|Quoting airplane09 (Reply 6):|
Not much of a camera expert can you please explain carefully!
Where to start? You need a certain amount of light to create an image. In routine daytime operation the camera metering can determine a combination of shutter speed and aperture that will give the correct amount of light to hit the CCD (the sensor in the camera). The various camera settings, sport/landscape/etc give different combinations. For sports you want a fast exposure (leading to a large aperture), for landscape a small aperture (leading to a slow shutter speed). If you imagine the amount of light to give the correct exposure as a cylinder, it can be a short, fat cylinder or a long, thin cylinder. Short+fat = short shutter speed and large aperture, long and thin = long exposure and small aperture.
There are other technical issues that come into play with small and larger apertures that are related to the lens performance, as well as the size of the CCD. A large aperture gives a small depth of field so only the main object on the photo is in focus - a single sportsman, for example - whereas a small aperture has a large depth of field - everything in the field of view is in focus,
The preset settings on some cameras (sport/landscape/etc) will give the camera's selection of the best combination of shutter speed and aperture, i.e. one choice.
If the camera has aperture priority, shutter priority or manual, this lets the photographer have more control over the exposure, i.e. you tell the camera what shutter or aperture to use and the camera chooses the corresponding aperture or shutter, according to the light level. And manual control is complete control over the camera, so you choose both the shutter and aperture.
I've just had a look at a 1000IS review and it has a longest shutter speed of 15 seconds which I expect will be set up in the Special Scenes Night Scene (6th image down on the page below).
There doesn't seem to be a shutter speed displayed (but the camera's manual should help here), but there is a +/-0 indicator on the left side of the view which will be an exposure compensation, to allow longer or shorter exposures than the camera decides.
Beware that at night, streetlights and other bright sources of light can mislead the camera's metering. It's better to overexpose at night to stop grainy images. Each element in the CCD (and there are 10million in this camera) needs a certain amount of light (individual photons) to hit it to register any light intensity at all and at night there aren't as many.
Some cameras, but not this one, have B as the slowest shutter speed and this allows the shutter speed to be anything you want for long exposures, but this generally needs a remote shutter release and lock, and good batteries.