Distance is very deceptive and if one is accustomed to judging the distance of an object of a certain size and then has to change to a similar looking object of a different size the difficulty is increased.
When I have been shooting at stags and judging their distance with fair accuracy and then change to roe deer shooting, the roe always seems much further off than the real distance, because a roe at one hundred yards looks the same size as a stag at two hundred yards off.
This difficulty is increased if the objects are mistaken for each other.
Suppose a river with steep banks, fifty yards broad, in a flat meadow, and you stand in clear atmosphere and full sunshine at a spot twenty yards from the nearest bank. From where you stand you cannot see the breadth of the river; the two banks looking like one line on the green of the meadow.
A faded, weatherbeaten, red fire bucket, is standing on the edge of the far bank, and a flower pot on the near bank.
Both objects look identical in size, shape, and colour because of the linear and aërial perspective at these distances, and it is impossible, unless they are studied very carefully with a telescope or field glass, to know which is which and therefore which is the further off. If you are accustomed to judging the distances of flower pots you would think the fire bucket was a flower pot and therefore only twenty yards off instead of seventy.
Be sure you know what the object is when using it as a means of judging distance, it may be something much larger or smaller of a similar appearance.
A pony, when seen through a thick haze, mistaken for a horse would entirely upset your calculations.
The use of being able to judge distances accurately is to enable you to decide how much to aim above a distant object to make up for the distance the bullet drops in going that distance.
The drop of the bullet increases rapidly as the distance increases.