Home » Digital Rearview Mirrors Are Garbage And I Can Explain Why With Science

Digital Rearview Mirrors Are Garbage And I Can Explain Why With Science

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There are some things that sound great on paper, but in practice are awful, just awful. Things like, say, a popsicle made of chowder or a genetically-engineered flying chihuahua. When it comes to cars, I think one of the best examples of this concept is the inside rearview mirror that is actually an LCD screen connected to a camera. In principle, these should be terrific. But I hate using them. Why? What makes them so awful? To figure it out, I had to reach out to a scientist who studies visual cognition.

Are you familiar with these sorts of digital rearview mirrors? They’ve been around for a few years, and they seem to offer a lot of benefits: because the “mirror” (again, it’s not really a mirror at all, it’s a screen in the shape and location of a traditional inside mirror) is fed via a camera mounted at the rear of the car, nothing inside the car – passengers heads, luggage, pillars, errant balloons, whatever – can block your rear vision.

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Also, the camera can provide a wider field of view, and prevents getting dazzled when someone has their brights on behind you. Here, I’ll let Toyota try and explain why these should be great:

I just tried one out myself in the 2023 Toyota Sequoia, and while the technology itself was impressive and well-executed, actually using the screen-mirror for its intended purpose was awful, almost literally unusable. It’s terrible.

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(I should mention that while my examples here are from Toyota, this is not a Toyota-only thing. Other carmakers have these, and there’s aftermarket options as well.)

It’s terrible in an unusual way, too, in that there’s nothing actually wrong with the design or execution, it seems to be terrible in concept. I wasn’t exactly sure why, but I had some suspicions. I also wasn’t sure if maybe there was just something wrong with me, but I asked a couple other Toyota reps (whom I won’t name, of course) and got confirmation that I’m not alone in finding these digital mirrors unusable.

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Part of what seemed to be going on had to do with my own particular vision situation. Ever since I made the decision to Become Old, which happened a few years ago in my late 40s, my once-perfect-vision eyeballs decided they’d help my more mature look by suddenly refusing to be able to see anything close to my head.

This is known as presbyopiaand happens to pretty much everybody, so all you beautiful, sexy young readers out there holding your phones inches from your gorgeous, clear, vivid eyes can just fuck right off, because it’ll happen to you, too, and then you’ll be having to get used to yourself in glasses just like I did.

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Anyway, because this condition only really affects your close-up vision, I don’t wear glasses to drive, as my distance vision is fine, and I just deal with the fact that my dash instruments are a bit blurry. But, significantly, when I drive and look in my conventional rearview mirror, inches from my face, the images reflected in that mirror do not look blurry. They look as clear as any of the other distant cars through the windshield look.

But when I look at the digital rearview mirror’s screen, it’s blurry as hell. It’s the same distance as a normal mirror, so what’s going on, here? And, even with my glasses on, I can see the screen-mirror image clearly, but it still feels unsettlingly wrong compared to a normal mirror. How can I make sense of this?

Well, maybe can’t, but Dr. Jay Pratt, who specializes in perception and visual cognition and is a professor at the University of Toronto, can.Mir John

Of course, he did refer to me as “John” in all our correspondence, but I think he still knows what he’s talking about when it comes to this sort of visual puzzle.

Dr. Pratt gave some really interesting explanations for what I was experiencing, so I’ll let him explain. First, the issue of why the digital mirror image was blurry when a regular mirror, the same distance away, is not:

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Issue #1: the mirror image is crisp but the LCD image is blurry.  I think this is an optics issue and probably due to your farsightedness.  The mirror is essentially reflecting light from distant objects plus the distance between your eyes the mirror (i.e., everything is distant viewing).  The LCD screen, however, is taking images from distant object, going through some processing, and then projecting it at the relatively short distance from your eyes (i.e., a relatively short viewing distance).  Depending on your correction, putting your reading glasses on might bring the LCD into focus – but you’d lose focus on the sideview mirrors.  Indeed, I wear progressive lenses; reading viewing at the bottom, distance viewing at the top.  This is good for driving because dashboards are down and windshields and mirrors are up.  But an LCD screen above my head would require me to tilt my head a lot to get my reading correction onto the screen.
Try this.  Stand a foot or two away from a mirror in a bright space.  Hold something with writing at the mirror; it’s probably fuzzy without your reading glasses.  At the same distance, now hold the object with writing beside your head and look at it through the mirror.  By doubling the viewing distance (object to mirror to you) the writing is probably crisper (albeit reversed).
Or think of it this way.  If you had a picture of a distant mountain on an LCD screen near your head, you would need glasses to see it clearly.  If it was a mirror at the same distance, with a mountain in the distance, you would not need your reading glasses.

This fits with something I was sensing intuitively, that I look into mirrors, but at screens. The image and light on a screen is generated and displayed right at the surface of the screen, but a mirror is actually bouncing light off the surface in such a way that

“A reflection appears to be the same distance from the “other side” of the mirror as the viewer’s eyes are from the mirror. Also, when light is reflected from a mirror, it bounces off at the same angle in the opposite direction from which it hit.”

…which just means that when I see a car in the rearview mirror behind me, to my eyes it’s the same as if I was looking directly at that car at a distance directly. I’m not looking at an image of the car on a flat plane four inches from my face, I’m actually looking at the car with my distance vision, because that’s how far the light actually traveled.Mir Mirrordiagram

Does that make sense? I don’t have to re-focus from distance vision to close vision because the mirror is distance vision.

