The Official Blog of Braden P. S. Leigh

I recently realized something interesting about after images. You know, when you look at something for awhile and an image of it seems to get burned into your vision, but in the opposite colors.

If you don’t know what I’m talking about, here is an example. Give it a try.

Stare at the white dot in the middle of this flag. Try to keep your eyes and head still. After about 30 seconds, look at a white wall. Blink a few times.

Did you see it?

Another good example is when you look at a bright light and it leaves a spot in your vision. Or, if you’ve been on your computer for awhile, you might be able to close your eyes and see a faint rectangle where your screen has been.

The traditional explanation for this is basically: our eyes get tired (and/or damaged). For example, if part of our vision has been green for a long time, the corresponding area of our retina gets tired of seeing green, which causes that part of our vision to appear more red (the opposite color).

This would make sense except: this phenomenon actually helps us see better. I think our eyes are doing this on purpose all the time.

What I mean is, if there’s something really bright in our vision, our eyes will put a spot over it that makes it less bright. If our world is very orange (like it often is under artificial light), our eyes will put a blue after image over everything so it doesn’t appear orange. This allows us to resolve colors with much more precision.

For you photographers out there, it’s similar to adjusting iso and white balance on a camera. But unlike cameras, our eyes can do it differently for different areas of the image.

Pictures will make explaining this much simpler. It’s easy to do an experiment at home with a cfl (spirally) light bulb (It works with any type of light, but if you’re looking at a clear bulb through which you can see the filament, give it some extra time because that filament is very bright). Try this and tell me how it goes:

When you first look at the bulb, it will probably look something like this:

It’s too bright to see any detail on the bulb. Your eyes are used to seeing the room around you, and the room around you is quite dark compared to the bulb itself. This puts the bulb far outside your visible range of light, rendering it a nondescript blob.

But! If you keep your eyes on it and stare at it for a minute or so, you will find it eventually looks like this:

You can see it now! You can see detail in everything. The ceiling, the bulb, it’s all clear.

Now, we all know about how our pupils dilate and contract to accommodate different levels of light, but something different is happening here. If our pupils contracted when we looked at the bulb, it would look more like this:

The whole world would get darker. I think you will find this isn’t what happens. The room will stay as bright as it was and only the bulb will change. This is something different.

It’s the after image. I dub the spot that forms over the bulb your ‘see really bright things spot’. And indeed if you now look at something that isn’t so bright, say your computer screen, you’re going to find it very difficult to see anything in that spot. But if, while you still have your spot, you look at another bright light bulb then it will also be clear.

Of course if you stop looking at bright things for awhile, your vision will readjust to looking at dim things and your ‘see really bright things spot’ will fade away.

It’s easy to confirm that it is the spot doing it. While you still have the spot in the middle of your vision look just to the side of the bulb. Do it so you can still see the bulb in your periphery but the spot isn’t covering it anymore. It will be too bright to see any details on again.

Also (once the after image from the first experiment fades away) pick a point just to the side of the bulb and stare at it. Use your willpower not to look directly at the bulb. The after image will, of course, form over the bulb, so it won’t be in the center of your vision this time. After awhile you will be able to see the details of the bulb in that spot in your periphery, but, if you look strait at it, it will be too bright again.

It works something like this:

This thing that people have always looked at as a nuisance is actually a relic of something quite helpful. Of course the only reason we see after images is because our eyes can’t adjust fast enough for our ever changing field of vision. It would be nice if we didn’t have after images, if our eyes were always perfectly adjusted to whatever we were looking at, but I’m sure it is some kind of chemical processes that takes time. With that in mind, I think it’s super neat that our eyes can do it at all. It’s a good reminder of how far cameras have to go before they approach the awesomeness level of our eyes.

I feel like this is something people, at least eye doctors, should be aware of, yet, as far as my googling can tell, no one is. I’m not sure what to do with it. So…here’s a blog post!