550d back focus

Hi chaps, hope I'm posting in the correct place, I have set up my 550d to use the * button to focus, and the shutter to get exposure, I have looked everywhere but cannot find how (if at all possible) to 'lock' exposure - ie say if I have the camera on a tripod, fairly constant lighting, using spot focus I half press the shutter to get a reading from the best place in the shot, recompose and release the shutter and sit back and wait for the bird (or whatever) to appear if I then, when the subject is Ok, hit the shutter it's going to reassess the exposure and alter it ... my question really is, if I'm using the * button for focus is it possible to establish an exposure value and lock it ?

Regards Roger

If you're going to meter for ambient then wait for the subject, why not try using manual? Look through the viewfinder and adjust the aperture/shutter to what you want and set the focus when you need it, that way the exposure won't change when you hit the shutter.

AAAAAhhhhhh manual - I've only looked at the little "M" on the dial so far ... this may be just the time for me to dig in ... it can't be that daunting after all I am so old I go back to the days when there was nothing but manual and seemed to work it out, we're all too used to having our hands held these days ... thank you, I will get on with experimenting with manual. Best Roger