Dr.Pratt elaborates on this a bit more:

Mirrors work differently because they are reflecting distant light.  Imagine you’re sitting at your desk looking at a computer monitor.  The monitor might be 2 feet away from you, and for that you need your glasses (well, maybe not now, but likely eventually).  If we replace the monitor with a mirror, it’s reflecting whatever is behind you, say that back wall of your office some 10 feet away.  So 10 feet between the mirror and back wall, plus another 2 feet between you and mirror, and your viewing distance is 12 feet.  At that distance, you don’t need glasses to see in the detail the back wall in the mirror.  It’s all a matter where the light information is coming from; a nearby screen or reflected from a more distant object.

Oh, here’s another way to think of it.  Suppose you have a mirror that has a sticker on it. You stand 5 feet from the mirror, looking straight at it.  Your image is 10 feet away; 5 from you to the mirror and 5 from the mirror back to you.  The sticker viewing distance is only 5 feet (from the mirror to you).  So depending on one’s eyesight, one might be in focus and one out of focus. The LCD is like a sticker on a mirror.

So, even if you have perfect vision, looking from the windshield to the LCD screen of the digital mirror will still require your eyes to re-focus, like they do when you look down at your dashboard, and that’s not nearly as seamless as you’d think. You notice it, partially because most drivers are used to having no need to re-focus going from windshield to mirror and back.

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There’s more to it, though; it’s not just an issue of focal distance, as screens have other factors in play:

Issue #2: Transitions from LCD to windshield are hard. I’m guessing that the reverse transition, from windshield to LCD is much easier. This may be due to the attention capturing properties of the LCD.

LCD screens are very luminant with lots of light energy, and changes in luminance are very good at automatically capturing our attention. For another driving example, normal roadside billboards are pretty easy to ignore, but LCD billboards are much more distracting. Depending how much light energy the LCD is pumping out, being right near our focus of gaze (unlike the LCD center consoles with are away from where we should be looking), these things might be harder to disengage attention from that a mirror.

After all, the mirror is just reflecting light from outside the car; it’s the same luminance out the front and back windshields). Turning down the contrast of the LCD should equate luminance inside and outside, thus reducing the transition difficulties.

This is a big factor, too, at night more so, as the LCD screen is actually a light-generating source. And, if that’s not enough, there’s the issue of why the image is a bit disorienting from the digital mirror as well, even when you can see it clearly. The good doctor has an answer for this, too:

And yes, only a camera mounted on the rearview LCD monitor would give the same perspective as a rearview mirror.  A camera anywhere else would give a different perspective. Now that could be offset with some fancy processing of the images, but do to so would be a lot of computing power.  With enough practice, the new camera perspective would come to seem normal.

That one is a bit obvious – a camera on the back of the car has a different perspective and field of view than one mounted at the point of the mirror – but it’s definitely a factor in why using these digital mirrors feels so off.

Not having your view blocked by headrests or cargo is good, and having a wide field of view is good, but it’s still a view that’s discordant with where you are, as you drive, and that shift is a bit disorienting. This particular issue is likely the easiest one to get used to, but it’s still not great, and doesn’t feel as seamless or natural as a conventional rearview mirror.

It’s an interesting problem, why digital rearview mirrors suck so profoundly, and it’s not really the sort of problem that better technology can fix, because it’s an issue with the physics of light itself.

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What complicates things even more is the fact that similar tech, rearview cameras used when reversing, don’t suffer from the same issues even though they’re essentially the same basic technology. The reason is that we don’t use them the same way. A back-up camera is focused on for the short period while reversing; it’s not glanced at with regularity over a long period of time like a rearview mirror is, so the transition issues just don’t come up.

Despite how great they sound when described or shown in a promotional video, I’d have to suggest that anyone, especially if you need reading glasses, should save their money and stick with good old cheap real rearview mirrors. Digital ones are just the wrong application of too much technology in a place where it doesn’t work.

I don’t know why or how these ever made it past the initial testing phase, though I suppose you could get used to them, and maybe in some cases the benefits could outweigh the negatives. But I’d encourage everyone even considering one to get in a car that has one and try it out, so you can actually feel what I’m talking about here, because words can’t really describe the suckiness properly.

Until Toyota or some other carmaker changes the fundamental nature of light, these overcomplicated non-solutions to the problem of poor rear visibility are best avoided.

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Cheery Swede
Cheery Swede
2 months ago

A while back test drove the Renault Megane e-Tech. The digital rearview mirror was the deal killer. But couldn’t put my finger on it. But this explains exactly what I felt when trying to use it. Thanks Torch! Excellent write-up. Now, looking at the R5… (please, please, analogue mirror!)

Last edited 2 months ago by Cheery Swede
Pip Swann
Pip Swann
1 year ago

The big scientific thing that has not been mentioned is “depth of field” If you focus on an object 100metres ahead and then move your focus to 50metres ahead you’ll possibly not be aware of your eyes refocusing. However were you to focus on an object at 1metre and then change to 51metres then a massive refocusing operation is apparent even the the distance difference is still 50metres. This is down to the depth of field differing.
So, this would come into effect when the distance from the monitor is greater, as it is in a truck or a motorhome.
The Toyota that you’ve featured gives a different experience, maybe, to a large vehicle due to this factor.
I’m not a scientist at all but I do remember this (and not much else) from gcse physics and I’m considering a mirror mounted monitor for my campervan.
Your input in your feature is a vital consideration in my decision whether or not to go for it. I’ll have to witness it for myself I reckon.
Thank you
Pip.

Brent Hays
Brent Hays
1 year ago

The “Full Display Mirror” system claims to act as a regular mirror OR as a digital mirror, “with the flip of a lever”; I have no idea whether or not it works, just tossing it out there:
https://www.fulldisplaymirror.com/about/

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