When we’re thinking about vision, the central question is always: given a certain object, can we see it?

Given a certain object, can we see it?

We traditionally break that down into three main object parameters. Namely:

• Is the object big enough? Is the finest-detail part of our vision able to pick up that level of detail?
• Is the object bright enough? Is our vision able to detect objects that dim, in that level of lighting?
• Is the object bold enough? Is our vision able to detect the object standing out from its background? That is, does it have enough contrast?

To these, we need to add a fourth parameter:

• Is there anything getting in the way of the object? Do we have an unobstructed view? Technically put, is it within our visual field? Is it in view, or out of view?

I know that this last one seems kind of silly (I mean, of course we can’t look through walls, or see things that are right behind us), but it becomes a really important question to consider when we are dealing with a visual field that might have blind areas.

Let’s go through the classic three parameters in turn. We’ll do more — a lot more — on fields later.

When we ask the question “How good is my vision?”, this is the first parameter most people think of, particularly with reference to “How far down can I read on the eye chart?”

As we covered earlier, Our ability to discriminate fine detail is measured by Visual Acuity (VA). Whenever you see a notation like 20/20, 6/6, 20/400, 6/36 and so on, this is a measure of visual acuity. Remember, visual acuity not a test score. 20/20 is not like getting 20 out of 20 on an exam — it doesn’t mean you have ’20 out of 20′ vision. All it means is that you can see at 20 feet (the first number) letters of a size that a ‘normal’ eye can see at 20 feet (the second number).

If your ability to see fine detail is something other than normal, the second number will be different. For instance, a person with VA 6/24 (which is the same as 20/80) isn’t seeing so well, because at 6 meters they can only just see letters of a size that people with normal vision could see much further away, at 24 meters.

It’s important to be clear with what a visual acuity measurement tells us. It tells us how good your very best bit of vision is, when given beautiful black and white single letters in a well-lit room. That’s it. It doesn’t tell us anything else — and there’s a lot more we need to know.

VA is actually a good indicator of overall vision in a healthy eye, which is why optometrists and doctors use it. If you just have optical blur (you’re short-sighted or have some astigmatism), your VA will be reduced, and how much it’s reduced will be a reliable indication of how strong your glasses will need to be. VA is the main measure used in determining whether you have to wear glasses for driving, and is quite valid for that, for healthy eyes. 

Visual acuity is only a good indicator of overall vision in a healthy eye.

But I need to repeat: in many eye conditions, high-contrast single-letter well-illuminated detail vision is one of the last things to be affected. If you’ve got an eye condition/disease and your VA is poor, you definitely have an impairment. But having a good VA does NOT mean you have good vision. You can have a severe vision impairment and still have a good VA.

Many of my patients express frustration that they go to their eye care professional for help with what feels like very poor vision, but they are told they still have very good vision, based purely on the fact that they can still read a long way down the chart. Sometimes it almost feels like gaslighting. When I use tests that demonstrate there are problems with other aspects of their vision, it can be a relief.

Eyes are better than any camera

The human eye is able to see over a remarkably wide range of illumination, from a bright sunny day all the way down to a moonless night. Even the very best cameras can’t match the eye’s ‘dynamic range.’ That’s not to say that our eyes see equally well in all light conditions though, there is definitely an optimal range.

All light is detected by special cells in the retina called photoreceptors. One of the reasons we have such a good range is because we have two different systems of photoreceptors.

Rods are for night vision

The first, and simplest system is made entirely of photoreceptors called rods. They work best in low light conditions, and are responsible for our night vision. They are spread fairly evenly across our retina, but there aren’t any at all in our fovea (which is why, if you’re looking at a faint star in the night sky, you’ll see it better if you look a tiny bit to the side of it). Rods are not densely packed, so they don’t provide much in the way of detail vision, but they’re good for getting around in low light conditions.

Rods don’t work at all in bright conditions, or even in what most people would consider normal lighting, so people who have conditions where only their rods are working need to wear dark glasses to see.

