Is the sharpness of imaging different within the depth of field? Where is the sharpest? What I am asking is the distance from the lens to the object, not the image distance in the lens. How to calculate this distance?

  • Why do you think DOF is relevant? What do you mean by "when the image is clearest"? What type of object are you concerned about?
    – xiota
    May 26 '20 at 3:30
  • @xiota Isn't the image as clear as the entire depth of field? So where is the clearest place?
    – enbin
    May 26 '20 at 3:46
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    1. What do you mean by "clear"? Do you mean focused and sharp? Or something else? It's possible for an image to be in focus, but not "clear" (eg, haze, soft focus). 2. DOF is a zone of acceptable sharpness /focus. It varies for different people and viewing conditions. What viewing conditions are you concerned about? (eg, web, print, poster, billboard, pixel peeping) 3. What type of object you're concerned about. When taking someone's portrait, are you satisfied if only their eyes are in focus, or would you want the rest of their head to also appear sharp?
    – xiota
    May 26 '20 at 3:55
  • @xiota I modified the question
    – enbin
    May 26 '20 at 6:08

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".

  • So what is u in the imaging formula?
    – enbin
    May 26 '20 at 11:32
  • @enbinzheng I didn't reference the formula, but you can easily google "thin lens equation" and find multiple pages that explain how to use that. Just be aware, it has nothing to do with depth of field. DOF is solely a measure of how far you can be from the plane of focus (as defined by the thin lens equation) and have "acceptable" level of defocus.
    – Zeiss Ikon
    May 26 '20 at 12:25

The clearest image of an object is always when the object is in focus — at the plane of focus. That is, for a given lens of focal length ƒ, when the lens is positioned a distance v from the camera sensor, then an object at distance u from the lens is in focus in accordance with the thin lens approximation, ƒ-1 = u-1 + v-1.

  • Do you mean that the object at that position of u will be the clearest? This position is generally somewhere within the depth of field?
    – enbin
    May 26 '20 at 5:04
  • 1
    Exactly. It is within the depth of field.
    – scottbb
    May 26 '20 at 5:26
  • 2
    This distance defines the depth of field. The farther you are from that plane, the blurrier things get. DOF is just defined by how much blur you tolerate.
    – xenoid
    May 26 '20 at 6:31
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    Ignoring distortion, aberrations, etc, this works for flat objects parallel to the film/sensor plane. It may not work well, or at all, for other objects, depending on how their positions are defined. (If the position of a sphere is determined by its center, no point on its surface would be in focus using this method.)
    – xiota
    May 26 '20 at 8:31
  • @xenoid Do you mean the distance of this plane is u?
    – enbin
    May 26 '20 at 10:12

The image is sharpest at the focus distance. Many lenses have markings that roughly indicate that distance but you can also extract it from EXIF data shot from a digital camera.

Sharpness decreases away from the plane of focus. Depth-of-Field is simply a concept to describe a range of distances which are sufficiently sharp. Sufficiently sharp depends on the image viewing size and viewing distance, so most charts and calculators for Depth-of-Field assume a certain print size (10x8" is common) and viewing size.

If you want to know the distance from the lens to the object then you have to subtract the flange distance and length of the lens from the focus-distance. Keep in mind that most lenses change in length depending on the set focal-length (for a zoom) and focus-distance.

There is one more minor catch called Field Curvature. This is an optical default causing the focus-plane to be curved rather than an actual being actually flat as a plane would normally be. Only some lenses show this and it can vary by the set aperture and focus-distance. When Field-Curvature occurs, the distance to the sharpest points in the scene will depend on its position within the field-of-view.

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