ED Measurement Information from Mike Bonacci 1-2013

The ED can never be less than the A measurement. The A is the measurement across the 180 line of the eyewire and should be between the longest two points along the 180. The ED (effective diameter) is by definition two times the longest radius of the eyewire. This radius can be anywhere within the circumference of the eyewire. Most opticians are taught to measure the ED along the diagonal of the eyewire. That is top temporal to bottom nasal. This is not always the longest radius therefore, the measurement at this point can sometimes be less than the A. It is impossible for the ED to be less than the A. It can be equal but not less.