Can EXIF metadata show the exact focus distance for each image in a focus stack?
Asked 5/9/2014
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I’m working with a focus stack where each frame is focused on a different part of the scene. The EXIF metadata shows the same focal length for every shot (for example, 9.7mm), even though focus clearly changes from frame to frame.
Is there any metadata field that records the actual focus distance used for each image? I’m trying to estimate depth from a stack, so I need the focus setting for each shot rather than just the lens focal length.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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I'm not absolutely sure of what you're asking, but I'll try to answer.
The focal length is not a measure of at which distance the lens system has been focused but rather a measure of how strongly the system converges or diverges light. In photography this translates to how wide the lens is and what field of view you'll get by using it.
When you change the focus of a lens the focal lens stays (ideally) the same. In practice (such as lenses with internal focusing) the focal length does change depending on where you focus, but this is just a minor drift and doesn't even have to appear in the metadata depending on the model of the lens/camera. The phenomenon is called breathing or focus breathing and can be a problem for videographers trying to do racking focus, but is usually not a problem for still photographers. Focus stacking could be an exception to this.
All in all there's nothing surprising or strange that the focal length stays the same no matter where you're focusing. Depending on the camera it's sometimes possible to get additional information such as what focus point(s) were used during the exposure or the depth at which the focus was locked. I recommend you shooting in RAW and to use the bundled software from the camera maker in order to have the best possibilities to extract the information you want.
Originally by user21986. Source · Licensed CC BY-SA 4.0
user21986
12y ago
0
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The EXIF focal length value is not the focus setting. It describes the lens’s angle of view, so it usually stays the same across a focus stack if you did not zoom the lens.
What you’re looking for is focus distance / subject distance, sometimes stored as a non-standard EXIF field such as “Subject Distance” or “Distance to Subject.” Many cameras and lenses do not record it at all, and when they do, it may be coarse or unreliable rather than precise enough for depth-map work.
Also, some lenses change effective focal length slightly as focus changes (“focus breathing”), but that small change often is not written to metadata.
So in practice: no, you usually cannot get precise per-shot focus information from standard metadata alone. If your camera does not record a subject-distance tag, you’ll need another method, such as measuring focus positions externally or including a known scale/reference in the setup.
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