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This question already has an answer here:

Assuming that the person stands parallel to a wall. The person and the wall are at the same ground level. The person takes a picture of the wall (Considering that the person always captures the bottom edge of the wall).

Hypothesis:

It is obvious that when the person is closer to the wall, the bottom edge of the wall tends to be at the bottom in the image. As we move farther, the edge moves more closer towards the center in the image.

So, there exists a relationship between "Distance from wall" and "Position of edge of the wall in the image".

The known parameters are:

-> Height of the camera from which the image is captured

-> Angle(Orientation of the camera)

-> Position of the edge in the image

How can I formulate the distance(depth) based on the above parameters? Are there any other parameters that affect the above relationship?

Note: I don't know any of the size of the objects in the screen.

marked as duplicate by mattdm, Hueco, Michael C, scottbb, inkista May 18 '18 at 20:51

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    @osullic You could also assume a person to be a line and a wall to be a surface. – Ian May 17 '18 at 11:20
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    This is pretty basic Photogrammetry. I'm afraid to answer though because I don't want to waste 20 minutes answering a question that will be closed for off topic. – PhotoScientist May 17 '18 at 13:29
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    Worth noting, though, that this is a question about angular subtense whereas the proposed duplicate is about spatial subtense. The givens in this question aren't solvable via the solution linked unless this question is reworded. – PhotoScientist May 17 '18 at 16:59
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    I'm voting to close this question as off-topic because it is about using a camera as a measuring device, not to take creative photographs. – Michael C May 17 '18 at 17:40
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    There it is. I thought the close vote would be quicker on this one. here is angular size and if you don't know how to determine lens FOV. The rest is just trig. – PhotoScientist May 18 '18 at 13:30
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You can't. Too many unknowns.

Another big variable you have not included is the current zoom of the lens.

Your question isn't clear:

Situation 1. You are trying to determine the person/wall distance.

Situation 2. You are trying to determine the camera/person distance.

In general this can be done if you have two images taken from different positions. This is the whole idea of getting topographic maps from aerial photographs. They take a series of pix from a plane with a 60% overlap. Each stereo pair shows the near objects changing relation to the further ones.

Sometimes you can use shadows to make a determination. The image has to include an object where the whole object and the shadow can be seen. And your person has to cast a shadow that hits the wall. From this you can figure out how many person heights the person is from the wall, or if his shadow doesn't touch, a minimum number of person heights.

If this is significant, there are laser range finder devices that will work in daylight at reasonable distances (up to a few hundred feet, more if you use a corner reflector)

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