testing distances greater than 20 foot

[ellivron55]

My brain is getting more fuddled as i get older.If you have a testing distance greater than 20 foot/6 metres do i need to make any adjustment to the final rx.
thanks

[Echo419]

no .. 20 feet and beyond is optical infinity

[Dave Nelson]

There is a compensation to be made for testing at distances shorter than 20 feet, but it is made to the letter size of the snellen chart, and the 20 foot distance can only be shortened a rather small amount before the vergence of light skews the test results. You ask a rather odd question... why would anyone test beyond 20 feet? You could test shorter due to room size limitations, but longer?

[Aarlan]

There shouldn't be any difference w/ the script since 20 ft and over should have a similar vergence (both are at or near optical infinity)

What it will affect is the accurate documentation of V/A, since most charts are calibrated to 20 ft or 6M. If the distance is different, the subtended angle will change, so you have to account for that when recording BCVA.

For example if a person is a -4 myope, they will see best w/ a -4 whether it is 20 ft, 30 Ft or 40 ft. They will, however not be able to see as far down the chart at longer distances, so you have to convert to record, so if at 40 ft, they can only read to the 20/40 line, they are actually seeing 20/20.

AA

[Dave Nelson]

Aarlan, you are 2 minutes to late.:bbg:

[drk]

Not to poop on your party, but 20 ft or 6M is NOT optical infinity, but it will do.

[ellivron55]

Someone may test longer for eg using a 11 foot room and a mirror

[Dave Nelson]

10 or 11 foot room and a mirror=20 feet. (approx) ?? also, not to split hairs, but Aarlen did say "at or near."

[chip anderson]

I actually knew a very, very good ophthalmologist who on request would have his patient's bring thier rifle to his third floor office and refract over thier glasses while they were sighting through the gun. He had a lot of
very satisfied patients.

Chip:cheers:

[HarryChiling]

If the distance is different, the subtended angle will change, so you have to account for that when recording BCVA

I think this is what made the difference in what she said the chart has optotypes which have specific subtended angles, for instance the "E" big one you know where was at 20 ft the angle would be 5 degrees.

If that distance changed to lets say 30 feet the angle would be smaller.

tan (angle) = text size/distance

So to keep the angles the same you would need to increase the text size or chart size see the example bellow

[ellivron55]

On the new computerised test charts you just type in your testing distance and it automatically changes the size of the optotypes. However before I was testing at 3 metres/10 foot and mr x for eg reads the 20/20 line easily but put the same chap on my new system at 22 foot and the px struggles on the 20/20 line.Now at both distances the optotypes have been exactly worked out correctly so they correspond to the classic convention none of this using a 20 foot snellen chart at 10 foot etc

[Dave Nelson]

I wonder what the closest distance you can test by changing the letter size on your computer? At some point accomodation will come into play. I would guess 10 feet is about the max.

[Aarlan]

I think this is what made the difference in what she said the chart has optotypes ....

If you're referring to me, I'm a he. Yes it's a goofy name.

...instance the "E" big one you know where was at 20 ft the angle would be 5 degrees.

The angle being subtended is 5 minutes of arc which is 1/60th of 5 degrees (60 minarc per degree). If it were 5 degrees your 20/20 letter would be almost 2 feet tall. Instead at 6 M it is 8.726 mm.


AA

[HarryChiling]

If you're referring to me, I'm a he. Yes it's a goofy name.

Sorry, obviously not the first mistake I have made. You are right about the minutes of arc, I looked at a chart near by and it has a (^o) symbol instead of a (') which designates minutes arc. The point is still the same that the angle changes if you are moving the chart back.