Rods can’t see colour at all, only the overall brightness of the colour, so our night vision is just shades of grey — just like converting a colour photo to black & white. And they can’t detect red light at all, so red objects appear black.

Cones are for everything else

Our vision in normal light conditions is handled by a more complex system of three different types of photoreceptors known as cones.

Each type of cone is sensitive to a different colour of light. The fact that there have different sensitivities allows the retinal nerve fibres process the information from all three types of cone together to determine the colour of an object, so this system gives us our colour vision.

Cones are spread fairly thinly across most of the retina, but have a much higher density in the macula, which is how the macula is able to give us fine detail vision. The very densest area of cones is in the fovea, where they are packed absolutely as tightly as possible, in order to give us that one spot of ultra-high detail vision.

Cones have evolved to work best in daylight conditions. They don’t work so well in lower light, and they drop out completely in very low light conditions (such as night-time), which is when we rely on the rods. Vision from rods is reasonably poor though, so we prefer adding extra light to our environment so we can see with our cones instead — we switch on the light, we use a torch (flashlight), we turn on the car headlights, and the authorities switch on the street lighting at night.

Glare — too much of a good thing

We use the word glare to describe any situation where there is too much light, but it’s worth considering the differences between various types of glare.

Ambient glare is when everything is simply too bright. At a certain level vision becomes less comfortable, and at an even higher level the cone photoreceptors just get overworked and can’t keep up (we say that they are ‘bleached’). Simply using sunglasses works well to bring things back to a light level that is comfortable, or we move somewhere else, into the shade or inside.

Spot glare is when there is something uncomfortably bright in otherwise comfortable surroundings. Good examples include driving towards the sun when it’s low in the sky, seeing the sun reflected off the windscreen of the car ahead of you, or talking to someone who is silhouetted in by a bright window. You can wear sunnies, but really the best thing to do is to move (yourself or something else) to get the bright light source out of your field of view — in the car you’d put down the visor, or move your position relative to the person you’re talking to so the window is no longer directly behind them.

Veiling glare is when bright light illuminates something that is on a clear surface in between you and what you’re looking at. An example would be when we have scratches on our glasses, or sunglasses, or even the car windscreen (windshield). It might not be too bad, but when direct sunlight strikes the scratches they reflect the sunlight around, and it’s hard to see past them to whatever we are supposed to be looking at — especially if that thing is dim. You could try wearing darker sunglasses, but that won’t help much. What really makes a difference is to shield the glasses or windscreen from the sun — if it’s your glasses, hold your hand up to shade them, or wear a hat. If it’s your windscreen, it’s time to replace it.

Contrast — Does it Stand Out Enough?

Contrast is about whether the thing you’re looking at stands out from its background, or just blends into its surrounds. Lots of animals use low contrast to camouflage themselves against predators. But flowers and fruit use high contrast to stand out and attract animals to them.

Mostly, we find things easier to see if they have good contrast. The best contrast is black and white, but sometimes that can appear a bit stark, and a softer contrast is used for comfort. Pale colours and shades of grey can feel more refined, pleasant and artistic, but when things become too pale they can become difficult or impossible to make out.

It’s frustrating when we encounter (say) food labelling text that is black print on a grey background, or a bank form all done in pale blue on white. This is just careless design. An important part of inclusive and thoughtful design is avoiding low contrast whenever possible, even if it looks arty/cool.

I once attended a lecture by a world-renowned glaucoma specialist. He illustrated some of his charts with yellow labels on a white background. None of the audience could read it. I was amazed that he — an optometrist! — could make such a careless design choice.

But other tasks are low contrast by design. For instance, when sewing clothes people often want to use white thread on white cloth, or black thread on black cloth. It’s not bad design to do that, but it can be very challenging to see.

Even those of us lucky enough to have perfect vision still experience situations in which our vision is not good enough for what we want to do. Of course, that’s not vision impairment, so let’s call it vision insufficiency. And before we move on to thinking about true vision impairment, let’s see what we can learn from these annoying situations.

