Setting exposure with focus/recompose

I took some pictures of 2 people in a room with a window behind them. I focused on the most important subject, kept the shutter button half-way down, then recomposed so both of them were in the frame, putting the focus point between them. They were moving a little bit so the focus point at the time I took the exposures was sometimes on a wall and sometimes on the window glass, with the light coming in from outside. I assumed that the exposure was set when the focus was set but it wasn't. In the exposures where the focus point was on the wall, the exposures aren't great but they're okay. When the focus point was on the window at the time of exposure, the window light caused the exposure to go way down so the subjects were silhouetted.
Is this normal behavior or have I caused this by setting something off the default?
Thanks
I'm using a Rebel XSi in aperture-priority mode and spot-metering.

Check your manual & see if you can lock exposure with a back button. Better yet, use M & try this: Need an exposure crutch?

Normally the shutter button does not lock exposure...use the * on the back to lock exposure. Read the manual to find out more about behavior of your camera with that button, as the exposure will be held only certain circumstances or be cleared.

Okay, thanks guys.

Hang on, if you're in One Shot AF, half-pressing the shutter should lock the AF, but not the exposure if you're in Av, is this not true with all these DSLRs?

If, though, you are in AI Focus or AI Servo your movement will affect the focus.

As to the back button AF, it's useful, but pressing then releasing the back button is like half-pressing and holding down the shutter button in One Shot AF.

tonylong wrote in post #15531029
Hang on, if you're in One Shot AF, half-pressing the shutter should lock the AF, but not the exposure if you're in Av, is this not true with all these DSLRs?

This is exactly what the OP said.

Wilt advised using the * button to lock exposure, not AF.

I thought that the Canon default was to lock both focus and exposure with a half press of the shutter button. This is locked only while the button is pressed. I though that you then used the * button to lock the exposure in one point, before then moving the camera and locking focus with a half press, which you can then recompose or not as you wish while holding the half press. The * button can be used to both AE lock and recompose/focus or to lock exposure between shots because it actually locks the exposure while allowing the shutter button to be used separately.

Alan

The manual is actually confusing on this. It says, in part, "Press the * button to lock the exposure, then recompose and take the shot. This is called AE Lock. It is effective for backlit subjects." I was in a backlit situation and it sounds like that's what I should have done.
But the manual also says, before that, "Use AE Lock when the area of focus is to be different from the exposure metering area or when you want to take multiple shots at the same exposure setting." Well, the area of focus was exactly where I wanted the exposure to be read from, I just wanted to recompose after setting both.
So I'll try the * button and see what happens.

ncjohn wrote in post #15532085
The manual is actually confusing on this. It says, in part, "Press the * button to lock the exposure, then recompose and take the shot. This is called AE Lock. It is effective for backlit subjects." I was in a backlit situation and it sounds like that's what I should have done.
But the manual also says, before that, "Use AE Lock when the area of focus is to be different from the exposure metering area or when you want to take multiple shots at the same exposure setting." Well, the area of focus was exactly where I wanted the exposure to be read from, I just wanted to recompose after setting both.
So I'll try the * button and see what happens.

In that case, if you are using the default button settings, you'd want to both keep the shutter button half-pressed and the AE Lock button pressed.

If you decide to move the shutter function to the * button, then the AE lock function will change -- I don't recall the various options, I don't have a camera manual handy...