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Feb
13

How the Eyes See

Any discussion of “flat” is incomplete and uninformed without an understanding of how the eyes actually see. I’m not talking about “we see at 30 frames per second” or anything quite as simple as that. I’m talking about understanding how shot calling really works, and how that affects not only what we perceive as “flat” (both from behind the gun, and while observing someone shoot) as well as understanding how best to leverage those effects in our gun setup choices.

You may have read or been told various different things about how we see. The internet is full of various descriptions and analogies about how we see. The most common one is that we see at 30 fps (that’s “frames per second”). In terms of an oversimplification that’s easy to understand, it’s not a bad analogy. It’s just that, though – an oversimplification. The truth is, we see constantly in our field of vision, within limits. What we actually process into what our brain perceives as vision is perhaps another story…

The notion of a 30 fps limit comes from the world of motion pictures, television, and video games. If you take smooth movement and chop it up into a number of steps, you can view those individual steps back and see simulated motion. Show enough discrete steps fast enough, and our eyes get fooled into thinking they’re seeing constant motion. 24 frames per second seems to be pretty effective – every time you see a movie in a theater, you’re seeing 24 discrete steps a second. Well, not quite – you’re actually seeing 24 steps, each shown twice at a total rate of 48 frames per second (more on that in just a second). But, still, 24 discrete steps a second seems to be enough to allow us to perceive smooth, constant motion.

The thing is, it’s not quite that simple. In the case of the movie, what shows up on the screen in between frames is … nothing. Black. The theater is dark, too, which also helps us not really perceive the sudden drop down to the dark. Remember that the movie is really going at 48 fps, though? That’s because you actually can perceive that black frame at 24 frames per second. You can see what appears to be a “flicker” on the screen. So, they double each frame – you still see 24 discrete frames per second, but the flicker rate is double that, and your brain doesn’t process the faster dark frame.

But, what if what appeared on screen between frames what *white* instead of black? Imagine that for a second – you’re sitting in a dark room, and suddenly a bright light flashes on for .02 seconds (48 fps flicker rate). Do you think you’ll see it? Various lab tests show that you can see white frame flicker rates above 200 frames per second! And you can see a photographer’s flash, right? The duration of the light from a photographic strobe is frequently .001 seconds long or shorter. Not only can you see it, but if you’re looking at it, it will frequently stay imprinted on your retina for quite a while. This is actually why video game hardware and software developers are constantly pushing the frame rate envelope – the eyes are still able to see flicker in games running at 200 frames per second.

So, depending on the relative intensity of the image, you can see things that happen a lot faster than 30 frames per second. In fact, the main limitation on how fast you can see is actually the speed of chemical reaction in the rods and cones that are on the surface of your retina. The rods and cones are the actual cells that detect light striking the retina and and convert that signal into an electrical impulse that gets sent up the optic nerves to the brain. The cone cells are primarily what we’re using for shooting. The center of your vision is primarily cone cells. Cones are the reason we can see color, and in fact are quite a bit like the pixels on a monitor. There are 3 different varieties that are sensitive to red, green, and blue light, respectively. We have about two times the number of green cones as we do red or blue cones, making our vision more sensitive to green hues than any other color.

Rod cells are more concentrated around the periphery of our vision. They only see luminance – that is, black and white. They’re smaller than cones, too, which effectively allows them to see more sharply. This is one way that birds see much more crisply than humans do – that eagle with disgustingly sharp vision has retinas that are comprised solely of rods. They trade that clarity for color vision. Rods are also much more sensitive than cones, taking only a single photon to stimulate them (cones take 4-5 photons). This is why you tend to lose color vision in dim lighting – the cones aren’t getting stimulated enough to generate a signal.

Rods and cones are constantly being stimulated, signaling, and then resetting through a series of chemical reactions. Each of those rods and cones are working independently of the others and sending signals to the brain individually. Unlike your TV or computer monitor, where the whole screen updates at effectively the same time over 60 times per second (usually, more than 75 even), your eyes update each discrete pixel constantly and independently. This means that you don’t see discrete frames. So, you can see that any notion of “the eyes see at 30 frames per second” is simplistic at best.

This brings us back to the question – can we actually see the gun cycle? Yes… and no. Most handguns will cycle on their own at a rate of about 1200 rounds per minute, or 20 rounds per second. That’s about .05 seconds for the slide to unlock, move to the rear of it’s travel and stop, then start moving forward again, strip the next round out of the magazine, and return to battery. The thing is, we tend to see that movement as a blur, especially if we’re looking at it with the center of our vision (those relatively slow cones, remember?). Remember that those individual cells/pixels in our vision update constantly and independently, right? As the image of the of the gun in motion projects across the cells of the retina, they all fire and reset on their own schedule, giving us a scrambled look at the gun. The brain reassembles all of those signals as blur. We don’t see things clearly when they move that quickly.

Next up, we’ll talk about how this new found understanding interacts with our perception of the gun… complete with slow motion videos!

About the author

DaveRe

USPSA Grand Master, NRA Instructor, http://re-gun.com/about/

Permanent link to this article: http://re-gun.com/2012/02/how-the-eyes-see/

3 comments

4 pings

  1. Sean says:

    Awesome post Dave. This is the stuff that really intrigues me ever since trying to increase my overall eyespeed and focus. Thanks

  2. FireboltXL says:

    Dave, if I interpret this correctly, you should crank up the brightness of your dot to max so your eye picks it up faster. Or is it the opposite, minimum brightness so you don’t have total dot focus? Great article, thanks for sharing.

    1. DaveRe says:

      John, actually, what you want is to have the maximum possible brightness while still retaining a crisp dot. You don’t want it to flare out, and have fuzzy edges. Even with a very bright, crisp dot, you shouldn’t be shooting with a dot focus – you should always focus on the target – but dot brightness generally doesn’t affect your ability to shoot with a target focus, either way…

  1. DR Performance says:

    2nd part of the "Flat" series – How the Eyes See http://t.co/c8c37qS8

  2. xre says:

    2nd part of the "Flat" series – How the Eyes See http://t.co/c8c37qS8

  3. “Flat” and Calling Shots | Re-Gun says:

    [...] « How the Eyes See [...]

  4. xre says:

    RT @drperformance: 2nd part of the "Flat" series – How the Eyes See http://t.co/eXuobe3Y

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