Contrast has to be maximized

Contrast has to be maximized. The way most vision charts achieves this is to have black symbols on a white background.

This is because

it is easier to distinguish black from white, rather than black from a dark shade of gray. Having maximal contrast eliminates the issue of

contrast sensitivity in acuity testing.

Illumination of the object that you are seeing must be standardized

The room or area where you are having your eyes tested must be lit brightly enough for you to be able to see clearly. There must be

sufficient light and brightness for the cone photoreceptors at the macula to be able to function at its optimum. On the other hand, too

much brightness may introduce glare and affect the test. It has been recommended that a luminance level for acuity testing be equal

or  higher than 85 candelasper square meter.

Distance must be standardized

For distance acuity, you will need to essentially be able to see at infinity, i.e. at an unlimited distance away. Conventionally, 20 feet or 6

meters is taken as infinity from an optical perspective. This is the accepted distance when measuring distance acuity. If you test your

vision nearer than the stipulated 6 meters, the chart letters will appear larger and you will be able to read further down the chart (and

that's cheating, by the way...).