Can you tell from a blurred photo whether focus was in front of or behind the subject?
Asked 2/26/2019
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If a subject is out of focus because the actual focus plane fell in front of it (front focus) or behind it (back focus), is there any way to tell which happened by looking only at the final image? I’m especially interested in practical clues for film/manual-focus shooting, where there’s no EXIF or instant review. Are there visual signs in the blur itself that can indicate whether focus was too near or too far?
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 depends. In many cases, it may actually be possible without any further visual aids in the picture.
Many lenses, if not most, will show different longitudinal chromatic aberration in front of and behind the focus plane. If you scroll down just a little bit on the linked page, you will see this demonstrated with a picture of a focus test chart. With this lens, the blur in front of the focus point will have purple colour fringing, while the blur behind the focus point will seem greenish.
If you know the characteristics of this lens, you could look at just a very small part of the image like e.g:
... and say for sure that this out-of-focus area is behind the focus plane.
Originally by user10009. Source · Licensed CC BY-SA 4.0
user10009
7y ago
0
Generated from our catalog & community — verify before relying on it.
Sometimes, but not always. A blurred image alone does not universally reveal whether focus fell in front of or behind the subject.
Useful clues:
- Relative sharpness of nearby objects: if you have foreground and background detail, see which side of the subject looks closer to sharp. If the background is sharper, focus was likely behind the subject; if the foreground is sharper, it was likely in front.
- Longitudinal chromatic aberration: many lenses render out-of-focus areas differently on each side of focus. Often one side shows magenta/purple fringing and the other greenish fringing. If you know your lens’s behavior, this can be a strong clue.
- Bokeh shape orientation: with some lenses, out-of-focus highlights in front of and behind the focus plane are effectively rotated 180° relative to each other. This is only useful if the aperture/bokeh shape is asymmetrical enough to recognize.
If the image has no depth cues, no identifiable color fringing, and no distinctive bokeh highlights, then you usually cannot tell just from the blur.
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