SiuBLoDau

22# mucc1928

If there is a lens that covers every need, then most camera manufacturer will close down for sure.  LOL

The focal length ( in mm ) is another indication of how much angle it covers.  See here

http://www.usa.canon.com/app/html/EFLenses101/focal_length.html

The aperture, in F, is the diameter of the 'hole' of aperture wide open when divided by the focal length ( e.g. 50mm f1.2 means the max hole is 50mm/1.2 = 41.67mm in diameter ).  The larger the shallower the depth of field.

Some more ideas on Depth-of-field:

http://en.wikipedia.org/wiki/Depth_of_field

It will be very helpful to understand what is meant shutter speed, aperture and focus length and how they affect your picture.  No matter what camera / system you use, those basics will last forever!!!

My 2 stupid cents.

SiuBLoDau

mucc1928.... to decipher a Canon lens:

EF 55-200mm F/4.5-5.6

EF = the mount.  All nowadays Canon lens are EF ( for full frame ) or EF-S ( for non-full frame ) lens.  Older manual focus are FD lens.

55-200mm = focal length = angle of view.
F4.5-5.6 means aperture at 55mm is f4.5, while 200mm is f5.6

Depth is a combination of focal length and aperture.  The longer the focal length ( e.g. 200mm ) and the larger the aperture ( i.e. a small number, say, f1.2 ) will give you the most shallow ( blurred ) result.  

I would say, to make things less complicated, the cheapest way to see what sharpness and shallow depth of field are -- 50mm f1.8.  Can't go wrong.  It is so light weight that you just get a lens pouch and throw it in your backpack -- you can still have your 18-55mm for landscape, and when you need a portrait lens just search your backpack lor.  That 55-200mm is not expensive -- just the 50mm f1.8 is too cheap.

Well.. this guy got a web page that compares many lenses and how they render the background:

http://www.kenrockwell.com/nikon/85mm-bokeh.htm

Oops... talk too much I think :S