i spy...with my little eye...something that begins with...i!

i spy

At some point, if you've achieved a certain level of mastery in photography, the question occurs to you: why does a camera function so differently from the human eye? More to the point, what are the chief mechanical differences between the human eye and a camera that prevents the design of a more capable camera? Or perhaps your questions are more specific: how does my camera's resolution, in megapixels, compare to my eyes?

eye vs camera

The main problem with addressing this question is that it requires likening things that are not alike. Unfortunately, the human eye and a camera are different beasts that tend to frustrate nearly every attempt at comparison. This is in large part due to the fact that when most people say the "human eye" they actually mean the "eye-brain system," which is far more complicated than just the eye, which is itself already complex enough to do plenty of the frustratin'.

acuity is not "degree of cuteness"

Throwing the brain into the mix creates a problem because it does a lot of "post-processing" on the images that stream in from the eye, giving us a mental picture much different from what the eye itself is actually detecting. Also, the eye has different kinds of vision—in the center of the field of view, in a very narrow range in fact, we see with acuity. Outside that very narrow range, our brain fills in a lot of the details that we think we see from moment to moment, but is actually not being "seen" in the same sense as what's in the center of view. (Of course, this comment will inevitably beget the philosophical discussion: what does it mean to "see," exactly?)

If you doubt that your eyes only see with acuity in a fairly tight circle around the direct center of your field of vision, try this experiment: pick up a book, open it to a random page, and fixate your eyes on a word somewhere in the center. Now, see how many words you can read around that word without moving your eyes to look directly at those words. The words you can make out fall in your acute vision field. (You'll find that if you move the book farther away, you can read more words because your acute vision covers a constant angle of view—this works up until the book gets far enough that the overall level of acuity you enjoy isn't high enough to make out any of the words at all.)

offsides...five yard penalty

The rest of your field of view is in your non-central field (at last, that's what I'm calling it). Your peripheral vision is comprised of the part of your field of view for which your brain does not bother filling in any detail—you're only vaguely aware of it in the visual sense provided it's not moving.

What our non-central vision lacks in acuity it makes up for in motion detection. Hunters often say when you first spot prey in the distance that's fairly well camouflaged with its surroundings as it moves about, don't continue to look directly at it, but look slightly to the side. That way, when it starts moving again you'll see it and you can put it in center vision, but once it stops, look off to the side again. Stargazers often use this trick as well—if you look directly at a faint star, after a couple of seconds you'll begin to question whether it's actually there. But if you look slightly off to the side, your eyeball moves around and twitches enough that it creates apparent motion of the faint star you're trying to see and you can once more pick it up.

(Incidentally—this is the reason why our eyes in are constant motion...if you've ever tried to make your eyes exactly still you know how difficult it is to keep them from twitching. It's because our brain requires that motion to keep the motion-detecting parts of your visual field feeding your visual cortex the detail it craves. You'll also find that if you are able to keep your eyes from twitching for an extended period, ten or fifteen seconds, the level of detail in your non-central vision noticeably falls off, sometimes even fading to black. Even so, it's not obvious until you start twitching again and suddenly see color and some detail spring back in your periphery.)

frustration abounds

No matter what one says about the eye in relation to a camera, someone is bound to argue (and, in some sense, almost certainly be right). It's a useless endeavor to try to figure out a megapixel rating for the eye, or figure out what it's dynamic range is, etc. Whereas it makes sense to discuss megapixels for a camera, the concept of measuring resolution in the eye depends upon the aspect of vision under study.

A more fair comparison would be hooking a camera up to a computer, then periodically having the camera move slightly and snap a shot, then the computer takes it and stitches it into a composite of the entire scene comprised of several such shots pointed in slightly different directions. Add some expert post-processing to clean up the image and fill in details that don't even necessarily show up in the image at the camera's given resolution based on knowledge about the objects in the scene stored in the computer, and you've got yourself a system that's starting to get into a comparable space. As you continue adding to the system to make the camera more directly comparable to the eye, you add more technology that holds roughly the same incongruity with biology that you're trying to avoid by building the system.

but wait...there's more

There is a practical consideration that comes out of this discussion. Just as the fundamental differences between the eye and the camera make such questions nonsensical, there are lesser differences between different cameras that nonetheless even make comparisons between devices difficult.

Those who have deep knowledge about digital photography immediately identify newbies to the field by the first question they always ask upon seeing a camera bigger than a pack of cigarettes: how many megapixels is that thing? Consumers uninitiated to details of photography ask this question to salespeople first as they internally flesh out a mental list of the cost/benefit pecking order. Camera manufacturers know that consumers do this, of course, so to an extent it is a self-fulfilling tautology: the higher the megapixels, the better the camera. Manufacturers will not include a more expensive image processor in a model with a low megapixel sensor even if that low megapixel sensor takes images with overall higher quality than a higher megapixel model, so in practice higher megapixels most often indicates overall better image quality. However, this correspondence of image quality to megapixels is not true in principle, which means that there is the occasional sleeper model that separates itself from its megapixel brethren with which it's been lumped.

The fact is, there is no single measure that sums up the image quality it can produce in contrast to a different model. There are five and six megapixel cameras that produce better images than eight megapixel cameras. Likewise, there are eight megapixel cameras that produce images of vastly different quality from each other. Clearly, if resolution were the be-all and end-all that many people suppose it to be, this would not be the case.