This is a microfiche reader, a relic from the time of rotary phones and cassette players. There are probably a lot of (younger) readers who have never encountered one. In the days before extensive digitisation of resources, libraries would deal with the problem of storing vast quantities of newspapers, magazines and other documents by taking very high resolution photos of them, and then storing the miniaturised slides of those photographic images, which took up only a fraction of the space.

The reason I’m showing this device is that those images were truly tiny, an entire page being much less than a centimetre high. There was no way our eyes could see such fine detail, so we used these microfiche readers to project a highly magnified image of the slide on to a screen. That was how we dealt with encountering detail that was too small — we used a device that made it bigger.

This is an extreme case, but we’ve all encountered unreasonably small legal fine print (presumably they don’t want you to read it), or a splinter in our finger, or an interesting-but-tiny insect.

“It’s too far away” is just another variety of “it’s too small“. Things that are far away look small, and so we go closer to make them look bigger.

When it’s too small — we look for a way to make it bigger.

We all know this one. When what we’re looking at is too dark, we find a way to make it brighter. Lighting design is all about making sure we have adequate light in our day-to-day lives, but our standards are calibrated around normal vision.

As soon as the light levels drop to a point that we are not seeing comfortably, our first response is to seek extra light. We turn on the room light, or move to a brighter spot, or bring a torch/flashlight to whatever we are trying to see.

It’s worth underlining here — our intuitive (and correct) response is to reach for a light, not a magnifier. This will be an important lesson to keep in mind.

When it’s too dark — we look for a way to make it brighter.

Adjusting contrast is easy if we can manipulate the brightness of the object and the background separately. For instance, if we are trying to thread a needle using black thread, we can make it easier to see the thread by putting a white background behind it. And we might make life easier by choosing to sew with a thread colour that’s different to the cloth we’re sewing. If we’re taking a photo of a person with a bright background behind them, we would use a fill-in flash to make the person brighter.

With print it’s not so easy, because the object is the dark print and the background are part of the same physical object, so the contrast can’t be changed.

So, what can we do when we encounter those examples of text with really poor contrast print?

Our first step is generally to seek out really good light — perhaps take it over near a sunny window, or use a craft lamp to illuminate it really well. This can help, but not because it improves the contrast. It helps because we’re using the absolute optimal level of illumination, the level that gives our retinas peak sensitivity to low contrast.

But if that’s not enough, we need to cheat — we substitute the low contrast text with higher contrast text. This needs hight tech. Cameras are much more sensitive to low contrast, so a camera linked to a computer system can scan the document, process it and then show us a new version, one with brighter background and darker text. That is, we replace the faded text with bolder text.

When text is too pale — we do what we can with light, but nothing can truly make the text bolder. High-tech solutions replace the text with a bolder version.

Again, we wouldn’t normally consider this a problem with our vision as such, but it will be very relevant later when we’re dealing with the issue of blind spots in the visual field, so let’s discuss it a little it here.

I took the above photo at the cinema. It wasn’t a great view. So, what did I do?

I moved seats. Problem solved.

Here’s a related example. Consider this young person with paint-spattered glasses. She’ll see some parts of her visual field just fine, but other areas will be ‘blind’. Still, she could move her head around so she’s looking through the clear parts of the glasses, or just take the glasses off.

If something is blocking our vision, we try to restore an unobstructed line of sight. We move either ourself or the obstruction.

But there are times when that simple solution won’t work — and this is when we can learn a lot. A common example is when we accidentally look at the sun, or when someone takes a photo with a flash and we were looking right at it. Suddenly we have an afterimage right in the middle of our vision, one that gets the way of whatever we’re looking at.

This is one of the few times when we get to experience having a blind patch in our vision. Just a temporary one, but annoying all the same. Our normal ways to fix an obstruction don’t work.

• We can’t move ourselves away from the afterimage.

• We can’t move the afterimage itself.

What do we do? There isn’t anything we can do. We just have to wait until our eyes recover and the afterimage disappears.

Having to wait is frustrating. But now imagine what it would be like if that afterimage never disappeared. That gives you some idea of what macular field loss is like.

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