calculate changes in "lines of vision"

[NSAspy]

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I need some help figuring out how many lines someone's vision changed. I get that a change from 20/25 to 20/30 represents a drop by one line etc. I'm struggling with changes in between or beyond the graduations in the snellen chart, say 20/100 to 20/150, or 20/150 to 20/800.

I'll really appreciate your help, and yes i'm searched around.

 

[orbitsurgMD]

Using LogMAR indices may make calculating improvement or decrease more manageable.

 

[NSAspy]

Using LogMAR indices may make calculating improvement or decrease more manageable.

Thanks for the prompt response! It's appreciated! You're absolutely right about logMAR, it makes things a LOT easier!!

Quick followup, for CF and HM, does this paper represent consensus approach regarding converting CF/HM to logMAR?
http://www.docholladay.com/publications/097 Visual Acuity Measurements JCRS2004.pdf (Table on pg 3/4)

Thanks again for your help!!