Eye test maths?

beyond the blue

beyond the blue

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Neil
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Can someone just confirm that l'm not going crackers by saying that if I'm sat in the 10 foot long opticians room and l'm looking into a mirror on the far wall at a test screen above my head the furthest l will be seeing is 20 ft?
 
A bit more than 20 feet (assuming the mirror's actually on the wall (Not on a bracket of some sort). You'd have to apply a bit of Pythagoras's theorum.
If b is 10 feet you're sitting at ac, the mirrors at cb and the charts at a x 2
nb. The face of the mirror would have to be perpendicular to b or parallel with a
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Wow, thanks for the replies. The reason l ask is because l have received a new pair of distance glasses for driving when l had the eye test done everything looked clear in the test room at either 10 foot or 20 foot however the equation works (hence the question) but out in the open anything over say 40 foot my left eye struggles to focus. I am going back to see the optician again this afternoon so will ask her to let me see through her contraption in the real world and not just through a mirror in a 10 foot long room.
 
Imagine that the chart is 20ft away, no mirror. The light rays are coming towards your eyes and are all getting mixed up. At 10ft away they will be mixed in a particular way. That is what is being reflected from the mirror, the mixed up light rays which are the same as would come from a chart 20ft away.
It's a bit like those optical puzzles which were popular a few years back where you looked at a pattern, focused your eyes at a different distance and suddenly saw the Dolphins or whatever thing the mixed up rays were of
 
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I have the same problem as you, I always have to get the optician to do a test outside where I can look at a something further away and they hold up all the different lenses to see which is best
As one optician, an Aussie, said " you have some wierd eyes there mate"
 
beyond the blue said:
Wow, thanks for the replies. The reason l ask is because l have received a new pair of distance glasses for driving when l had the eye test done everything looked clear in the test room at either 10 foot or 20 foot however the equation works (hence the question) but out in the open anything over say 40 foot my left eye struggles to focus. I am going back to see the optician again this afternoon so will ask her to let me see through her contraption in the real world and not just through a mirror in a 10 foot long room.
Click to expand...
Let me guess! Specsavers?
 
Depends how much your prescription has changed, you may have to "settle in" to your new specs...but yes, go back to your optician to check.
All the best. Graham SMC Tech. (37 years member of The Worshipful Company of Spectacle Makers.) :giggle:
 
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