Ooh... so this turns out to be not so simple to define -- it turns out everything runs afoul on the definition of an "average human eye". When you try to look up that definition... you'll find all kinds of answers (been there. done that.)
You'll get definitions based on the eye's ability to "look around" vs. what it can see looking straight ahead. You'll get definitions based on both eyes. You'll get definitions that your ability to notice peripheral movement from the SIDE is much better than your ability to notice peripheral movement from, say, above (something that wasn't necessary for us to evolve because there are many birds that can pick us up and carry us away for food -- I suppose.)
Then I watched Brain Games (it's still on Netflix) and they had a whole episode on how the eye really works. It turns out absolutely nothing like you might imagine. What you see... or rather what you think you see... is a composite image that's been assembled in your brain based on your eye having had an opportunity to "look around". Your ability to see something sharp only exists in a rather tight area directly in front of your eye and if you get very far away from that your eye is very good at noticing that "something" is there, but it's ability to resolve what it is is rather lousy. They proved this to test subjects by having them stare at a target while they swapped people standing off to the sides of the target who were clearly "in" the field of vision but not so well that the person be tested could tell if the person was even a boy vs. a girl (most test subjects performed very poorly.) You ARE quite good at noticing "movement" in your extreme periphery ... but you can't tell what the movement is caused by.
You can try this yourself. Looking at this message thread, look at just one word in any sentence, any paragraph... make sure your eye STAYS focused on that word. Now... can you actually resolve fine detail in the letters a word immediately next to the word you are looking at? I am able to look at a possibly two short words or one long word and resolve it fine detail... beyond that I must let my eyeball move to see the detail.
Consequently, your eyeball scans the scene and your brain is basically building a mosaic image in your head of what it thinks it sees -- and a clever magician exploits that weakness for all it's worth and can take your money all day long if you put them to the test.
ANYWAY... from all the digging I did back when I was trying to find this stuff... the average human eye seemed to be expected to be able to observe a field of view which might be between 40º and 50º and of view depending on the person (even though the Brain Games shows explain that it's not even that good). If you use a 27mm lens on an APS-C camera (which has roughly a 27mm diagonal) you end up with about a 45º diagonal angle of view -- which pretty much fits the average human eye -- whatever that is.
Tim Campbell
5D III, 5D IV, 60Da
