Fhyujk

If you focus on a plane that is in front of your subject, or past it (relative to depth of field), the subject will appear blurry. Is there some way of telling purely from the resulting picture if focus was too near or far?

EDIT: to clarify, I don't mean plane as in airplane, I meant the "plane" in space that is, for a given focus setting, sharp. As determined by the lens and film/sensor. The focal plane. The question comes down to "can you detect from blur whether it was back-focus or front-focus?"

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.

When the aperture is positioned to minimize vignetting, the bokeh shapes for objects that are too far is upside down compared with objects that are too near. If the arrangement of your aperture blades is not horizontally symmetrical, you can try finding "reference bokeh" that are clearly nearer or further than the focus plane to compare with highlights on the subject to see whether the bokeh shapes are the same or upside down.

^{My answer only deals with "human" ways in differentiation - that is: No software, only your eyes and hands.}

If I have no reference (as in: you blind me, you set the focus distance, and then I can only look through the viewfinder, but cannot change a thing), the answer is: it depends on what I can see.

Take, for example, an alley of trees: one in front, one where the subject stands, and one in the distance. When I see that the subject is not sharp, I can try to tell whether the tree in the foreground or the one in the background is more in focus.

If I have no reference - e.g., as Yaba mentioned, when taking a photo of a aeroplane with blue sky in the background and no foreground, then I have no way to know exactly (my guess would be that focus is too close, however, as planes tend to be somewhere near infinity).

The easiest way to find out usually is to slightly change the focus and see where it is (again, this works better with a reference than without).

But generally speaking, there is no sure way to differentiate - too short a focus distance does not lead to (significantly) different blur compared to a focus distance that is too long.

If the plane is too far or to close is just from the blur hard to say. the easiest way is to have objects in front and behind and see which one is sharp and so you could determine if it is too far or too close... this is the same way as it is done by calibrating the autofocus of a lens with a lenscal tool like this.

without such objects its hard to tell it.

With a plane in the sky it get's hard. Unlike there are other objects at different distances available (Birds, Clouds,...) you cannot compare it easily visually.

Some lense/camera combinations can track the focus distance and will write it into the EXIFs. However this is not very reliable, but could give you a hint. When you know the plane type and therefore can look up it's real size you can calculate it's aproximate distance and compare this to the focus distance.

This page with its calculator can help you with this: https://www.scantips.com/lights/subjectdistance.html

No

it is technically not possible to detect from the blur alone how far or in which direction the image is out of focus.

This is the reason why contrast-detection autofocus systems have to "hunt" for focus by repeatedly changing the focus distance and checking whether the image got better or worse.

In contrast, phase-detection AF systems know (theoretically) exactly how far and in which direction they have to change focus to achieve optimal sharpness.

Of course the image changes if you take context clues from other objects in the picture into account (i.e. "guessing"), but that is something which currently ony organic viewers can do. This might change with AI algorithms in-camera, but i suspect other advances will improve on pure CD-AF before that. (see @szulat 's link from acomment:https://www.imaging-resource.com/PRODS/panasonic-gh4/panasonic-gh4TECH.HTM)