Can you estimate the focal length used in a photo without EXIF metadata?
Asked 8/24/2018
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Is it possible to tell what focal length was used just by looking at the image itself, without reading metadata? What clues in the photo can help, and are there cases where it’s impossible to know exactly?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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It would be more accurate to say that you can estimate the angle of view demonstrated by a photo. Angle of view is a result of both focal length and sensor size. In the case of a cropped photo, only the part of the sensor that has information included in the cropped photo should be considered as the 'sensor size'.
Even then, one must have context clues to estimate the angle of view at which a photo was shot.
If the photo is of a flat test chart perpendicular to the optical axis of the lens and parallel to the imaging sensor, then we can't really tell, even if we know the exact size of the chart. In such a case we can only determine field of view. But the same field of view can be obtained by shooting from different distances with differing angles of view.
For example, assuming the same 35mm/full frame camera/sensor size, we could get the same field of view of a flat subject by using a 50mm lens from 10 feet, a 100mm lens from 20 feet, or a 600mm lens from 120 feet. In the first case the 40° horizontal angle of view from 10 feet with the 50mm lens shows the same field of view (i.e. the same width and height) of the flat test chart as the 20° horizontal angle of view from 20 feet with the 100mm lens and as the 3.5° horizontal angle of view from 120 feet with the 600mm lens.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
7y ago
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Generated from our catalog & community — verify before relying on it.
Usually you can only estimate, not determine the exact focal length. What you’re really judging from the image is the angle of view, and that depends on both focal length and sensor size. Cropping also changes the visible field, making exact identification harder.
Without reference clues, it may be impossible. A photo of a plain wall, for example, gives you almost nothing to work with. Even with a known subject size, the same framing can often be made from different distances with different focal lengths.
Useful clues include:
- field of view: wide, normal, telephoto
- perspective clues from known parallel lines, like hallways or buildings
- obvious distortion, such as fisheye-style curved lines
- apparent compression of distance in telephoto-looking shots
- relative subject/background size and how much background is included
If the scene contains measurable references, you may be able to model it in 3D software and estimate the angle of view more closely. But in most real photos, exact focal length cannot be reliably recovered from image content alone—only a rough category or angle-of-view estimate.
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