# How to calculate visual acuity?

Having mastered the concept of the angular size of an object, you can now understand how to calculate visual acuity, and even perform such a measurement.

Draw on a sheet of paper twenty equal black lines with a length of a match (5 cm) and a millimeter of thickness, so that they fill the square. Having attached this drawing to a well-lit wall, move away from it until you notice that the lines no longer differ separately, but merge into a solid gray background. Measure this distance and calculate - you already know how - the angle of view, under which you cease to distinguish strips of 1 mm thickness. If this angle is equal to one minute, then your visual acuity is normal, if it is three minutes, then acuity is one third of normal vision, and so on.

Let's solve the following problem. Look at the drawing. The lines merge for your eye at a distance of 2 meters. Do you have normal visual acuity?

We know that from a distance of 57 mm a strip of 1 mm width is visible at an angle of 1 degree, that is, in 60 minutes. Consequently, from a distance of 2000 mm, it is visible at an angle x, which is determined from the proportion:

x: 60 = 57: 2000

x = 1,7 minutes

In this case, the visual acuity is below normal and is:

1: 1,7 = about 0,6.

Now we have learned that the stripes viewed from an angle of view of less than 1 minute cease to differ separately from the normal eye. This is true for every object: whatever the outlines of the observed object, they cease to differ with the normal eye, if they are visible at an angle of less than one minute. Each object turns, at the same time, into a barely perceptible point, into a speck of dust without dimensions and shape. Such a property of a normal human eye: one angular minute is the limit of its visual acuity. This applies equally to large but overly distant objects, as well as to close ones, but too small.

The magic effect of microscopes and telescopes is due to the same reason. The purpose of these devices is to change the course of the rays of the object in question so that they enter the eye with a steeper diverging beam. Due to this, the object is presented from a large angle. When they say that a microscope or a telescope magnifies 100 times, it means that with their help we see objects at an angle 100 times larger than with the naked eye. And then the details, hidden from a simple eye beyond the limit of visual acuity, become available to our eyesight.

By virtue of this feature of our vision, every object removed at 3420 (57x60) of its width ceases to differ in its outlines and merges into a point. If one accepts that the distance between the eyes of a person is only 3 centimeters, then both eyes merge into a point already at a distance of 3x3420 cm, that is, 100 m. The gunners use this for a visual assessment of the distance. According to their rules, if the eyes of a soldier seem to be two separate points from a distance, then the distance to him does not exceed 100 steps (60-70 m). We got a greater distance - 100 meters: this shows that the sign of the military means a slightly reduced (by 30 percent) visual acuity.