Because these experiments run on monitors with different pixel sizes, you will (at least for the moment) have to adjust your viewing distance from the screen to assure consistency.

Measure the following line, then use the table [or the formula given] to estimate the correct viewing distance for your monitor.

 If you find this line length [cm] Sit at this distance [cm] If you find this line length[in] Sit at this distance [in] 5 5.25 5.5 5.75 6 6.25 6.5 6.75 7 56.5 59.3 62.1 64.9 67.7 70.6 73.4 76.2 79 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 22.6 23.7 24.8 26 27.1 28.2 29.4 30.5 31.6

The figure to the left shows the relationship of distance to visual angle for an object of fixed size. To calculate the distance, we use the fact that, for a triangle with one right angle, the ratio of the length of the opposite side to the length of the adjacent side equals the tangent of the angle. Thus,

a     x/2
tan  -  =  ---
2      d .

We have designed our experiments expecting this line to be about 6.2 cm long. At our typical viewing distance of 70cm, the line fills an angle of .0885 radians (5.07 degrees). (Some numbers are rounded.)

a     6.2 /2                  x/2
tan  -  =  ------      tan .0443 = ---
2       70                     d

Given this target angle, tan .0443 = .0443, and the actual length of the line, we can calculate the viewing distance.

x/2       x
d = ---    = ---
.0443    .0885

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