The human eye sees a prominent field of view about 43° wide; you can only clearly see things to the side of this field by turning your eyes in the appropriate direction. I believe this is a function of the curvature of the eye, or more precisely, the curvature of the eye's lens, and the distance from the lens to the retina. To be sure, you get visual input from beyond that narrow range; your total field of view is quite wide, perhaps 160° or more, but most of that does not result in a clear image or detailed information for your brain to use. Movement in this area will attract closer inspection from your brain (which requires turning your eyes, as well as your attention, towards the movement); this ability to detect motion at the periphery of your vision is what saves you from being broadsided by little old ladies who see their world through, rather than over, the steering wheel.

As the structure of your eye determines the field of view that you see, the structure of a camera lens determines the field of view that the camera lens "sees". The view through the lens of a camera will be sharp from edge to edge, but there is no peripheral vision to provide additional information. Cameras usually come with what is referred to as a "normal lens" which has a focal length (analogous to the distance between your eye's lens and the retina) of 50 or 55mm. These are called normal lenses because they have a field of view of about 43 degrees, and provide a view similar to the primary field of view of your eye. Of course you can buy additional lenses with longer or shorter focal lengths; those with a shorter focal length are called "wide-angle" because they show a wider field of view, all the way to 180 degrees in some cases. This larger field of view is displayed on the same size film and viewfinder, of course, so things seen through a wide-angle lens look smaller.

I bring this all up because while waiting to buy something in a camera shop once, I was listening to the discussion between the salesman and the customer ahead of me. The salesman was cleaning the customer's camera and lens so it would be in good shape for the wedding the customer was going to photograph the next day. At some point during the conversation, the customer asked the salesman why things looked smaller through her wide-angle lens. The salesman patiently explained it to her, and eventually she went on her way. I expressed some surprise at her lack of knowledge, as she was going to be shooting a wedding, and the salesman said that she had business connections, but didn't really shoot weddings as a living. I later mentioned this to a friend who is a professional photographer and does shoot weddings for a living; I expected that he would deride her lack of understanding of what to us were simple photographic concepts. Instead he responded that he didn't need to know how his car works in order to drive it.

My friend was not entirely correct, as it happens. In the case of the car it is not necessary to be even an incompetent mechanic to simply drive a car. This does not, however, mean that it is unnecessary to understand how the car works. Even if you never thought about it consciously, if you've driven in the snow you surely understand that the accelerator doesn't make the car go faster, the steering wheel doesn't turn the car, and the brakes don't slow the car down. What really happens is that the accelerator makes the drive wheels turn faster, the steering wheel points the front wheels in different directions, and the brakes reduce the speed at which the wheels rotate. If this sounds like merely a semantic difference, the next time you're driving in the snow find a big empty parking lot, then while cruising at forty miles an hour give the steering wheel a good hard turn and slam on the brakes. Actually, you can learn a great deal in this manner, and that knowledge could serve you well someday.

The point I wish to make is that there is a difference between simply knowing how to work things and understanding how things work. When I take a beginner caving (you didn't think this article was about caving, did you?) and they use a carbide lamp, I teach them how to make it work, but I don't teach them to understand how it works; if they enjoy themselves and wish to continue caving they have plenty of time to learn, and if they have a lamp problem on the beginner trip I fix it. Unlike a properly educated and experienced caver, these beginners are still limited by merely knowing how to work the lamp and any other equipment that is new to them. Surprising as it may seem, there are some cavers with much caving experience who are also working with these types of limitations.

Over the Memorial Day weekend I had the good fortune to go caving in Buffalo Creek Cave in Mammoth Cave National Park in Kentucky, as part of a Cave Research Foundation survey party. All of the cavers I was with have much caving experience; three of them (I leave myself out of the present discussion) have extensive experience under their belts, and I am sure that they have an excellent understanding about most aspects of caving and caving equipment. There was also another member of our party, one who has been caving for a few years and has had much experience in that time. This person has done vertical caving here in the northeast, and has attended at least one project caving trip in Mexico or Central America. One might presume that with all that experience this person must understand much about caving; one should, however, be careful as to what kinds of presumptions one makes as one bad experience can ruin your whole day.

Buffalo Creek Cave is one of the park's "small caves", which means that by northeastern standards it is a substantial cave. It has, at present, nearly two miles of mostly walking passage. If a passage is ten feet wide and fifteen feet high with a rectangular cross section one might assume this to be walking passage, which would be a reasonable assumption if the passage didn't have eight feet of water in it. As it happens, for a distance of some two hundred feet there is, in fact, a swimming passage. All of the cavers on the trip knew this ahead of time, and preparations included the presence of three life jackets and an inner tube as part of the necessary equipment. For most of us, preparation also included the waterproof packaging of things we wished to keep dry. Our group swam through this section with only one minor problem - not all five of us had our extra carbide stored in a waterproof container. Now, my beginning cavers probably know that a carbide lamp has a flame coming out of it, and even an electric caver probably understands that this flame is the result of the acetylene gas produced when water and calcium carbide are mixed in the lamp. Perhaps all five of us present in Buffalo Creek Cave knew what causes the flame, but it soon seemed that one of us didn't understand.

Shortly after our swim I noticed the smell of acetylene; my lamp wasn't leaking, but I figured somebody else's was, and I didn't give it much thought. About the time I wasn't giving it much thought, one of the others discovered that her pack was behaving in a most unusual fashion. The pack was becoming slightly tumescent, and had a smell somewhat like rotten eggs; a quick investigation by the pack's owner led to the conclusion that her extra carbide was damp (to say the least). As she proceeded to rummage in her pack she mentioned that the carbide was no longer terribly useful to her, and asked if she should "just dump it in the water to get rid of it"? Before she could proceed to actually do so, one of the others told her not to. I have no idea how much acetylene is produced when a pound of carbide is dumped into hundreds of pounds of water, and I have no idea just how fast all that acetylene is produced. I also don't know how much oxygen there is in three hundred feet of passage that is ten by fifteen feet, and I don't know how much oxygen is consumed by the combustion of the acetylene produced by a pound of carbide. I do, however, know that if she had dumped her carbide in the water one of the two carbide cavers ten feet from her would have had the dubious honor of igniting all that acetylene, and I am sure that all three of us would have had our day ruined. I suspect that we might have had it much worse than merely having our day ruined, as I consider being unconscious in an environment with little or no oxygen to be a very bad thing. As if that was not enough our companion discovered that all of her plastic bags had melted, and her food was also less than useful. Now that her pack contained little in the way of useful stuff, she put her spare batteries in a pocket and left her pack behind. For some reason she was going to leave her Petzl carbide lamp behind until I suggested that the carbide left in the lamp was probably still good for five hours worth of light.

We got through the rest of the trip without major problems, and our intrepid companion came out on a borrowed light source, and according to another companion she did it with the spare batteries for her own light still in her pocket; he had apparently neglected to point out to her that she should have put them in her light.

This whole event, as you may have guessed, played itself out because somebody who knows how to work a carbide lamp seemingly didn't understand how carbide works. A little additional knowledge would have allowed for understanding how things work, and would have prevented the possible detonation of an entire caving party without the need for the intervention of another party member. Similarly, the photographer who understands how lenses work may be able to make better choices of which lens to use thereby taking better pictures, and the caver who understands their equipment will probably be able to use it more safely and enjoyably. The same is true of all endeavors, and I'm confident that you can think of some on your own.