actually these buttons are quite standard.. not only for canon..
most advanced camera models will have
extracted from canon eos 1000/ rebel manual:-
Program AE (auto-exposure) mode (P).
Turn the command dial to P. The camera will automatically select
shutter and aperture settings according to its built-in basic
program. Program AE mode uses continuous wind and also evaluative
metering unless you push the partial metering button.
Unlike the green mode you can program shift by turning the main
dial and adjust the exposure compensation by turning shift-main
dial. (see below)
If the cameras maximum shutter speed (1/1000 sec for the Rebel/EOS
1000/1000 QD and 1/2000 sec for the Rebel II/S II/EOS
1000N/1000FN/1000S-QD/1000S-QDP) and the value of the minimum
aperture of the lens flash in the viewfinder then theres too much
light. Either use slower film or put a neutral-density filter in
front of the lens. If a shutter speed of 30 seconds and the value
of the maximum aperture of the lens flash in the viewfinder then
there isnt enough. Use faster film, flash or bulb
mode.
Shutter priority AE mode (Tv).
In this AE mode you set the shutter speed (time) and the camera
automatically sets an appropriate lens aperture for you, based on
the selected metering mode - evaluative (default) or partial.
Turn the command dial to Tv, which stands for Time value. (Nikon
more sensibly refer to shutter priority as S mode.) Turn the main
dial to adjust the shutter speed setting and shift-main dial to
adjust exposure compensation. The camera flashes the aperture value
if youre out of range. (ie: if you havent got enough light itll
flash the maximum aperture of the lens, and if youve got too much
itll flash the minimum aperture of the lens.)
You cant go into bulb mode here - the maximum time value is 30
seconds. For bulb you need manual mode. The shutter values are
expressed as fractions of a second unless the " symbol appears, in
which case the value is expressed as a second. Thus 125 means the
shutter speed is 1/125 of a second whereas 1"5 means the shutter
speed is 1.5 seconds.
Aperture priority AE mode (Av).
In this AE mode you set the lens aperture and the camera
automatically sets an appropriate shutter speed for you, based on
the selected metering mode - evaluative (default) or partial.
Turn the command dial to Av (for Aperture value). Turn the main
dial to adjust the aperture setting and shift-main dial to adjust
exposure compensation. The camera flashes the shutter speed value
if youre out of range. (ie: if you havent got enough light itll
flash 30" and if youve got too much light itll flash 1000 or
2000, whichever is the maximum shutter speed for your particular
model)
You can choose any aperture value that falls within the aperture
range of the lens you happen to have installed. The wide-open
aperture varies a lot from lens to lens - f/1.4, 1.8 and 2.8 are
typical values for fast lens and 3.5, 4.5 and 5.6 are typical for
slower zoom or telephoto lenses. The largest aperture value is
usually printed on the lens itself, and is sometimes an aperture
range on zoom lenses which do not have constant apertures - eg:
f/3.5-5.6. The smallest aperture value on a 35mm lens is usually
f/22.
Metered Manual mode (M).
In this mode you set both the lens aperture and the shutter speed
manually. The camera will assist you by telling you whether it
thinks you have the correct metering based on the selected metering
mode - centre-weighted averaging (default) or partial.
Turning the command dial to M lets you shoot in metered manual. How
the camera behaves in this mode depends on which camera you
have.
The older 1000/Rebel viewfinders display a simulated needle match
mode. This sort of animated graphic tells you if your picture is
likely to be underexposed, overexposed or exposed correctly.
(correct exposure is obviously when the moving rectangle appears in
the middle of the scale)
However, the Rebel II/S II cameras the camera will display little +
and - arrows telling you whether your picture is correctly exposed,
overexposed or underexposed. Minus means that the image is
underexposed and that the camera needs more light; plus means that
the image is overexposed and the camera means less light. Both plus
and minus means that the camera thinks the exposure is correct.
This is not as useful, since the arrows dont tell you how far
youre under or over. This step backwards in functionality between
the version numbers was apparently done for patent licensing
reasons.
Either way, use the main dial to change the shutter speed and
shift-main dial to change the aperture. You can also go into bulb
(long time exposure) mode - its the buLb setting thats one step
past 30 seconds. However, since the camera lacks any provision for
remote shutter release it isnt a very useful feature unless you
rig up a homemade mechanical lever to keep the button pressed
during long exposures. Bulb mode means the shutter stays open for
as long as you keep the shutter release button pressed. Apparently
the name comes from the olden days of mechanical shutter releases,
when youd have a rubber ball-shaped bulb that youd squeeze to
trigger and hold the shutter.
I understand the camera switches to centre-weighted averaging
metering mode when you go into manual mode, which is annoying,
since you cant specify evaluative if you happen to prefer that.
You can always go into partial by pressing and holding the awkward
partial metering button.