Focal length of LCD screen as a car's rear view mirror

[Melbourne Guy]

TL;DR Summary

I need glasses to read, but not for driving, and have no issue seeing out my review mirror in the car. But would that be the same for one of those LCD camera rear view mirror units?

I noticed on the commute home today a car with a LCD rear view mirror and I wondered whether, if I were driving it, I'd be able to focus on it the same as I focus on my car's glass review view mirror? I need glasses to read, and that includes a computer, so figured a LCD screen that close, even though it's showing a view out the back, would be as fuzzy to me as a book is without my reading glasses. Is that right?

 

[Drakkith]

Yes, you may need glasses to view the screen since the distance to the viewed object is a few feet for an LCD screen vs several dozen feet for a mirror. Try taping a piece of paper with words or an image to your rear view mirror. If the paper is blurry and can't be read/seen easily, you will likely need glasses if you switch to an LCD screen rearview mirror.

 

[berkeman]

Summary: I need glasses to read, but not for driving, and have no issue seeing out my review mirror in the car. But would that be the same for one of those LCD camera rear view mirror units?

Maybe look through the hit list for this Google search on car hud rearview mirror -- Head-Up-Displays generally will project the reflected image with focus at infinity...

https://www.google.com/search?q=car+hud+rearview+mirror&ie=utf-8&oe=utf-8&client=firefox-b-1

 

[Melbourne Guy]

Maybe look through the hit list for this Google search on car hud rearview mirror -- Head-Up-Displays generally will project the reflected image with focus at infinity...

https://www.google.com/search?q=car+hud+rearview+mirror&ie=utf-8&oe=utf-8&client=firefox-b-1

Thanks @berkeman, while I hadn't thought of a HUD, those items don't appear to be a replacement for the standard rearview mirror so aren't exactly what I wondered about.

 

[Melbourne Guy]

Try taping a piece of paper with words or an image to your rear view mirror. If the paper is blurry and can't be read/seen easily, you will likely need glasses if you switch to an LCD screen rearview mirror.

Yep, I'd struggle with a LCD rearview mirror, @Drakkith. Sometimes, technology doesn't make things better!

 

[DrJohn]

Remember, you are looking at a flat 2-D screen just a few feet away. Like reading a book or magazine that distance away. It has no real 3-D depth, just an illusion - the camera has done the focusing to give you a flat image. A bit like looking at a photo of a landscape showing items miles away, but the flat image itself is feet away.
So if you need glasses to read a book or view a computer screen, you will need them for the rear view camera screen.

 

[Bruzote]

Remember, you are looking at a flat 2-D screen just a few feet away. Like reading a book or magazine that distance away. It has no real 3-D depth, just an illusion - the camera has done the focusing to give you a flat image. A bit like looking at a photo of a landscape showing items miles away, but the flat image itself is feet away.
So if you need glasses to read a book or view a computer screen, you will need them for the rear view camera screen.

True. Kind of like how some interior decorators and even architects are thinking that projecting images of distant views on walls will simulate being outdoors. It fails. The distance to the walls is close, and we need to focus on them more intently, straining the eyes. :-b

 

[Godot_]

Reading glasses are for hyperopic people, or people after age >45-ish (the lens hardens with age, only 0.5dpt accomodation left). If you only need reading glasses, I stronly suspect your in the latter group...

So your vision should be fine at distances >one to 2-ish meters. (dioptrie := 1/ focal distance in meters)

Myopic people - which would be the ones having issues with in-car distances - only need them when they have really high-grade myopia >3 dpt,

 

[Melbourne Guy]

Reading glasses are for hyperopic people, or people after age >45-ish (the lens hardens with age, only 0.5dpt accomodation left). If you only need reading glasses, I stronly suspect your in the latter group...

I don't understand the angry face, @Godot_, but yes, I need reading glasses, which I noted in the OP.

So your vision should be fine at distances >one to 2-ish meters. (dioptrie := 1/ focal distance in meters)

Yes, my vision is fine in this 1m to 2m range...with reading glasses! Otherwise, the view is blurry.

Myopic people - which would be the ones having issues with in-car distances - only need them when they have really high-grade myopia >3 dpt,

I do not understand this observation. Isn't myopia where distant objects are blurry but you can see close objects fine? That's the reverse of my sight and if this was the case, I'd not be concerned about an LCD screen replacing my rear vision mirror.

I'm not sure whether you're replying to my question or something else along the way?

 

[Godot_]

I don't understand the angry face, @Godot_, but yes, I need reading glasses, which I noted in the OP.

Sorry. I assumed it meant *blush*.

Yes, my vision is fine in this 1m to 2m range...with reading glasses! Otherwise, the view is blurry.

So you might probably be slightly hyperopic, aged lenses should still handle that.
IDK the proper English term for hyperopic... ...farsighted? Always thought that to mean the metaphoric thing.

When you look at faraway stuff, you can probably see that all right ?

Myopic translates to nearsighted. Then you would need the glasses not for reading but for e.g. driving. Unless the nearsightedness is really strong.

If you need glasses for the 1-2m range, you're probably mildly farsighted. If you're <45y old, you should see an optometrist. Because this can be compensated in younger years - by flexing the lens in the eye with attached muscles. That's called accomodation.

With age, the lens gets stiffer -> loses accomodation capability. And at age 45, give or take 5y, it gets too stiff to adapt to the typical reading distance of 50cm or so. This would be a 2dpt accomodation. End stage is an accomodation width of 0.5dpt => 2m distance is fine, closer is mildly blurry, but reading adequately large letters should still be possible. So, unless your age is >60y, you should still be able to see stuff at that range.
Because - if you're <40y old - when you age further you'll no longer be able to compensate in long distance vision, too.

So, tough news: You _might_ need glasses for all-day wearing soon. Probably bifocals / varifocals. The optometrist should tell you.

HTH

 

[hutchphd]

I do not understand this observation. Isn't myopia where distant objects are blurry but you can see close objects fine? That's the reverse of my sight and if this was the case, I'd not be concerned about an LCD screen replacing my rear vision mirror.

I think there is some confusion here about whether the diopter refers to the eye or the correction lens.
Diopter=1/( focal length in meters). So a +2dpt eyeglass lens has a positive focal length of 2 meters and will correct for hyperopia. You are both correct.

 

[Melbourne Guy]

Sorry. I assumed it meant *blush*.

Oh, it might, @Godot_, these emojis can sometimes fool, your comment makes more sense in that context, thanks for saying.

When you look at faraway stuff, you can probably see that all right ?

Yes, my distance vision is still pretty good.

So, tough news: You _might_ need glasses for all-day wearing soon. Probably bifocals / varifocals. The optometrist should tell you.

As it happens, I had my eye test this week and the optometrist did try and upsell me on bifocals. I declined, I'm not quite at that point but it's definitely approaching fast

I think there is some confusion here about whether the diopter refers to the eye or the correction lens.
Diopter=1/( focal length in meters). So a +2dpt eyeglass lens has a positive focal length of 2 meters and will correct for hyperopia. You are both correct.

Any confusion is entirely on my side, @hutchphd, I am clueless about diopters!

 

[Tom.G]

Diopter=1/( focal length in meters). So a +2dpt eyeglass lens has a positive focal length of 2 0.5 meters and will correct for hyperopia.

https://www.britannica.com/technology/diopter-optics

...diopters is numerically equal to 1 m divided by the focal length in metres.

 

[hutchphd]

Whoops. Thanks.