frame equivalent focal length.
- To get the real focal length, you need sensor size information from manufacturer specs, teardowns, or trusted spec databases.
In short: equivalent focal length may be estimated; actual focal length cannot be accurately determined without sensor data.
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