I need some lab guys to level with me.
Mine is telling me with a straight face to just measure corner-to-corner of the FT reading segments to "verify" NPD values and disregard where I'm dotting up the OC of the reading segment. Is there some obscurity of advanced optics that makes either my lensometer or prentice's rule unreliable in a reading seg, at any power range? Is there some esoteric caveat in ANSI that says FT reading segs are inscrutable for horizontal imbalance?
Assuming a standard line placement at the limbus, am I incorrect that the vertical fix for such a read should be 4-5mm below the line (just in case an oblique axis makes it trickier to lock onto?)
I'm only talking about horizontal prism. I'm not talking about vertical prism, image jump, or which way to trap a sagittal reflection.
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