Hey, I'm not sure I invented this, but I don't think in terms of seg heights with segmented multifocals, I think in terms of seg "drop".
I originally thought of this to keep my newly-trained wife/optician's head clear: just dot everone's pupil center and take split pd's with the pupillometer, and we'll work out the details later. (I've since been fired as a boss, by the way:) )
For MRP's and progressives, this is all we needed, so I empirically determined that a normal BF seg line is 8mm below pupil center, and a TF is 4mm. Either one can be manipulated 2 mm higher or 2 mm lower, depending on need (submitted for your amusement):
E.g.
BF standard drop 8mm
BF for reading mostly 6mm
BF for distance mostly 10mm
(BF for sunglasses 12 mm)
TF standard drop 4mm
TF for distance mostly 6mm
TF for near mostly 2mm
(computer TF 0mm)
What this has done, other than simplifying things, has also given me the ability to do this: If a segmented multifocal is being fit into a small frame, and you are worried about cutting out too much seg, if you look the dot on the lens like a fitting height for a progressive, the same values apply for optimal fitting height. 18 mm is pushing it, below 18 is no good, 25 is nice. A FT-28 seg's vertical dimension is 20 mm, so fitting it a 8 below gives you the full vertical dimension at 28 mm high, if you follow me. It's halfway cut out at 18mm high, and 3/4 at 25.
I thought these numbers would give me some sort of a standard to compare progressive designs to. While true corridor length may be difficult to ascertain due to different definitions, the theoretical corridor length of about 8 mm would put the reading area in the traditional position.
But when I take out my ruler and lay it on the progressive lens identifier charts, from the fitting cross to the top of the near power circle is always 15mm, regardless of brand!
I'm confused! Why are the circles so low, and why don't they vary by manufacturer? I would expect them to be close to 8mm below the fitting cross.
Help me out, here, please.
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