Excerpt from Practical Approach

I'm having a great time updated our Practical Approach Web site. 
I can see I'll be adding to this for a quite a while — it's quite addicting. I'll keep you updated here as to when new entries are made as well as on facebook.
Example of latest entry: 




ABOUT THE APERTURE

As the aperture is opened, more light enters the camera. The aperture is inside the lens and is made of movable metal leaves that make straight edges around the hole.

The aperture is a variable sized hole in your lens. The size of this hole directly influences how much of the scene in front of you will appear in focus. An artist-with-a-camera chooses his aperture size in the beginning of his image making process. This choice is not taken lightly. Aperture size will ultimately dictate or influence most every other decision that needs to be made — including that of required shutter speed, focal length, flash use, I.S.O choice and more.

This hole can be adjusted via controls either in-camera or on the lens itself (depending on the age of your lens). Each major step is numbered, with each number representing a specific aperture size. These numbers are called f-numbers. The (f) refers to the focal length from which the number is derived.

Every In Focus photographer to know the basic series of f-numbers.  It's actually an easy thing. ... all you really have to do is remember two: 1 and 1.4.  Simply double them alternately and you've got it. Yes, I know that 5.6 and 11 aren't quite right ... but it's close enough and easier to remember than the actual sizes. The other apertures available are fractions of the above.

Often these f-numbers are called f-stops. The word 'stops' hails from photo history. Long before the automatic aperture system was developed photographers had to insert various metal plates into their camera with very specific size holes. If the photographer used smaller holes then the light would be stopped more so than when he would use a plate with a larger hole. Hense the word 'stop' became synonymous with f-number.

Often you'll hear photographers use the words 'stopping up or down' when referring to adjustments made to the aperture. They may also use the word 'step' or the words 'shrink the hole' or 'widen the gap'. All refer to the same thing. For instance if you hear the phrase "I stopped down 5-stops." This simply means that the photographer made his f-number 5 sizes smaller.


Doubling or 'halving' the amount of light is an change of one stop.