Introducing important concepts that explain how we see, and the everyday experiences that can give us all insight into vision impairment.

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

Given a certain object, can I 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? Is it within our visual field?

I know that this last one seems kind of silly (I mean, of course we can’t look through walls), but it takes on a lot of importance when we are considering tasks like reading or recognising faces, in which it’s important to see not just individual details, but how they relate to each other.

Let’s go through the first 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. 

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.

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 tell see colour. Our night vision is just shades of grey. They can’t detect red light at all, so red objects appear black.

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.

Contrast is about how well the thing you’re looking at stands out from its background.

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 black food labelling on a grey background, or a bank form all done in pale blue on white. Even worse is trying to read an old receipt where the print has faded out almost completely.

Even those of us lucky enough to have perfect vision still experience situations in which our vision is not sufficient for the task at hand. Consider that as, not vision impairment, but vision insufficiency. Before we get on to true vision impairment, it’s instructive to consider situations of vision insufficiency to see what we can learn from them.

This is a microfiche reader. 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, each page much less than a centimetre high. There was no way our eyes could see such fine detail, so we used a microfiche reader to project a highly magnified image of the slide on to a screen. This 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.

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 light to whatever we are trying to see.

It’s worth underlining here — our intuitive response is to reach for a light, not a magnifier.

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

Challenging low-contrast situations are commonly encountered in arts & crafts. If you’re looking at print, it’s most often black print on a white background, but when you’re sewing (for instance) it’s more commonly black thread on black fabric, or white thread on white fabric.

Still, we encounter annoyingly low-contrast print at times, in the form of food packaging or magazine articles that use dark print on a dark background, or light print on light. This is just careless design. An important part of inclusive design involves 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 labels in yellow text on a white background. None of us could read it. I was amazed that he — an optometrist — could make such a careless design choice.

Adjusting contrast is easy if we can manipulate 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.

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

Looking at low contrast documents in better light often helps a bit, but it’s not because it makes the contrast any better — it’s just simply that our eyes function better when we have better light.

Sometimes documents are so faded they seem impossible, even with the best light and magnifiers — for instance, archaeologists and historians sometimes have to try to read extremely faded documents (or that annoyingly faded receipt). To read them, we need the big guns, a computer/camera system that can scan the image and then make the print darker and the paper lighter, which results in a substitute image that is easier to read. That is, we make the print bolder.

When text is too pale — we do what we can with light and magnification, but sometimes the only thing that will make things bolder is a high-tech solution.

We wouldn’t normally consider this a problem with our vision, but it will be very relevant later, so let’s consider it here.

In the absence of having Superman’s x-ray vision, when something gets in the way of what we want to see it (kind of) takes away some of our peripheral field. It might be peripheral field (side vision), as in the case of these sunglasses and the horse blinkers, or it might be more central, as in the case of the big head in front of me at the cinema.

Image Credit

In other cases, it might be a more complex and patchy obstruction. One of the following images shows the view of a face partially obstructed by branches. The other shows a person with paint splatters on her sunglasses that will definitely leave some areas of her vision obstructed.

Image Credit

Whatever the obstruction, our response is the same — we move ourselves, or we move the obstruction. We’d take off the sunglasses or blinkers, or we’d move to a better seat in the cinema or around the branches.

When something’s in the way — we move it, or we move ourselves.

But there are times when that simple solution doesn’t work. An example is one we’re all familiar with — when we accidentally look at the sun, or a someone takes a photo with a flash, and suddenly we have an afterimage in the way of what we’re looking at.

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 will give you some insight into what macular field loss is like.

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