- Focal length (Wikipedia)
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system’s optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system’s optical power.
- Big Four Mountain (summitpost.org)
Big Four has big fame in the Northwest. At least that’s what I’ve always thought. Its fame stems from two seeds of awe: the stupendous North Face and the ice caves at the foot of the North Face. The North Face is plainly visible from the highway (Mountain Loop Highway) at several locations and from the viewpoint/picnic area near the highway. This face is 4,000 ft high. It rises like a palisade from the ice caves in a series of high-angle ribs and gullies. The ribs rise up to form five distinct nubs that look like brass knuckles. Indeed the symbolism is apt, for there is no easy way to the top of this peak and it might beat you up if you try. The peak is everywhere rugged. The easiest route is from the east (from Dry Creek). The North Face relents with difficult ice climb routes but it is generally avoided in summer (loose rock). The NE Ridge contains a hard rock route called the Tower Route.