You Can Spot a Star Millions of Miles Away… So Why Can’t You Read a Street Sign at Dusk?
A heads-up before you dive in: this is a deep one. The topic of contrast sensitivity is genuinely complex, and I have done my best to explain it in plain language with interactive tools you can actually play with. If parts feel dense, I would encourage you to stick with it, take the included vision test, and spend some time exploring your own results. If you make it through and truly understand what is happening, I can promise you will walk away knowing more about the nuances of human vision than many eye care professionals do.
Imagine you're lying in bed on a summer night and you hear a low thrum coming through the walls. Bass. Somewhere a few blocks away, a band is playing. You can detect that music is happening, the deep bass frequencies travel far and pass through walls easily. But can you name the song? Not a chance. The high-frequency details like the lyrics, the melody, the crisp snap of a cymbal, all got absorbed by distance and brick and air long before they reached you.
Now walk a block closer. You can make out a rhythm, maybe even the genre. Closer still, and suddenly you recognize the chorus. The song snaps into focus, not because the band got louder, but because your ears finally received enough of the ingredients to reconstruct the whole thing.
Your vision works the same way. And just like music, what you see isn't a solitary signal but a recipe made of many ingredients. Some of those ingredients travel far and are easy to pick up. Others fade quickly. The question isn't just "how small can you see?" It's: which ingredients can your visual system still pick up?
That question is what the Contrast Sensitivity Function measures. And it reveals something that a standard eye chart never can.
What 20/20 Actually Means
When an optometrist says you have 20/20 vision, they mean you can read a specific line of letters on a chart from 20 feet away. Each letter on that line is designed so that its critical features, like the gap in a C or the crossbar of an A, subtend 1 arcminute of visual angle at your eye. One arcminute is 1/60th of a degree.
To picture this, imagine drawing two straight lines from your eye to the top and bottom of the letter. Those two lines form a narrow triangle, with your eye at the tip and the letter forming the base. The angle at the tip of that triangle is the visual angle. That angle, not the physical size of the letter, is what we are measuring. A small letter up close and a large letter far away can produce the same visual angle if that angle at your eye is identical.
So 20/20 is really a statement about angular size. It tells us the smallest detail that creates an angle of about 1 arcminute at your eye that you can reliably resolve under ideal conditions: black letters, white background, good lighting.
It is a useful number. But it only measures one thing: the finest detail your eye can distinguish when contrast is essentially perfect.
Here's where it gets strange.
The Things You Shouldn't Be Able to See
If 20/20 vision means you can resolve details down to about 1 arcminute, then you shouldn't be able to see anything that subtends less than that. But you can. Easily.
Stars. The nearest star beyond our sun is so far away that it subtends roughly 0.00001 arcminutes — thousands of times smaller than the resolution limit of human vision. Yet you step outside and see thousands of them.
A golf ball at 200 yards. At that distance, a golf ball subtends barely 1 arcminute which right at the 20/20 threshold. Yet experienced golfers routinely track their ball at 250 yards and beyond. If 20/20 defined the boundary of what we can see, that ball should be invisible.
A car on a long desert highway. At 5 kilometers, a sedan also subtends about 1 arcminute. Yet you can spot that car with no difficulty whatsoever and you might even be able to tell it's a truck, not a sedan.
Something else is going on. The eye chart measures resolution, whether you can tell a B from an 8, an F from a P. But most of the time, we aren't trying to read letters. We're trying to detect things. And detection follows completely different rules.
Detection vs. Resolution
Here's the key distinction:
Resolution means identifying the fine details of an object like reading its letters, recognizing its shape, distinguishing it from something similar. This requires your visual system to pick up the high-frequency information in the image: the sharp edges, the fine lines, the small features that make a B different from an 8.
Detection means noticing that something is there like a bright dot against a dark sky, a white speck on green grass, a silhouette on the horizon. This mostly depends on low-frequency information: a broad patch of brightness that differs from its background.
A star is a point of light against darkness. You don't need to resolve any detail, you just need to detect a bright spot against a dark background. A golf ball on a fairway is a bright blob against green. A car on a highway is a dark shape against pale road and sky.
The eye chart completely misses this distinction. It tests only resolution, under only ideal contrast. It has nothing to say about whether you can detect a pedestrian in fog or read a highway sign at night.
But how does this actually work? How can the same visual system that struggles to read a sign at 100 feet easily spot a white ball at 600 feet? To understand that, we need to look at what an image actually is.
Every Image Is a Recipe
When light reflects off a golf ball and travels to your eye, it creates a tiny image on your retina. That image seems simple — a white circle on green. But it's not simple at all. Like a chord in music, it's actually made up of many overlapping waves.
Not light waves. We're not talking about the electromagnetic spectrum here. We're talking about patterns of brightness across the image.
Think of it this way. Take any photograph and draw a line across it from left to right. Now plot the brightness of each pixel along that line. You'll get a squiggly curve (bright here, dark there, bright again). That curve, like any curve, can be broken down into a combination of smooth sine waves. Some of those waves change slowly (the brightness drifts gradually from light to dark). Some change rapidly (sharp edges where brightness jumps).
