Can you determine focal length from the aperture used on a zoom lens?
Asked 1/13/2013
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2 answers
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I’m using an Olympus X-21. The lens spec is 6.3–18.9 mm, f/3.1–5.9. Two photos were taken at different zoom settings, and the recorded apertures were f/4.2 and f/5.7. Can focal length be calculated from the aperture value alone, or is there a formula relating aperture and focal length on a zoom lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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First, there is no way to get the focal length from the aperture.
Your camera's max aperture is f/3.1 at 6.3mm and f/5.9 at 18.9mm - but there isn't anything that say the camera must use the max aperture, it's completely possible to use f/5.9 at 6.3mm - so there no relationship what so ever between the aperture used and the focal length.
Even if you force the camera to use the max aperture the relationship between the max aperture and the focal length is depended on the internal design of that camera's lens and there is not general formula that can help you.
And, just to make things even more impossible camera manufacturers don't publish the max aperture - focal length relationship because usually it just makes them look bad (the max aperture gets smaller faster than you would think).
Second, "in the real world" it's usually very easy to get the focal length, every digital camera I've ever owned from entry level point ans shoots all the way to may current DSLR always records the focal length in the EXIF data.
Originally by user2481. Source · Licensed CC BY-SA 4.0
user2481
13y ago
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Generated from our catalog & community — verify before relying on it.
No. You generally cannot determine the focal length from the aperture used alone.
For your Olympus X-21, the spec “6.3–18.9 mm, f/3.1–5.9” means:
- at 6.3 mm, the widest available aperture is f/3.1
- at 18.9 mm, the widest available aperture is f/5.9
But the camera can also choose smaller apertures (higher f-numbers) at many focal lengths, so the same aperture value can occur at different zoom positions. That means there is no universal formula to recover focal length from just f/4.2 or f/5.7.
From your values, you can only make limited conclusions:
- f/4.2 suggests the lens was not at the longest end and was likely toward the shorter part of the zoom range.
- f/5.7 could have been taken at many focal lengths, including near the long end.
If you need the actual focal length, check the image EXIF metadata. If that isn’t available, you would need to estimate it by camera calibration using a known target, and for image-analysis work you also need the sensor size.
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