Cannot see big road signs wearing full prescription glasses

I can see 20/15 (outdoors in the shade) on the 20 feet Snellen eye chart with my full prescription glasses.

However when I wear the same glasses and look at a big road sign outdoors, if I back off enough (maybe 60 feet?) I cannot see those big letters.

Is this normal? Shouldn’t I see infinity clearly with 20/15 wearing my full prescription glasses?

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Seeing something, and seeing it perfectly clearly are not the same thing. Being able to read a 6 m Snellen line is not the same as being able to see each letter on it with perfectly sharp, clear edges. This is one of the disadvantages of trying to measure one’s visual acuity on a Snellen chart. The blur that one may not notice on a Snellen line, will show up at greater distances. Measure your naked eye distance to blur, check on the diopter calculator what your full diopter needs are, and compare this to what you now consider full correction based on Snellen lines. You are likely to find that they are not the same.

Seeing infinity clearly is a bit misleading. After a certain distance most things just get too small to see clearly, even for the sharpest-eyed bird of prey.


@Ursa Thank you so much! I do use cm measurement but I have a hard time telling where the blur starts so it varies. I also worry about “cheating” because subconsciously I want to produce a higher number. Same with my daughter. Sometimes her numbers are 38 cm, sometimes 27 cm. I cannot fully trust those numbers. But I do keep records to track progress.


You will get better at telling where blur starts, and cheating doesn’t matter, as long as you always cheat by the same amount. It is the progress that matters, and you will only see that when you have collected enough data to see the pattern through the ‘noise’ of daily variations. But kidding yourself about how well you see when driving is not a good idea, so take care that you do not undercorrect for that.

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@Ursa Thank you for your encouragement. I was trying to get off my dependence on glasses by driving without glasses on familiar local streets, because I can always see a vehicle or a person when they are close enough. But now I know blur adaptation is not a good idea and I resumed using full prescription glasses for driving.


I did the same, although what I needed for driving also came down steadily and now I drive without, but only on small local roads. I still have a pair of glasses I would use if I had to drive at night or on fast roads I do not know well. But this is very rare indeed. You will get to that stage as well.

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In your introduction you say you have -5.0D?!?
That’s not the same as @Ursa driving without glasses!

If that’s the opto’s current prescription for you, you should have full clarity for driving with them as a starting point in my opinion. (Or have you missed the last few checkups and you may as well be -5.50 or -6.00 by an opto’s measurement?)
60ft for a big road sign is a good reference point for clarity…

From your intro and topics / posts I guess you are an enthusiastic beginner. I’d recommend getting a pair of glasses that gives you full clarity for the most challenging vision situation (e.g. night driving) and use them for distance vision, and especially when clear vision is crucial. Or if your current full prescription is fresh from the opto, then just wear it to remind your eyes what distance clarity is.
Otherwise it’s advised to spend a few weeks working out the centimetres for close up - how much it changes (with different lights, within the day, between days). You’ll need good data to work out the best first differentials for you and for your child.

And when you have full clarity at distance and you have reduced differentials successfully, that’s when it’s advised to reduce normalised first, but not to zero diopter…


@BiancaK Thank you so much for your advice! I have stopped driving without glasses once I saw that all of you are using the full prescription glasses for driving; so that is not the time to relax eyes and use distance vision. I’m waiting for differentials to be ready.

I want to stress this point. Perish the thought that anyone is encouraged by me to drive with inadequate correction.

  1. My binocular vision was never worse than -3. I always drove with full correction, which I removed as soon as I got out of the car.
  2. When my binocular vision was down to -1, checked on naked eye cm and Snellen, I experimented with driving without glasses, but with what I needed on the passenger seat in case of feeling unsafe.
  3. Now that my binocular vision is close to -0.75, again checked on naked eye cm distance and Snellen, I am confident driving without, but the glasses are in the glove compartment.

You may have picked up that my left eye is much weaker, but it still adds more than adequate stereopsis for driving.


Wow you’ve made so much progress! Got to learn from you :slight_smile:

We can’t really learn from each other, as our eyes and our vision habits are so different, but we can certainly be inspired by others to keep on at it. Bianca’s latest update post is truly inspiring.