Fixed Aperture. Aperture remains constant regardless of the lens' focal length. I.e. The Canon "L" series have a constant fixed aperture when zooming.
Fixed Focal Length. Basically a non zoom lens. 100mm, 50mm, 200mm etc.
Fixed Aperture - Normally when a zoom lens goes from wide angle to telephoto the aperture changes. If the camera has an option to fix the aperture value then it remains constant regardless of focal length.
While a basic model might have a single fixed aperture/focus lens, a better version might have three or four lenses of differing apertures and focal lengths on a rotating turret.
Turning Fixed Aperture On and Off
Setting the Zoom Startup Position
Using Digital Zoom
Turning Digital Zoom On and Off
Using Lens Converters
Using Auto Flash
Adjusting Flash Power
Combating Red-eye
Using Fill Flash
Using Slow Sync Flash ...
Typically Mirror lenses are quite slow so you may live with the limitation of a fixed aperture.
Many types of lenses, though not usually those sold for use with EOS cameras, have fixed apertures. Mirror lenses, for example, fall into this category since they lack (adjustable) diaphragms.
It is not the shutter, which is a seperate mechanism which controls how long the aperture remains open. Fixed aperture cameras use apertures where the opening size does not change. Many point and shoot cameras, and pinhole cameras are fixed aperture.
The Sunex 5.6mm f/5.6 fixed focus, fixed aperture circular fisheye was the most interesting thing I saw at the show. (The Nikon D3 and D300 were last year's news.) It's $799 and fits either Canon or Nikon. It's a solid metal and glass lens.
The classic Minox sub-mini had a fixed aperture of f/3.5 (plus a curved film plane), and my Digital Ixus, at least, always shoots wide open (there is an ND filter that is used in strong light; the EXIF data reports its use as a smaller f stop).
Problem 4: Aperture. No problem. The telescope has a fixed aperture. Whatever it is, that's it!
Problem 5: Focus. Also no problem! Simply look through your camera's viewfinder and focus the telescope.
For a fixed aperture, as the EV increases 1, the shutter speed increases one step; for fixed shutters speeds, as the EV increases 1, aperture decreases one f/stop. Long exposures are for (or have) negative EV.
The Light Value System is a method of using the LV/EV on a camera, where the exposure is set by dialling the LV or EV; the camera may then have a fixed aperture/shutter combination, ...
In practice, unless you are dealing with a fixed-aperture lens (many simple point-and-shoot cameras have only one fixed aperture), the aperture of a lens is usually expressed as a range of f-stops.
If you are using a fixed aperture zoom lens, you can zoom in temporarily on the part of the image you want to meter. If you meter off the brightest areas, open the lens aperture between 1.5 to 2 stops.
In Auto Aperture mode the K-x will adjust the aperture if the brightness of the recorded scene changes. In Fixed Aperture mode the camera achieves the same goal by only adapting the sensor sensitivity.
Though a mirror lens ins't an ideal macro lens since you have fixed aperture and can't stop down for DOF, it's nevertheless quite useful in some situations. Getting the EF500/4.5L to focus down to 1.7m would need an awful lot of extension tubes! ...
 See also: Aperture, Light, Lens, Camera, Shutter
  
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