All kidding aside, there are times when people or even you as well want to know how far something was taken from. Lets say in terms of telephotography.
There is actualy a simple formula that everyone can use to figure out the distance a photo was taken.
As an example we take a simple setup where a man is standing and photographed landscape with a 135mm lens on a full frame body.
The formula is DISTANCE equals HEIGHT(subject) devided bij the TANGENS of the VERTICAL OPTICAL ANGEL.
We know the persons height (1,8m)
We know the vertical optical angel (for a 135mm lens = 10° - see Canon documentation)
We know the tangens of this 10° (windows calculator says 0,176326981)
Sollution : 1,8 / 0,176326981 = 10,2 meter
So in order to have a standing man full frame on, lets say a 5d(II) body with a 135mm lens we need to be 10,2 meters away. As simple as that.
For a crop sensor you need to change the lens setting but the priciple is the same. The same thing for feet lovers.
Hope this is making any sense. Good luck and greetings from Ghent.