will some one please explan this i need to know thanks.

I'm crossing the great divide between Canon and Nikon (there isn't one, really) because the process is the same.

In most PASM modes, you can tell your camera to take three shots: one "under-exposed", one at 'normal exposure' and one 'over-exposed' (as defined by your camera's lightmeter in the viewfinder). This is used when you're not sure about the correct exposure, for example in situations where there are multiple light sources or where there are shiny reflective surfaces (such as water or snow), etc. You then choose the 'correct' one from the three shots you take.

You can also do this with flash exposure, although try to avoid because you can burn out the flash unit if used continuously and repeatedly (not to mention, temporarily blind your subject!).

You can set the exposure increments (one-third or one-half stops) in the setup menu to narrow or widen the range. FWIW, Nikon has a convenient Bkt button for this. Canon users have to access fiddly menus.

If you shoot using a tripod, bracketing is a half-way measure towards full HDR photography.

AE bracketing is deactivated if you change the aperture or speed. Otherwise, remember to switch it off when not needed.

The reason for bracketing is that cameras guess the correct exposure and most of the time get it right, but you are given this function to cover all your exposure bases.

I hope this helps.

Just to add:

Bracketing is the *only* way to get real HDR photos, just in some cases 3 shots are not enough - you need to do a burst of -2, -1, 0, +1 and +2 to cover the entire range.

Also if lighting is ok (i.e. exposure is 1/250 or faster) and camera is capable of doing 5 fps or more, you can easily do it without any tripod - current HDR software is clever enough to compensate minor shifts.

Also it is recommended to use bracketing in A or M modes only.

My reply may have raised more questions than answers about exposure. The following may help.

All digital cameras feature a lightmeter. This decides how much light should be recorded by the film/sensor. It may sound obvious but until recently, you had to guess this or use an external lightmeter. Depending on the amount of light hitting the sensor, the camera decides the amount of time (shutter speed) and the amount of light (aperture [f/ value]) in full auto mode. In program mode, if you change the ISO or aperture or shutter speed, the camera will automatically adjust the other parameters to prevent a shot being too dark or too bright. In A or S mode, changing one value will make the camera adjust the other(s) according to a pre-defined set of values. In full manual mode (which has nothing to do with autofocus), you have to make all the decisions on your own.

Getting the exposure right is the one part of photography where we all get it wrong. For example, shoot a landscape and the sky is (nearly) always brighter than the land. How do you find the right balance? What about a portrait with the subject bright and the background dark? Too dark or too bright and the subject loses detail. One way of getting the right exposure is the rather empirical approach of bracketing.

Another way refers to the method cameras use to measure light *before* taking the picture. Cameras usually measure light in one of three ways:

* Firstly, the camera measures light for the entire picture as a whole. This is the default option and is usually called “evaluative”. It works in most situations

* Secondly, the camera can be set to give priority to the centre third of the image (usually referred to as “centre-weighting”, which is useful for portraits

* Thirdly, you can set the camera to measure the exact centre of the image or one of the focus points. This is known as ‘spot metering’.

All digital cameras can be adjusted to do this. For example, stick a lit candle in front of a dark background. Shoot it using the evaluative, centre-weighting and spot modes and you’ll be surprised at the results.

Light metering is measured in EV or “stops”, which is a bit (but not quite) like the f/ stops in aperture (these are the values Rokas mentions above). Cameras usually display metering as a horizontal scale in the viewfinder (SLRs) or on the LCD (SLRs and compacts). The centre mark is considered the ‘correct’ exposure. Shift the metering to the left and your image is darker; to the right and the image is brighter. The important thing to remember is that the centre mark isn’t always the ‘correct’ exposure. This is where the histogram comes in very handy, but that’s a whole different story.

The general idea - it's called a rule, but it's more of a guideline - is to have a generally well exposed image without the highlights (bright areas) burnt out or the dark areas (shadows) losing detail. Again, I stress, this is a general idea.

A very good introduction to the fascinating – and difficult – subject of exposure is Bryan Peterson’s “Understanding Exposure”.

Yes i think i have an idea of what it is thanks guys i need that.