Where is an image sharpest within depth of field?
Asked 5/25/2020
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I’m trying to understand whether sharpness varies across the depth of field. Is there a single distance that is the sharpest, and does sharpness gradually decrease in front of and behind that point? I mean the subject distance from the lens/camera, not the image distance inside the lens. How can that distance be determined or calculated?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Never forget: depth of field is a myth. The term is a convenient shorthand for a standard of how much defocus is acceptable, and is dependent on the ultimate size of the print or level of magnification in examination, as much as the magnification on the negative or sensor.
At the highest resolution examination, even with a very small aperture, the plane of focus is effectively a mathematical plane, having no thickness (and ignoring field curvature, which is an aberration of real lenses). All depth of field refers to is the thickness of the zone in which the level of defocus is "acceptable".
Originally by user89902. Source · Licensed CC BY-SA 4.0
user89902
6y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. The image is sharpest at the actual plane of focus — the specific subject distance the lens is focused on. As you move closer or farther than that distance, sharpness decreases.
“Depth of field” is not a zone of equal sharpness. It is only the range of distances that appear acceptably sharp under some viewing condition. That acceptable range depends on enlargement, print size, and viewing distance.
In an ideal thin-lens model, the focused subject distance u and image distance v are related by:
1/f = 1/u + 1/v
where f is focal length. For a given lens/sensor position, only one subject distance is exactly in focus.
In practice, the easiest way to know that distance is the lens focus-distance scale, or sometimes EXIF metadata. If you specifically want distance from the lens itself rather than the camera’s marked focus distance, you’d need to account for the camera’s flange distance and the physical length of the lens, noting that many lenses change length with zooming or focusing.
So: the sharpest point is the focus distance; depth of field is just the acceptable blur range around it.
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