Originally Posted by
jpways
I hope this is the answer you're looking for but:
The easiest way to think of a line bifocal is to think of it as fusing one lens blank on top of the other (which is one way it used to be made). So if you have 2 lenses of equal, but opposite powers (-1.00/+1.00,-1.25/+1.25,-1.50/+1.50...) The lower segment will always read as plano as that is the total reading power (the same will hold true for lined trifocals and progressives at the NRP)
Similarly a -1.00/+1.25 will read +0.25 when you just read the bifocal segment.
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