This is the same math that breaks a musical chord into its component frequencies. A chord is "just" an A and a C# and an E, all at once. An image is "just" a stack of brightness waves at different frequencies, all at once.
The slow-changing waves are low spatial frequency. On a CSF chart, they live on the left side of the x-axis in the low cycles per degree (cpd).
The fast-changing waves are high spatial frequency. They live on the right side, the high cpd.
Every object you look at, no matter how big or small, contains both.
Why a Tiny Golf Ball Has Low-Frequency Content
This is the part that feels paradoxical. A golf ball at 200 yards is tiny . It’s barely a speck. How can its image contain low-frequency waves?
Here's how. Picture the image of that golf ball on your retina. Now think about what's happening to brightness as we scan across it:
At the edge of the ball, brightness changes fast. We go from dark green grass to bright white ball in a very short distance. That sharp transition creates high-frequency content. It's the visual equivalent of a cymbal crash. Fine detail, rapid change.
But once we're on the ball itself, what happens to brightness? Almost nothing. The ball is white. It stays white across its entire surface. That gentle, sustained patch of uniform brightness is a low-frequency signal — it's the bass note. Slow, broad, easy to detect from far away.
The edge tells you the shape. The sustained brightness tells you something is there.
This is exactly like hearing that bass from blocks away. The low frequencies — the broad brightness differences between the ball and the grass — travel well. Your visual system can pick them up even when the ball is far away and tiny. The high frequencies — the sharp edges, the dimple texture — fade out with distance. At some point, you can detect the ball (you still hear the bass) but can't make out its dimples (the lyrics are gone).
Your CSF Curve: A Personal Sensitivity Profile
This is where the Contrast Sensitivity Function becomes powerful.
Your CSF curve maps how sensitive your eyes are to each spatial frequency — each "ingredient" in the recipe. At low frequencies (left side of the curve), it measures how well you detect broad, gradual patterns. At high frequencies (right side), it measures how well you resolve fine detail.
The curve has a characteristic shape for humans: sensitivity rises from low frequencies, peaks somewhere in the middle (typically around 4–8 cpd), and then falls off at high frequencies. The high-frequency cutoff is essentially what the 20/20 eye chart measures. But the chart ignores everything else, the entire rest of the curve.
And the rest of the curve matters enormously.
The Recipe Test
Now, back to our recipe analogy. Every object, at every distance, under every lighting condition, can be broken down into its sine-wave ingredients, each with a specific spatial frequency and a specific contrast.
If your CSF curve is above all of an object's ingredients, you can see every detail. You can read the letters, count the dimples, identify the make of the car. You can resolve it. You heard the whole song. Bass, melody, lyrics.
If your CSF curve is above some but not all of the ingredients, you can detect that something is there. You see a speck on the fairway. You see a shape on the road. And if you have context: I'm on a golf course, I just hit a drive. Your brain fills in the rest. Detection plus context equals recognition. You heard the bass and the rhythm, and you know this band only plays one song in that key.
If your CSF curve is below all of the ingredients, the object is invisible. The contrast at every frequency is too low for your visual system to pick up. The concert is too far away, the walls are too thick, and even the bass doesn't make it through.
This is why two people with identical 20/20 acuity can have completely different real-world vision. One might have a strong, tall CSF curve that easily detects pedestrians at dusk. The other might have a curve that passes the eye chart but dips in the mid-frequencies — and they struggle to see a person in dark clothing on a dark road. The eye chart would never know the difference.
See It for Yourself
I built the CSF Explorer to make this tangible.
At the top of the screen, paste the results from your CSF test. The explorer loads your personal contrast sensitivity curve, which is the exact shape of what your eyes can and cannot detect.
Then choose a real-world object: a license plate, a golf ball, a pedestrian, a highway sign. Set the distance. Choose the conditions (daylight, dusk, fog, rain). The explorer breaks that object into its sine-wave ingredients and overlays them against your curve.
Green means you can see it. You've got all the ingredients, full resolution. Yellow means you can detect it but not resolve the details. You know something's there; context will do the rest. Red means it's invisible to you at that distance and contrast. The recipe is missing too many ingredients for even detection.
It's your vision, tested not with abstract letters on a wall, but with the actual objects and conditions you encounter every day.
Your eye chart says you're 20/20. Your CSF curve tells you what you can actually see.
This tool is for educational and informational purposes only and is not a substitute for a professional eye examination. Results may vary based on screen calibration, ambient lighting, and testing conditions. Do not use these results to make medical decisions. If you have concerns about your vision, please consult a licensed eye care professional.
Dr. Robert Burke is an optometrist at Calgary Vision Centre. The thoughts, opinions, and analogies shared above are intended for education and entertainment purposes only (think of them like a friendly explainer, not a personal consultation.) Every set of eyes is different, and the right testing protocol depends on your specific vision needs, health history, and lifestyle. So if you're experiencing symptoms or just have questions about your vision, don’t rely on internet content alone, talk to your optometrist or health care provider directly. We’re here to help, but nothing beats an in-person exam with someone who knows your eyes.