This applies to everyone who recently bought a Canon Rebel series or D-series camera and have not touched their metering modes yet:

When we refer to the camera's metering modes, we are referring to the one of 3 modes your DSLR uses to determine the correct exposure for the scene you are looking at.

From the factory, your Rebel XTi comes set for Evaluative metering. If you refer to your manual, you'll see that this metering mode essentially treats the entire scene in the viewfinder as the "metering area". It takes all the lights, all the darks, everything in between, and averages them out to get you an exposure right in the middle of everything.

Now, before I continue, you must understand how the camera reads light. Have you ever been out walking in snow on a bright sunny day? It's an extremely bright scene and most people need to wear shades just to stop from squinting. If you were to take a photograph of this, you would expect that photograph to come out with the ground a bright white ... because you, as a thinking human being, understand that the brightness level of what you're looking at (pure white snow) is supposed to be very bright.

Your camera has NO IDEA what you're shooting at.

This is important. Your camera will try to expose EVERYTHING to what is reffered to as "middle gray" or "18% gray". Your camera doesn't know its snow ... it just knows that there is X amount of light coming from this object and I need to move exposure to X to capture it. Whether you are in Evaluative, Partial, Center-weighted, or spot metering modes ... your camera will try to expose whatever is in the metering area to an average of 18% gray.

In the first photo posted in this thread, the one of the flower, the background is filled with white aluminum siding and the majority of the foreground is filled with yellow (a shade that I would definitely consider brighter than 18% gray) petals. As the camera sees this "light" ... it doesn't know that that's a bright yellow flower and should be exposed brightly, it doesn't understand that that is white aluminum siding and should be exposed brightly ... it just takes an average (and in this case, the average is quite bright) light reading and exposes it so that it is rendered as 18% gray which, obviously, appears dark to human beings who expect white and bright-yellow objects to be just that .... bright.

When exposing any subject, in any metering mode, you must always keep in mind the luminocity of what you are metering off of. If you are taking a picture of a black table-cloth and that cloth takes up a considerable amount of the frame .... your camera is going to over-expose that table-cloth in an attempt to render it an 18% gray .... it will NOT try to render it black for you.

If you're still having difficulty understanding this, go to your bathroom right now, take your camera with you. Put your camera in Av mode (aperture priority mode), leave the flash down (do not use it), and set your ISO at whatever you want. Lay two towels on the ground, one white, one black.

Fill up the viewfinder with the black towel, half depress the shutter button, and take a look at what shutter speed the camera has decided is "correct" for that exposure. Now move over to the white towel and do the same. Note the extreme difference in shutter speeds that your camera has determined is needed to shoot these two different objects. Keep in mind that since both objects are in the same room and should be sitting in the same amount of light .... the correct EV (exposure value) for both is in actuality, identical.

Back to the flower picture. The correct way to shoot this flower would have been to set the camera to Partial metering ... partial metering uses a small area in the center of the viewfinder (a circle roughly the size of 9% of the total viewfinder coverage) as it's metering area. Now, since the subject, in this case, a flower, is something that should most likely be rendered slightly brighter than 18% gray ..... set your Exposure Compensation on the camera (refer to the manual if you are unsure how) to +2/3rds (approximately) of a stop and take the picture.

What you have done here is tell the camera that this is supposed to be a bright object, approximately 2/3rds of a stop brighter than 18% gray, and that's how you want the camera to expose whatever it is metering off of.

The bunny picture suffers the same fate .... the bunny takes up a large portion of the photograph and is stark white. The camera will underexpose him down to an 18% gray. Because he is stark white, I would suggest setting exposure compensation on him to approx +1 2/3rds of a stop (he should be rendered brighter than the flower).

For landscape shooters, it is important to keep this in mind because your "scene" will often be littered with a sky that is brighter or darker than your subject ... or in some cases, white clouds which can be even more devastating to your exposure value. Chose your metering mode wisely and take a moment to think about how your camera is looking at the scene.

For portrait shooters (like me), this is important because the difference in light between shooting an African-American and an un-tanned caucasian (as an example) can be great. I always meter off someone's face and I will use +2/3rds of a stop exposure compensation for pale white folks, -2/3rds of a stop exposure compensation for very dark folks, and I'll leave it right about dead center for well-tanned individuals.

I'd like to point out that this is not a bug, or a problem with your Rebel XTi ... this is a problem that all cameras face and is a limiting ability of their reflective light meter (which only reads the light that is reflected off an object). A much more accurate way to determine the correct exposure value for a particular situation is to use an incident light meter which measures the amount of light hitting your subject, instead of the amount of light reflecting off your subject.

Using the previous towel example, if you had taken an incident light reading inside the room, set your camera to that exact exposure in M mode, and shot both towels .... they would both be exposed properly, the white would look white, and the black would look black.

Let me know if anyone has any questions about this. Again, this is not a problem with the Rebel, all of the underexposure problems I have seen on these forums regarding the rebel have to do with the shooter not understanding metering or how the cameras sees light.