VA problems

[tbearOD]

I'm having a little difficulty figuring out VA problems, can someone help me with this problem?? thanks!!

If a patient can see a 2.4mm letter at 5 feet, what will their acuity be at 20 feet?

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[KHE]

Why is one measurement in feet and the other in metric units?

You can convert that and it's a simple problem but what a pain.

 

[cateyegal]

So to get the same VA equivalent at 20 feet as at 5 feet, you would multiply the letter size by 4 since 5ft is 1/4 of 20ft. So at 20 feet the equivalent letter size is 9.6 mm.
What does that letter size translate to as an acuity? a 20/20 letter at 20ft is 8.73mm tall. You could do a quick cross multiplying equation set up like this: 20/8.73mm = x/9.6mm. x=22 so the VA is 20/22. You would likely round this down to 20/20 depending on your professor.