Mr Vertigo wrote:

I'm a bit unclear on how the mm on a lens relate to its effects (i.e. the difference between a 200mm vs a 75mm vs a 30mm).  I understand that the higher the number the narrower the field of vision (obviously) but I was wondering whether it denotes any other effects as well (such as minimum focal distance).  I've been confused by this for a while.

Okay. The higher the number, the more "zoomed in" (the real term is 'longer') the lens is. The longer the lens is, the narrower its field of view is. However assuming you have the space, you can get basically the same field of view you got on a wider (opposite of longer) lens by placing your camera further back. The end result is a shot with similar composition that is more compressed; it has less of a sense of depth to it. Depth of field also becomes more shallow as you get further back, although it flattens each plane out somewhat, so for instance your subject that is in focus may actually get to be more completely in focus as the camera is further back on a longer lens.

Here's a comparison image of the width of the field of vision on various lengths of lenses, standing in the same spot. The image in my previous post in this thread illustrates the depth effects I've talked about as achieved by moving your camera further back as your lens gets longer.

This leads to some paradoxes in lens selection. For instance, conventionally if you want an indoor space to look bigger, you shoot it with a wider lens so that it stretches further away from us into the distance. And yet, if you photograph an outdoors, mountain landscape hovering over a city, you may need a long lens to achieve the sense of scope, because otherwise your mountains will be dwarfed in scale compared to the city in the foreground. A longer lens, further back will allow for a sense of scope wherein the mountains are huge and loom over the skyline by compressing the space.