Eye Test

Bones

Bones

Bones

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Random tequila fuiled question...

went for an eye test today (my first) and had to read the letters off a chart. had to look in a mirror at the chart , just wondered if that makes the distance you are looking twice as much?

pls admire the diagram below:

miror.jpg
 
The light has to travel twice as much, but the letters will appear the same size. So it's not the same as if they were twice as far away.
 
Crispy Pigeon said:
The light has to travel twice as much, but the letters will appear the same size. So it's not the same as if they were twice as far away.
Click to expand...

but that means that the room isnt technically twice as long with the mirror. im confused and its late.
 
Most people here must be almost asleep (im in the us, so 9:40pm)

There is a mirror so that the room can be smaller. It is exactly the same as if the chart was about twice as far away, will look smaller than if it was up close, since it will look as if it is twice as far away.
 
Ok, so why not just have the board on the wall where the mirror is and make the letters half as large.


Same result?
 
PinkPig said:
Probably not quite when you want to test long / short sightedness.
Click to expand...
Actually it wouldn't make a difference. AFAIK, having the mirror allows the optician to both look at the patient and the chart at the same time, so he can assess what the patient is doing whilst simultaneously seeing the chart

edit: also the parameters for how the letters change size are quite strict, and having the chart half size would lose some resolution - and so make the test invalid (i think)
 
Last edited:
j00ni said:
Actually it wouldn't make a difference.
Click to expand...

Yes it would. I can read tiny text if its close enough, but I cant read large text on the opticians chart. Its all to do with the distance to the point on which you eyes are focused.

To clear things up:

The mirror has the same effect as making the chart twice as far away. Your eyes actually focus on a point which is the room's length behind the mirror, not on the mirror itself.
 
Lagz said:
Yes it would. I can read tiny text if its close enough, but I cant read large text on the opticians chart. Its all to do with the distance to the point on which you eyes are focused.
Click to expand...
But the chart would still be sufficiently far enough to be testing distance vision as it's beyond the natural focal length
 
j00ni said:
But the chart would still be sufficiently far enough to be testing distance vision as it's beyond the natural focal length
Click to expand...

If that is the case they wouldn't bother with a mirror. Its obviously not there so the optician can see the chart for the following reasons:

1) Its back to front.
2) The optician has to strain her neck to look up at it.

If this was the only reason they would have surely come up with a better idea. Also - why is the mirror ALWAYS on the side of the wall the patient looks at. If this is the only reason they could equally well put the mirror behind your head the the actual board infront of you.
 
Lagz said:
1) Its back to front.
Click to expand...
so? Can you not read charachters back to front

Lagz said:
2) The optician has to strain her neck to look up at it.
Click to expand...
hardly

Lagz said:
Also - why is the mirror ALWAYS on the side of the wall the patient looks at. If this is the only reason they could equally well put the mirror behind your head the the actual board infront of you.
Click to expand...
What, so the charachters would appear a quarter of their current size? I think most people would struggle to read that 6/6 vision or not
 
The way the scale works for testing your eyesite is what is the minimum size font you can read at a distance of 10m. Since most opticians cannot afford to use a 10m room for eye tests, they use a 5m room and effectively double its length using a mirror.
 
Crispy Pigeon said:
The light has to travel twice as much, but the letters will appear the same size. So it's not the same as if they were twice as far away.
Click to expand...
Huh?
It's exactly the same as if they twice as far away, the letters are roughly half the height that they would be on the opposite wall,
 
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