The short answer is “You’re too close.” The slightly longer answer is…
In computer graphics, each point on the screen is called a pixel. When your screen is set to high resolution, the pixels are smaller, and curves appear sharper than the same image at low resolution. A comparison can be made to a mosaic art piece, which is made up of small squares placed to form an image. If you get close to the mosaic, you see the individual tiles, and see that what forms a smooth image from a distance, is actually a collection of jagged pieces. The key point is to be far enough away from the image so that you cannot see the individual tiles, or in the case of eChart, the individual pixels on the screen. For almost every current computer monitor, resolution of 1024×768 or better is possible, and recommended. At a testing distance of more than 10 feet, the patient will be unable to see the individual pixels and will not see jagged lines that you might be able to see if you are sitting right in front of the monitor.
Further, eChart acuity uses “vector graphics” which basically means that every letter and number is displayed to the highest quality possible for any given monitor, regardless of the size of the letter being displayed.